INTRODUCTION Life History Newly settled blue crabs Callinectes sapidus gen- erally grow through a series of early juvenile instars (developmental stages punctuated by ecdysis) within seagrass and other settlement habitats of lower estu- aries (Orth and van Montfrans 1987;Williams et al. 1990; Olmi and Lipcius 1991; Perkins-Visser et al. 1996; Pile et al. 1996; Pardieck et al. 1999). They may also move upstream to settle in soft-bottom areas of some subestuaries, such as in lower Chesa- peake Bay (Seitz et al. 2003a; Lipcius et al. 2005) and Mississippi Sound (Rakocinski et al. 2003) (see also Lipcius et al., Chapter 13). Upon attaining the 5th to 7th crab instar and ~20 mm carapace width (CW), juveniles typically disperse from their settle- ment site to exploit an array of habitats throughout the estuary (Pile et al. 1996; Moksnes et al. 1997; Pardieck et al. 1999; Etherington and Eggleston 2000, 2003). However, dispersal may occur as early as the 1st crab instar shortly after settlement in some estuaries, such as in North Carolina (Reyns and Eggleston 2004). Dispersed juveniles use a variety of micro-habitats where they forage on diverse food resources and grow for a typical period of 0.5 to 1.5 y (depending on temperature and food availability) until they reach sexual maturity in the 16th to 20th crab instar at ~110 to 180 mm CW (Van Engel 1958; Tagatz 1968a; Rugulo et al. 1998b). After mating, inseminated mature females cease molting and migrate back to the lower estuary, produce broods, and incubate eggs until larvae are released and transported out of the estuary onto the conti- nental shelf (see Jivoff et al., Chapter 7; Epifanio, Chapter 12). By contrast, mature males may con- tinue to molt and grow for 1 to 3 additional instars (typical large size is 180 to 200 mm, but occasionally some grow to >250 mm CW) (Van Engel 1958; see also Smith and Chang, Chapter 6). Unlike females, mature males tend to remain dispersed in the upper estuary without migrating directionally along the salinity gradient (Van Engel 1958; Hines et al. 1990, 1995). Timing of life history events and life span appear to vary with latitude. In higher latitude estu- aries with cold winter seasons, juvenile and adult stages may move into deeper water in channels to over-winter. This is a period of little activity, with movement, feeding, and molting proceeding slowly if at all, especially at temperatures below 9? or 10?C. Because of the shorter warm season, maturation typically occurs in the second season after settlement in Chesapeake and Delaware bays (e.g., see Ju et al. 2003 and Smith and Chang, Chapter 6). Most blue crabs are thought to die after a lifespan of about 3 y, although there is some debate as to whether death ensues from senescence or from high rate of fishing capture. In Chesapeake Bay, a small portion of the population lives to be 4 to 5 y old, with individuals rarely (<1%) living to 6 to 8 y (Sharov et al. 2003; see also Fogarty and Lipcius, Chapter 16). This pat- tern of timing and dispersal of life history stages Chapter 14 Ecology of Juvenile and Adult Blue Crabs ANSON H. HINES 565 Copyright ? 2007 Maryland Sea Grant College. The Blue Crab: Callinectes Sapidus, Victor S. Kennedy and L. Eugene Cronin, editors All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage or retrieval system, without permission in writing from Maryland Sea Grant. along the estuarine salinity gradient is also evident in South Carolina (Archambault et al. 1990). However, in lower latitude estuaries of Florida and the Gulf Coast, blue crabs grow faster over a longer warm season to reach maturity within a year (Perry 1975; Tatum 1980). Although mature crabs in this region are presumed also to live to a typical age of 2 to 3 y and a maximum age of about 6 y (Perry et al. 1998; Steele and Bert 1998; Pellegrin et al. 2001), there are few data about lifespan along the Gulf Coast. Ecological interactions of juvenile and adult blue crabs begin when juveniles disperse out of their settlement habitat and continue through the 1 to 3 y period of growth, maturation, and adulthood (e.g., Gillanders et al. 2003). This chapter addresses these ecological interactions during this complex period in the blue crab?s life history. The interactions typi- cally involve crabs that range in size from 20 to 200+ mm CW. This range includes crab instars 7 to 22 and ages of about 2 months to 3+ y post-settle- ment. Other chapters that link with this topic of juvenile and adult ecology include the biology of larvae (Epifanio, Chapter 12) and the ecology of early juvenile stages from settlement until dispersal from the settlement habitat (Lipcius et al., Chapter 13). Additional supplemental information can be found in the chapters on reproductive biology (Jivoff et al., Chapter 7), ecological aspects of disease and parasites (Shields and Overstreet, Chapter 8), and environmental physiology (Tankersley and Forward, Chapter 10). Aspects of population biology are important components of the ecology of blue crabs that I consider in this chapter; however, the topic of population dynamics, especially considered from the perspective of fishery models and stock manage- ment, is addressed in a separate chapter (Fogarty and Lipcius, Chapter 16). Overlap among these chapters is inevitable and valuable. Previous Reviews and Historical Progression of Research Previous reviews of the ecology of blue crabs in the context of their life history provide an historical perspective on the shifting emphasis of blue crab research. Early work focused on the life cycle and large-scale habitat use, primarily in Chesapeake Bay where the fishery was concentrated (Hay 1905; Churchill 1918, 1919; Cronin 1954; Cargo 1958; Van Engel 1958, 1987). A comprehensive review by Millikin and Williams (1984) not only summarized blue crab biology and ecology but also provided a key benchmark that integrated physiology and behavior into ecology by life stage in the context and terminology of modern demography, popula- tion dynamics, and standard crustacean physiological terms (e.g., molt stages, life stages) that were not idiosyncratic to fisher?s colloquialisms (e.g., pink sign, sook, jimmy). Millikin and Williams (1984) also invoked modern statistical approaches to describe variability in population biology. Further, they considered geographical variation of blue crab biology, providing summaries of the fisheries and data on some aspects of biology for populations in estuaries along the East and Gulf coasts of North America. As the blue crab fishery expanded geographi- cally, fishery biologists developed reviews with a focus on state management of stocks along the East and Gulf coasts (e.g., Perry and Van Engel 1982; Guillory et al. 2001), including New York (Briggs 1998), New Jersey (Stehlik et al. 1998), Delaware (Kahn et al. 1998), Maryland and Virginia (Van Heukelem 1991; Miller and Houde 1998; Rugolo et al. 1998a, b), North Carolina (Henry and McKenna 1998; Eggleston et al. 2004), South Car- olina (Archambault et al. 1990; Whitaker et al. 1998), Georgia (Evans 1998), Florida (Tagatz 1968a, b; Steele and Bert 1994, 1998), Alabama (Heath 1998), Mississippi (Perry 1975; Perry et al. 1998), Louisiana (Guillory and Perret 1998),Texas (Hammerschmidt et al. 1998), and the Gulf of Mexico (Perry et al. 1984; Steele and Perry 1990; McClintock et al. 1993; Guillory et al. 1998, 2001). Comparative analyses and proceedings flowed from meetings and colloquia that integrated scientists and managers across regions of the United States (e.g., Perry and Van Engel 1982), with proceedings 566 T HE B LUE CRAB of blue crab conferences appearing in the Bulletin of Marine Science in 1990, 1995, and 2003 (Smith et al. 1990; Olmi and Orth 1995; Eggleston 2003), and in the Journal of Experimental Marine Biol- ogy and Ecology in 2005 (Seitz 2005). Steele and Bert (1994) summarized information for latitudinal comparisons of reproductive and molting seasons, considering implications for population biology for the U.S. portion of the species? distribution. Inter- specific comparisons also provided valuable insights into the ecology of Callinectes sapidus as the most widely distributed member of the genus, triggered especially by Williams? (1974; later updated in Williams 1984) systematic monograph of the genus that included summaries of the ecology of each species. Comparative studies of species of Call- inectes in the Gulf Coast (Hsueh et al. 1992a, b, 1993) and Caribbean estuaries (Norse 1975, 1977, 1978a, b; Norse and Estevez 1977; Norse and Fox- Norse 1982; Haefner 1990a, b; Stoner and Buchanan 1990) extended the general ecology of C. sapidus . Comparative studies also recognized the valuable research contributions from Latin America (e.g., Paul 1981, 1982; DeVries et al. 1983; Ortiz and Guti?rrez 1992; Rosas et al. 1994; Oesterling and Petrocci 1995; Lazaro-Chavez et al. 1996; Oesterling 1998; Mantelatto and Fransozo 1999; Ch?zaro-Olvera and Peterson 2004). Limited reports also exist for C. latimanus in Africa (Kwei 1978). With the exception of a set of studies of C. similis (Hsueh et al. 1992a, b, 1993), reports for other species of Callinectes along North America remain unfortunately sparse and almost anecdotal (e.g., Daugherty 1952b;Tagatz 1967; Perschbacher and Schwartz 1979). Although the Chesapeake Bay population continues to receive the most intensive and comprehensive research on C. sapidus, blue crab research along the Gulf Coast is gaining in scope and breadth, and in this chapter I seek both to consider geographic variability as a key aspect of ecology and to encourage interspe- cific comparisons as a powerful tool to analyze blue crab ecology. POPULATION BIOLOGY Interaction of Population Dynamics and Community Ecology Blue crab populations are characterized by three fundamental features that affect their dynamics and community interactions: (a) a distinctly bimodal population size structure resulting from seasonal reproduction and recruitment, which has marked effects on size-dependent interactions as predators and as prey that in turn affect habitat use and move- ment; (b) spatial variation in sex ratio that reflects differences between mature females and males in habitat use and movement; and (c) marked annual variation in abundance, which significantly affects density-dependent interactions, such as movement, habitat use, trophic interactions, competition, and sources of mortality. Careful description of these three variables ? population structure, sex ratio, and abundance ? is essential to understand the interac- tion between population biology and other aspects of the community ecology of blue crabs (e.g., see Wahle 2003 for comparison of blue crab dynamics with clawed and spiny lobsters). Hence, I summa- rize population structure (size and sex ratio) and spatial-temporal variation in abundance as basic descriptive elements of blue crab ecology that typi- cally are prerequisites for mechanistic studies of complex community interactions, descriptions of which follow later in this chapter. The reader should refer to Fogarty and Lipcius (Chapter 16) for further detailed discussion of population dynamics. Sampling Artifacts and Gear Efficiencies for Population Variables The question, ?How many blue crabs are there in a population or habitat?? is straightforward as a fundamental aspect of their ecology and fishery management. Similarly, size and sex composition of populations are basic elements that interact with both ecological processes and the fishery. However, E COLOGY OF J UVENILE AND A DULT B LUE CRABS 567 quantifying and predicting variation in abundance and population structure of blue crabs are difficult, because numerous sampling artifacts confound accurate estimates of blue crab densities. The arti- facts vary interactively among gear type, habitat, crab size, and activity levels (Barkley 1972; Allen et al. 1992). A problem in the analysis of population size structure derives from sampling blue crabs with fish- ery gear, which is often designed to by-pass small juveniles, producing estimates of size-structure that are skewed to large, fishery-legal individuals (e.g., Caddy 1979). Thus, fishery gears selectively sample certain sizes and life stages of blue crabs that are tar- geted by commercial interests (e.g., Cargo 1954b; Van Engel 1962; Harris 1979; see also Kennedy et al., Chapter 15). Relative indicators of abundance have been derived from fishery catches, which may reflect fluctuations in fishery effort and size limits as much as variation in crab density (e.g., Rugulo et al. 1997, 1998a, b). Even when populations have been sampled consistently with fishery-independent sur- veys, artifacts in population structure and abundance also may be introduced by the sampling of habitats that do not include all sizes or ages of crabs. At best, gears have different catch efficiencies in various habitats. Many sampling gears do not func- tion well in structured habitat, where towed nets may become fouled or hung up. Traps deployed in structured habitat may have a different attraction to crabs than when deployed on bare sediment, where the trap itself provides more structure than the habi- tat. As noted, most sampling gears, including those employed in fishery-independent surveys, may fail to sample crabs of certain sizes (especially small juve- niles), so they must be fitted with cod-end liners with mesh as fine as 6 to 7 mm to capture small crabs. Most gears selectively capture crabs that are active, so trawls pass over sedentary crabs buried in the sediments and baited traps do not attract crabs that are not feeding. This selectivity can result in estimates of higher abundance in warm summer months when crabs are active and feeding than dur- ing cold winter months when most are buried. To sample buried crabs as well as those on the surface, the gear requires features like tickler chains on trawls, or teeth projecting downward on leading edges of dredges (e.g., Chittenden and Van Engel 1972; Sulkin and Miller 1975), but effects of these modifications also vary with crab activity level. Some studies have compared results among gears to determine which type of sampling will give the greatest relative index of crab abundance, or to consider trade-offs of sampling effort, mesh size, overall size of the mouth of the net, and the like (e.g., Sulkin and Miller 1975; Miller et al. 1980; Bishop et al. 1983; Rozas and Minello 1997). Com- pared to large mesh nets, small mesh nets create a large pressure wave in front of the gear when pulled through water or may clog with sediment, which may allow crabs to detect and escape the net. Large trawls may be better able than small ones to sample large crabs that evade capture by swimming out of the way. For seagrass beds with many small crabs, the gear that provides the best estimate of true den- sities has been suction dredges operated within enclosure rings or drop samplers (e.g., Orth and van Montfrans 1987; Ruiz et al. 1993; Pile et al. 1996; Rozas and Minello 1997; Rakocinski et al. 2003). However, although suction sampling works well for regions of higher crab densities where sampling area can be small, it typically does not work well for habitats (e.g., open sediment) that require large sam- pling areas because of low densities of crabs. Block nets provide good, unbiased samples of blue crab subpopulations in tidal marsh creeks (e.g., Hines et al. 1987; Hettler 1989; Rulifson 1991; Fitz and Wiegert 1992; Coen et al. 1999). The fishery-inde- pendent winter dredge survey in Chesapeake Bay, which provides the most comprehensive sample of the population, uses a Virginia crab dredge fitted with a fine mesh bag deployed at a time when crabs tend to be concentrated in deeper habitats lacking structure, and when crabs are not moving (Sharov et al. 2003). Because most types of gear do not catch all of the crabs in the sampling area, providing instead a relative indicator of crab abundance, gear efficiencies have been estimated to allow catch data to be adjusted for absolute densities (e.g., Stokesbury et al. 1999). Gear efficiencies are estimated either by placing known densities of marked crabs into the 568 T HE B LUE CRAB sampling area, or by repeatedly sampling a set area until all crabs are removed, allowing back-calcula- tion of the fraction of crabs caught in the initial sample. Orth and van Montfrans (1987) estimated 88% efficiency for 2 to120 mm blue crabs sampled with a suction dredge within a drop ring enclosure deployed in seagrass habitat. By contrast, catch effi- ciency of trawls is generally much lower: 25 ? 5% for crabs >25 mm in soft bottom (Homer et al. 1980); 2 to 20% for crabs in unvegetated soft bottom (Orth and van Montfrans 1987); and only 0.4% for crabs <25 mm CW and 1.4% for crabs >25 mm CW in seagrass habitat (Orth and van Montfrans 1987). Seines with 7 mm mesh had catch efficien- cies of about 50% for >25 mm CW, but only about 5% for crabs <25 mm in summer in shallow muddy bottoms of upper Chesapeake Bay (Davis and Hines, unpubl. data). Dredge sampling of blue crabs buried in sediments during winter samples have effi- ciencies of 22 to 47% (Endo 1992; Zhang et al. 1993; Zhang and Ault 1995;V?lstad et al. 2000). Mark-recapture methods may be used to esti- mate true abundance of animals (Cormack 1968; Pollock et al. 1990). However, these methods require restrictive assumptions that rates of mortality, natality, immigration, and emigration are either nil or known (Pollock et al. 1990), which is usually not the case for blue crabs. Modified approaches of these methods are being tested for the Chesapeake Bay population (Hoenig et al. 2003). Population Structure: Size and Sex Blue crab populations in Chesapeake and Delaware bays typically exhibit a bimodal size-fre- quency distribution that derives from the seasonal pattern of summer to fall larval recruitment, retarded growth during cold winter months, and a 2-to-3 y life span at this northerly latitude of the species? dis- tribution (Van Engel 1958; Hines et al. 1990). For example, seasonal cycles in the population structure of blue crabs in upper Chesapeake Bay (Fig. 1) show that crabs in the 0+ age-class enter the population in fall and disperse up the estuary from their settlement habitat as 20 mm juveniles, with possible further up- estuary dispersal in spring. The 0+ age-class grows to 80 to 90 mm during its first warm season of feed- ing in the subestuaries, and these juveniles become the 1+ age-class in their second summer, when they molt to maturity, with mature females ranging from 100 to 180 mm CW and mature males ranging from 110 to 200 mm CW. Males molt to maturity and may molt one to three times as adults, whereas females cease molting after attaining maturity. The winter dredge survey of crabs in Chesapeake Bay clearly shows the bimodal structure of the popula- tion, and allows for comparisons of age-class strength among years and estimates of annual sur- vivorship (Sharov et al. 2003). This pattern of bimodal population structure and fall recruitment of the 0+ age-class is also evident in Charleston Har- bor, South Carolina (Archambault et al. 1990). Although synchronous molting produces distinct modes for each instar in the population size struc- ture of some crab species, such as the snow crab Chionoecetes opilio (Robichaud et al. 1989), this is not evident in blue crab populations. At lower latitudes, a longer season of egg pro- duction, more continuous larval recruitment, and juvenile growth over a longer season results in less distinct age classes, making population size structure less bimodal (Steele and Bert 1994; Perry et al. 1995). However, bimodal population structure is still evident in Tampa Bay, Florida, where reproduc- tion and molting tend to have seasonal peaks in both late spring and late summer (Steele and Bert 1994). Populations in Florida and the Gulf of Mexico are also subject to a cool winter period of low repro- ductive activity, but the extended recruitment season and variable growth rates appear to result in a size structure that is not distinctly bimodal (Perry et al. 1998). Mean crab size of the 1+ age mode has declined significantly for males captured in pots in central Chesapeake Bay from 1968 to 1995 (Abbe and Stagg 1996; Abbe 2002) and for mature females in the spawning stock of the lower Bay (Lipcius and Stockhausen 2002). For males, the reduction in size appears to be attributable to fishery removals of large individuals (Abbe and Stagg 1996). For females, the cause arguably also may be related to short-term selection for physiological limits to growth (Lipcius and Stockhausen 2002; Sharov et al. 2003). E COLOGY OF J UVENILE AND A DULT B LUE CRABS 569 570 0.25 0.15 0.10 0.05 0.00 Male 0.20 0.15 0.10 0.05 0.00 0.20 0.15 0.10 0.05 0.00 0.20 0.15 0.10 0.05 0.00 0.20 0.15 0.10 0.05 0.00 0.20 0.15 0.10 0.05 0.00 0.20 0.15 0.10 0.05 0.00 0.20 0.15 0.10 0.05 0.00 0.25 0.15 0.10 0.05 0.00 0.20 0.15 0.10 0.05 0.00 0.20 0.15 0.10 0.05 0.00 0.20 0.15 0.10 0.05 0.00 0.20 0.15 0.10 0.05 0.00 0.20 0.15 0.10 0.05 0.00 0.20 0.15 0.10 0.05 0.00 0.25 0.15 0.10 0.05 0.00 0 20 40 60 80 100 120 140 160 180 200 0 20 40 60 80 100 120 140 160 180 200 0 20 40 60 80 100 120 140 160 180 200 0 20 40 60 80 100 120 140 160 180 200 0 20 40 60 80 100 120 140 160 180 200 0 20 40 60 80 100 120 140 160 180 200 0 20 40 60 80 100 120 140 160 180 200 0 20 40 60 80 100 120 140 160 180 200 0 20 40 60 80 100 120 140 160 180 200 0 20 40 60 80 100 120 140 160 180 200 0 20 40 60 80 100 120 140 160 180 200 0 20 40 60 80 100 120 140 160 180 200 0 20 40 60 80 100 120 140 160 180 200 0 20 40 60 80 100 120 140 160 180 200 0 20 40 60 80 100 120 140 160 180 200 0 20 40 60 80 100 120 140 160 180 200 Female Immature Mature April May June July August September October November Fr eq ue nc y Carapace width (mm) Carapace width (mm) Figure 1. Seasonal variation in the size structure of the blue crab population in the Rhode River subestuary of Chesapeake Bay, Maryland. Long-term (25 y) average size-frequency distributions of crabs sampled in monthly otter trawls are shown for males (left) and females (right). The bimodal size structure reflects the two-year cycle of recruit- ment and growth to sexual maturity. Note recruitment of 0+ age cohort in the fall as 20-mm crabs disperse up the estuary into the juvenile nursery habitat, as well as growth of the 0+ and 1+ age cohorts as the season progresses. For females, solid columns indicate juveniles and open columns indicate sexually mature crabs. Note disappearance of mature females at the time of their fall departure and migration back down the estuary. See Hines et al. (1987, 1990). The sex ratio is presumed to be balanced in most populations of blue crabs, especially at juvenile stages (e.g., Fitz and Wiegert 1992). However, quan- titative assessment of sex ratio is difficult because males and females partition the available habitats along the estuarine salinity gradient (Hines et al. 1987). The habitats extend over large distances in many systems like Chesapeake Bay, which thus inhibits standardized sampling for all segments of the population. Moreover, the migratory behavior of females produces seasonal cycles in sex ratios at indi- vidual sites (Hines et al. 1987; Fitz and Wiegert 1992; Steele and Bert 1994).Apart from these diffi- culties, long-term fishery-independent sampling indicates that the composition of males in the popu- lation in central Chesapeake Bay declined signifi- cantly from 1968 to 1982, but leveled off from 1983 to 1995 (Abbe and Stagg 1996). System-wide sam- pling with a winter dredge survey indicates that the sex ratio was balanced and stable in Chesapeake Bay during the 1990s (Sharov et al. 2003). Abundance Seasonal and Annual Variation Blue crab abundance in higher latitude estuar- ies, such as Chesapeake Bay, typically exhibits a strongly seasonal cycle (Miller et al. 1975; Hines et al. 1987, 1990; Orth and van Montfrans 1987; Par- dieck et al. 1999), which reflects natural behavior and demography of the crabs. Trawling in shallow portions of the estuary (Fig. 2) shows fluctuating abundances of juvenile crabs that result from disper- sal into the post-settlement nursery habitats up the Bay during fall and possibly spring (Hines et al. 1990). The seasonal cycle of catches also reflects declining abundances from the departure in fall of mature females that migrate down the estuary (Cargo 1958; Schaffner and Diaz 1988), as well as the movement of juveniles and mature males that shift into deeper waters over winter (Hines et al. 1987; Sharov et al. 2003;Aguilar et al. 2005). Since senescence of most crabs appears to result in death after about 2 to 3 y, the seasonal cycle of declining abundance may also reflect loss due to death of old crabs. However, there is little known about the sea- sonal timing of this source of mortality, except that cold temperatures interacting with low salinities during winters may take a significant toll, depending on crab age groups and distribution in the estuary (Sharov et al. 2003; Rome et al. 2005). Fishery catches also track the seasonal cycle of blue crab activity and abundance, with peak catches of intermolt (hard) crabs in pots during summer and ?hibernating? crabs caught with dredges in the lower Chesapeake Bay in winter (e.g., van Engel 1958; Rugulo et al. 1998; Sharov et al. 2003). Intense fishing pressure can also deplete the numbers of legal-size (mostly adult) blue crabs in large sys- tems like Chesapeake Bay, both seasonally and annu- ally (Lipcius and Stockhausen 2002; Sharov et al. 2003). Seasonal peak abundance of blue crabs in Chesapeake Bay exhibits marked annual variation that may differ by an order of magnitude or more among years in fishery-independent trawl surveys (Hines et al. 1987, 1990; Lipcius and Van Engel 1990;Lipcius and Stockhausen 2002), fishery catches (Rugulo et al. 1998b), and in the fishery-indepen- dent winter dredge survey (Sharov et al. 2003). Annual variation in blue crab abundance is signifi- cantly correlated with salinity and temperature as environmental variables that are linked to broad cli- E COLOGY OF J UVENILE AND A DULT B LUE CRABS 571 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 00 01 02 03 04 05 06 Year 250 200 150 100 50 0 M ea n m on th ly n um be r o f bl ue c ra bs p er tr aw l Figure 2. Long-term variation of mean monthly num- ber of blue crabs caught in otter trawls in the mesoha- line Rhode River subestuary of Chesapeake Bay, Maryland. Note seasonal fluctuation with peak abun- dances in summer, and large inter-annual variation. See Hines et al. (1987, 1990). matic factors influencing estuarine ecosystems (Ulanowicz et al. 1982; Steele and Bert 1994). Varia- tion in blue crab abundance among years mainly reflects large annual variations in larval recruitment (Orth and van Montfrans 1987; Goodrich et al. 1989; Olmi et al. 1990; van Montfrans et al. 1990, 1995; Lipcius and Stockhausen 2002), resulting in a significant recruit-stock relationship (Tang 1985; Lipcius and Van Engel 1990; Miller and Houde 1998) and in recruitment limitation at some low population levels (Lipcius and Stockhausen 2002). However, it is also recognized that factors affecting survivorship of juveniles after recruitment may decouple the recruit-adult relationship (Hines and Ruiz 1995; Pile et al. 1996; Rome et al. 2005). Long-term variation in blue crab fishery catches and scientific surveys indicate that abundance of the total adult population and mature females (spawning stock) declined drastically by 84% in Chesapeake Bay during the 1990s to early 2000s (Miller and Houde 1998; Lipcius and Stockhausen 2002). The extensive fishery-independent dredge survey throughout Chesapeake Bay conducted in winter also indicated significant declining crab abundance from 870 million to 240 million crabs during the 1990s (Sharov et al. 2003). Variation among Estuaries Blue crab abundance varies greatly among estu- aries, which is often manifested by variation of fish- ery productivity. Historically, Chesapeake Bay has sustained North America?s largest fishery catch of blue crabs, reflecting not only the large size of this estuary but also a highly productive combination of habitats (Van Engel 1958). The Chesapeake?s exten- sive array of key nursery habitats, including seagrass beds, oyster reefs, and many large subestuarine tribu- taries with irregular shorelines of shallow water, pro- vide refuge for juvenile blue crabs (Everett and Ruiz 1993; Ruiz et al. 1993; Hines and Ruiz 1995). Adja- cent Delaware Bay is about one third the size of Chesapeake Bay but has produced disproportion- ately fewer blue crabs (~10-15% of Chesapeake Bay), perhaps because extensive seagrass beds are lacking in Delaware Bay (Kahn et al. 1998). Blue crabs appear to depend more on extensive salt marsh habitats in estuaries along the southeast coast and much of the Gulf Coast than they do in Chesapeake Bay (Dudley and Judy 1973; Ryer et al. 1990;Thomas et al. 1990; Fitz and Wiegert 1991b, 1992; Zimmerman et al. 2000). However, abun- dances of newly settled juveniles (<10 mm CW) in Mississippi Sound were high (10-100 crabs m-2) in both seagrass habitats and in bare sediments (Rakocinski et al. 2003), indicating that the role of these habitats may be similar to that in estuaries like Chesapeake Bay. Crab abundance in Chesapeake Bay has been greater than in other estuaries despite its somewhat limited extent of salt marshes; however within Chesapeake Bay, salt marshes that occur pri- marily along the central Eastern Shore have sus- tained the greatest production of crabs in this estu- ary. Further, even in tributaries of the western shore of the Chesapeake, habitats associated with up-river salt marsh appear to have the highest food resources and produce the fastest growth rates of juveniles, rivaling seagrass beds (Seitz et al. 2005). Some of the variation in abundance among estuaries appears to result from differences in the dynamic balance of larval recruitment and rates of predation upon juveniles (Heck and Coen 1995). Gulf Coast estuaries experience levels of blue crab larval recruitment and abundance of early juveniles (<10 mm CW) that are one to two orders of mag- nitude greater than in East Coast estuaries, but pre- dation by a large guild of predators also imposes a higher rate of juvenile mortality along the Gulf Coast (Heck and Coen 1995; Morgan et al. 1996; Heck et al. 2001; Heck and Spitzer 2001; Rakocin- ski et al. 2003). Currently, published estimates of the total num- bers of blue crabs in an estuary are available only for Chesapeake Bay. The number of mature female crabs comprising the spawning stock in 1200 km-2 of the lower Bay was estimated using trawling as varying from July to August peak levels of 9 x 106 crabs in 1986 and 1.5 x 106 crabs in 1987, declining in fall to 9 x 105 crabs in 1986 and 6 x 105 in 1987 (Jones et al. 1990; Prager 1996). Based on the Chesapeake?s bay-wide fishery-independent winter dredge survey, Sharov et al. (2003) indicated that the total abundance of the Chesapeake population fluc- 572 T HE B LUE CRAB tuated between 240 million and 870 million crabs >15 mm CW from 1990 to 1999 (Fig. 3). During that decade, juveniles (0+ age class) fluctuated between ~100 million and 550 million crabs while adults (1+ age class) declined from ~350 million to 70 million crabs. Using a combination of 1990 to 1996 estimates from the winter dredge survey (Rothschild and Sharov 1997) and the Virginia sum- mer trawl survey (e.g., Lipcius and Van Engel 1990), Seitz et al. (1998) estimated the number of mature female blue crabs to be ~33 to 182 million crabs. However, only ~3.7 million (2-11%) of these females appeared to reside within the spawning sanctuary of the lower mainstem of the Bay in sum- mer, forming the core of the Chesapeake reproduc- tive stock. HABITAT USE Habitat use after post-settlement dispersal varies by size, sex, and molt stage in blue crabs, such that densities of blue crabs vary greatly among habitats. Although the spatial interaction of life history stage and habitat use has been known for a long time, recent developments in quantitative population modeling have only just begun to incorporate these elements in an explicitly integrated way for large- scale systems (Miller 2003; Jensen and Miller 2005). Generally, crab densities are highest in association with structured habitats and lowest on non-struc- tured soft-bottoms, but densities are also markedly affected by salinity, dissolved oxygen concentration, and other factors. Highest abundances have been recorded in seagrass habitats (up to 100 small crabs m-2), especially in lower estuarine zones, where early post-settlement juvenile stages are often concen- trated (Heck and Orth 1980a, b; Heck and Thoman 1984; Orth et al. 1984;Williams et al. 1990;Wilson et al. 1990; Etherington and Eggleston 2000; Rakocinski et al. 2003). Even where juvenile densi- ties are low, the extent of non-seagrass habitats can provide accumulative importance to the population, especially at subsequent life stages (Hines and Ruiz 1995; Blackmon and Eggleston 2001; Rakocinski et al. 2003; Lipcius et al. 2005;Posey et al. 2005; Seitz et al. 2005). Non-seagrass, usually non-structured, habitats are crucial for intermolt adults (e.g.,Wolcott and Hines 1989b; Hines et al. 1990). Thus, a wide range of estuarine habitats is required to complete the life cycle, typically involving sequential use of a series of habitats along the salinity gradient (e.g., Gillanders et al. 2003). Structured Habitats and Juveniles Juveniles >25 mm CW use an array of struc- tural habitats that provide them with refuge from predation and cannibalism (see sections on these topics below) and with food resources (see section below). Throughout their geographic range, juve- niles have used seagrass, including Halodule wrightii shoal grass, Z ostera marina eel grass, Thalassia tes- tudinum turtlegrass, and Ruppia maritima wigeon grass (Tagatz 1968a; Laughlin 1979; Heck and Orth 1980a, b; Penry 1982; Sheridan and Livingston 1983; Heck and Thoman 1984; Heck and Wilson 1987; Wilson et al. 1987; Thomas et al. 1990; Williams et al. 1990; Perkins-Visser et al. 1996; Par- dieck et al. 1999; Hovel et al. 2002; Rakocinski et al. 2003; Hovel and Fonseca 2005). Densities of early juvenile crabs (<20 mm CW) range from 5 to 50 crabs m-2 in North Carolina sounds, with highest densities in seagrass habitats (Etherington and Eggle- ston 2000). Peak densities of 50 to 90 small juveniles m-2 occur in seagrass beds of Chesapeake Bay and the Gulf of Mexico (Miller et al. 1980; Sheridan and Livingston 1983; Orth and van Montfrans 1987; Thomas et al. 1990; Williams et al. 1990; Perkins- Visser et al. 1996; Pardieck et al. 1999; Rakocinski et al. 2003). Recovery and restoration of submerged vegetation habitat has often been considered vital to sustaining blue crab populations and fisheries (e.g., Anderson 1989). However, submerged vegetation in lower salinities of upper Chesapeake Bay was not important habitat for juvenile blue crabs (Heck and Thoman 1984), whereas juveniles were attracted to patches of seagrass as refuge habitat in a lower meso- haline estuary of upper Chesapeake Bay (Ruiz et al. 1993), so use of this habitat may depend on salinity. Juveniles also obtain refuge in other vegetated habitats, especially drifting algae and salt marshes. Algal mats and drifting algae (Heck and Orth E COLOGY OF J UVENILE AND A DULT B LUE CRABS 573 574 A B C 70 60 50 40 30 20 10 0 45 40 35 30 25 20 15 10 5 0 100 80 60 40 20 0 1000 800 600 400 200 0 450 400 350 300 250 200 150 100 50 0 700 600 500 400 300 200 10 0 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 Year Year Year C ra bs p er 1 00 0 m 2 C ra bs p er 1 00 0 m 2 C ra bs p er 1 00 0 m 2 A bundance x 10 6 A bundance x 10 6 A bundance x 10 6 Figure 3. Annual variation in absolute density and population size of blue crabs in Chesapeake Bay, sampled with a fishery-independent winter dredge survey from 1990 to 1999. Data are adjusted for gear efficiency. (A) 0+ age class. (B) 1+ age class. (C) total population. Note large apparent annual variation in recruitment of juveniles (A) and long- term decline in adults (B). Error bars are 95% confidence intervals of the means; N = 1,500 dredge samples per year. From Sharov et al. (2003). 1980a), e.g., Ulva lactea sea lettuce (Wilson et al. 1990a, b; Sogard and Able 1991), may create impor- tant structured habitat for juveniles in some areas. Juveniles use salt marshes and associated marsh creeks throughout much of their geographic range, including New Jersey (Tupper and Able 2000; Jivoff and Able 2003), Chesapeake Bay (Orth and van Montfrans 1987; Lipcius et al. 2005; Seitz et al. 2005), Georgia (Fitz and Wiegert 1991b), Louisiana (Peterson and Turner 1994), and Texas (Zimmerman and Minello 1984;Thomas et al. 1990; Minello and Webb 1997;Akin et al. 2003). Salt marshes in lower estuaries are generally thought to support variably high abundances of blue crabs (up to 13 crabs m-2), particularly juveniles that move between marsh creeks and the marsh during the tidal cycle (Dudley and Judy 1973; Zimmerman and Menillo 1984; Hettler 1989; Ryer et al. 1990;Thomas et al. 1990; Fitz and Wiegert 1991b, 1992; Zimmerman et al. 2000). However, in sustained accurate sampling of a Georgia salt marsh, densities were relatively low: 1 to 10 small juveniles (<80 mm CW) ha-1, and 10 to 50 crabs ha-1 of a size >80 mm CW (Fitz and Wiegert 1991b). Although juveniles moved well into the interior of marshes on high tides of some Louisiana systems (Peterson and Turner 1994), use of the marsh surface in many places appeared to be limited mainly to the edge habitat (Lin 1989; Fitz and Wiegert 1991b; Micheli 1997a), and juvenile abun- dance remained high in adjacent tidal creeks (Orth and van Montfrans 1987; Mense and Wenner 1989). In New Jersey, the species of emergent vegetation affected marsh use by blue crabs, with native species of Spartina alterniflora apparently being preferred by crabs over the invasive species Phragmites australis (Jivoff and Able 2003a). Densities of juvenile (5-10 mm CW) blue crabs in Hudson River estuary salt marshes dominated by this invasive reed averaged 0.06 to 0.39 crabs m-1 in summer (Hanson et al. 2002). Up-estuary habitats associated with, and adja- cent to, salt marshes may be as important as sea- grasses for blue crab nursery habitat, due to availabil- ity of food and lower predation levels (Seitz et al. 2003a; King et al. 2005; Lipcius et al. 2005; Seitz et al. 2005). Blue crabs responded positively in growth, and in use of tidal creeks and other portions of restored salt marsh habitat in New Jersey (Jivoff and Able 2003b). Salt marsh restoration by increased tidal flushing also resulted in increased marsh use by blue crabs in Rhode Island (Raposa 2002). In Texas, however, densities of blue crabs were lower in cre- ated marshes than in natural Spartina alterniflora salt marshes, probably due to differences in elevation and tidal flooding duration of created marshes (Minello and Webb 1997). In addition to submerged vegetation and salt marshes, juvenile blue crab densities are higher in two other structural habitats than in nearby bare sediments. Eastern oyster Crassostrea virginica reefs are used by juveniles over a wide range of salinities (Menzel and Hopkins 1956; Galtsoff 1964; Carriker 1967) and over much of their geographic range, including Florida (Marshall 1954), South Carolina (Lunz 1947; Coen et al. 1999; Lehnert and Allen 2002), North Carolina (Eggleston et al. 1998a, b; Posey et al. 1999a), and Chesapeake Bay (Van Engel 1958; Galtsoff 1964). However, some reports indi- cate relatively low abundances of blue crabs in oyster reef habitats (e.g., Coen et al. 1999; Lehnert and Allen 2002), so the role of this habitat for blue crabs is not well defined. Coarse woody debris, which is especially common in shallow waters of forested shorelines such as upper Chesapeake Bay, has pro- vided still another structured habitat for juvenile blue crabs (Everett and Ruiz 1993). Habitat value for blue crabs depends on com- plexity of structured patches (i.e., density of struc- tural elements) (Bell and Westoby 1986). Blue crab response to habitat complexity varies with crab size and sex in habitat selection experiments (Williams et al. 1990; Schulman 1996). Juveniles 11 to 37 mm CW preferred high density of seagrass to low den- sity. In the absence of potential large cannibalistic blue crabs, juvenile females selected low-density sea- grass over high density while males selected high- density seagrass. In the presence of larger blue crabs, both female and male juveniles selected higher den- sity seagrass. Larger (>45 mm CW) blue crabs selected low-density seagrass. However, effects of shoot density of seagrasses may vary seasonally as other factors, such as crab density and abundance of E COLOGY OF J UVENILE AND A DULT B LUE CRABS 575 food resources within the seagrass habitat and the adjacent unvegetated sediment, come into play (Hovel and Lipcius 2002). Availability of rocky substrates has been very limited in estuaries within the distribution of blue crabs along the East and Gulf Coasts until recent times when humans have added large quantities of rock to estuaries to reduce shoreline erosion and to create breakwaters and jetties. There is almost no information available on blue crab responses to rocky substrates, but juvenile blue crab densities increased in small experimental patches of rock rip- rap compared to bare sediment (Davis and Hines, unpubl. data). Non-Structured, Soft-Bottom Habitats Non-structured soft-bottom habitats are typi- cally characterized by low crab abundances, with summer peak estimates (corrected for trawl catch efficiency) on muddy and sandy bottoms ranging from 0.08 to 0.63 crabs m-2 for juveniles >20 mm CW and adults in upper Chesapeake Bay (e.g., Hines et al. 1987, 1990), 0.02 to 0.36 crabs m-2 in lower Chesapeake Bay (e.g., Seitz et al. 2003a), and 0.1 to 1.7 juveniles m-2 on the Gulf Coast (e.g., Thomas et al. 1990; Rakocinski et al. 2003). In South Carolina, 15-mm juveniles were most abun- dant (0.32 crabs m-2) in sandy-mud substrates of salt-marsh creeks, especially in oligohaline zones; these densities were more than twice those collected on shell hash or in marsh grasses (Mense and Wen- ner 1989). Densities of large juveniles range from 0.01 to 0.6 crabs m-2 and adults from about 0.008 to 0.038 crabs m-2 (after gear correction) sampled in sediments with dredges during winter throughout Chesapeake Bay (Sharov et al. 2003). Recent studies (Seitz et al. 2003a; King et al. 2005; Lipcius et al. 2005; Seitz et al. 2005) indicate that shallow muddy habitats adjacent to salt marshes in the low salinity reaches of subestuaries are of great value to juvenile blue crabs, probably because of their higher food resources (infaunal bivalves) and lower predator abundance. Soft-bottom habitats >1 m deep are frequently used by large, adult crabs. In subestuaries, these habitats are the primary habitat for foraging males and females in the summer (Hines et al. 1987, 1990, 1995; Wolcott and Hines 1989a, 1990). Deeper (>10 m) soft-bottom habitats of the Chesapeake main stem are used extensively by females during fall migration (Aguilar et al. 2005) and by adult males and females for burial over-winter (Van Engel 1958; Schaffner and Diaz 1988; Sharov et al. 2003) and by mature females during the summer spawning season (Lipcius et al. 2003). In upper Chesapeake Bay, Mississippi Sound, and many other estuaries, most structured habitats have been lost due to destructive over-fishing and disease effects on oyster reefs, major declines in sea- grasses, and removal of woody debris as hazards to navigation (Orth and Moore 1984; Everett and Ruiz 1993; Rothschild et al. 1994; Moncreiff et al. 1998). In these systems, juveniles use non-struc- tured shallow (<70 cm deep) water as refuge habitat (Ruiz et al. 1993; Dittel et al. 1995; Hines and Ruiz 1995). In a subestuary of upper Chesapeake Bay, juvenile densities peaked at <40 cm depth and diminished significantly with increasing depth, whereas densities of larger crabs increased at depths >70cm (Hines and Ruiz 1995). Although densities in bare sediment are typically low (<1 crabs m-2), these soft bottom habitats may support large portions of the blue crab population because of their great extent compared to high den- sities in limited structured habitats. In Mississippi Sound, for example, small juveniles (<10 mm CW) are abundant on bare sediments in correlation with, but at lower densities than, nearby seagrass habitats where post-larvae settled (Rakocinski et al. 2003). However, habitat use of non-structured habitat depends interactively on blue crab size and water depth (Ruiz et al. 1993) and on salinity zone (Posey et al. 2005). Multiple Habitat Use Most studies have focused on juvenile blue crab use of a single habitat at a time, or on comparison of a structured habitat with bare sediment. How- ever, when multiple habitat use has been studied with balanced comparisons, juvenile blue crabs exploit all the available habitats in varying degrees. 576 T HE B LUE CRAB Juvenile blue crabs in Texas exploit submerged veg- etation, emergent salt marsh areas, and bare sedi- ment in differing degrees for feeding and refuge from predation (Thomas 1989). On the Texas coast, juveniles <40 mm CW were found at highest density in seagrass, at intermediate density in salt marsh, and at lowest density in bare sediment; how- ever, crabs were larger in salt marshes than in sea- grass or non-vegetated habitat (Thomas et al. 1990; King and Sheridan 2006). Thus, in Gulf coast areas where seagrass habitat is infrequent, salt marsh pro- vides important additional nursery habitat for juve- nile blue crabs (Thomas et al. 1990), but Vallisneria americana beds may provide crucial habitat on low tides when marshes are exposed (Rozas and Minello 2006). In areas of Mississippi Sound, where seagrasses have undergone a severe decline, early juveniles can be abundant in soft sediments adjacent to seagrass patches, with abundance of small juve- niles (<10 mm CW) in seagrass and bare sediment habitats exhibiting covariation (Rakocinski et al. 2003). Densities of post-settlement stages in struc- tured habitat were considerably higher than in nearby sediment habitats, suggesting either ?spill over? from settlement in seagrass patches or actual direct recruitment to bare sediments as a supple- mentary area. In South Carolina, juveniles used tidal creeks of salt marsh ecosystems, rather than going onto the marsh surface, probably avoiding exposure on low tides (Mense and Wenner 1989). Juvenile blue crabs in upper Chesapeake Bay used five habitat types (bare sediment, submerged vegeta- tion, woody debris, oyster shell, and rocky rip-rap) deployed simultaneously as experimental patches, with variable but significantly higher densities in structured habitat (Davis and Hines, unpubl. data). In New Jersey, juveniles used seven types of habitats including seagrass, algal habitat, and bare sediment as well as adjoining salt marsh at densities that var- ied temporally, but attained similar peak densities among all of the habitats (Wilson et al. 1990b; Meise and Stehlik 2003). However, another study showed greatest juvenile blue crab abundance in macroalgal Ulva lactuca areas compared to seagrass Z ostera marina and saltmarsh Spartina spp . creeks (Sogard and Able 1991). Shifting Habitat Use by Life History Stage Newly recruited juveniles (<5th instar) are found at highest densities in seagrass beds and only at very low densities in unvegetated habitats (Orth and van Montfrans 1987;Williams et al. 1990; Pile et al. 1996; Pardieck et al. 1999; Heck et al. 2001). A reduction in the density of later stage crabs (11-25 mm CW) in seagrass beds and increasing abundance in marsh creeks indicate that juvenile blue crabs undergo a shift in habitat use (Orth and van Mont- frans 1987; see also Pardieck et al. 1999; Rakocinski et al. 2003). The shift appears to be triggered by attainment of a size refuge from predation, because tethering of 1st through 9th instar juveniles showed that predation rates diminished significantly between 5th (7-9 mm) and 9th (14-16 mm) instars (Pile et al. 1996). Similarly, although small (<10mm CW) juveniles exhibited strong affinity for shoal grass habitat compared to bare sediment, larger juveniles did not exhibit strongly preferential association with seagrass along the Gulf Coast (Williams et al. 1990). However, other Gulf Coast estuaries showed vari- able, often high, abundances among seagrasses and bare sediment areas (Sheridan and Livingston 1983; Rakocinski et al. 2003). During summer, habitat use by blue crabs varies with size, sex, and molt stage as they feed, grow, molt to maturity, and mate, as illustrated within the Rhode River, a subestuary of Chesapeake Bay (Hines et al. 1987, 1995).There, large, intermolt, 1+ age class crabs (>100 mm CW) primarily use non- structured soft bottom habitat in deeper (1-4 m) water of estuarine channels and basins when forag- ing on infaunal prey, whereas 0+ age class juveniles (30-70 mm CW) primarily use shallow (>70 cm) water along the shoreline, where they escape preda- tion or cannibalism by large crabs (Hines et al. 1987, 1990, 1995; Ruiz et al. 1993; Hines and Ruiz 1995). Juveniles (30-70 mm CW) seek woody debris pri- marily along the shoreline as refuge for molting (Hines et al. in prep.). As prepubertal males approach their molt to maturity, they move up into tidal creeks, where >90% of the crabs are male and in active molt stages (Hines et al. 1987). These males E COLOGY OF J UVENILE AND A DULT B LUE CRABS 577 select the shallow tidal marsh edge along the creek as the microhabitat for molting (Wolcott and Hines 1990). After molting to maturity, males move back out into the subestuarine basin to forage and mate. In contrast, prepubertal females molt to maturity within the estuarine basin and especially at the mouth of the subestuary, where they couple with intermolt mature males. This mate guarding affords females protection from predation or cannibalism and allows males to block competitive insemination of their mates by other males (Jivoff 1997a, b; Jivoff and Hines 1998a, b; Carver 1999; Carver et al. 2005). Mated females remain to forage in deeper waters near the subestuary through the summer until the fall migration (Turner et al. 2003;Aguilar et al. 2005). During migration to the spawning area in the lower estuary, females tend to use habitat along the deep channel of the mainstem of Chesapeake Bay (Aguilar et al. 2005). Thus, the deeper waters of the mainstem of the estuary form a migration corri- dor for females (Lipcius et al. 2001). In lower Chesapeake Bay, over-wintering blue crabs are mostly mature females that are least abun- dant in shoal and spit habitats, at intermediate abun- dance in deep channels, and most abundant in basin habitats of the mainstem, especially at depths >9 m in sediments composed of 40 to 60% sand (Schaffner and Diaz 1988). This survey indicated that over-wintering females occur primarily in areas of the lower mainstem estuary characterized by moderate energy regimes and fine but sandy sedi- ments. In contrast, wintering juveniles and males bury into sediments of deeper channels and the mainstem of the middle and upper estuary (Sharov et al. 2003). During the summer spawning season in Chesapeake Bay, mature females exhibit peak abun- dance in the mainstem at depths of 6 to 14 m, with nearly half of all adult females in the lower Bay found deeper than 10 m (Lipcius et al. 2003). Salinity Zone Blue crab abundance varies along the salinity gradient, with highest densities occurring in the set- tlement habitats of the polyhaline zone (Orth and van Montfrans 1987; Fitz and Wiegert 1991b; Mok- snes et al., 1997; Zimmerman et al. 2000; King et al. 2005). After they disperse from their settlement habitat, the abundance of juveniles and males tends to be more evenly distributed across a broad range of salinities from polyhaline to mesohaline waters. In river-dominated estuaries of the southeastern United States (North Carolina to Georgia) where structured habitats for refuge are often largely absent, abundances of small juveniles (13-24 mm CW) were negatively correlated with salinity (Posey et al. 2005). Juveniles encountered lower predator abundance and lower mortality rates (tested by teth- ering), implying that low salinity areas may be important nursery habitat for food and reduced pre- dation. By contrast, abundances of larger juveniles (>24 mm CW) were uncorrelated with salinity or juveniles were more common at higher salinity sites (Posey et al. 2005). Juvenile blue crabs in Chesa- peake Bay can be abundant up-estuary in lower salinity zones where they feed on dense infaunal invertebrates associated with detrital production of adjacent salt marshes (Seitz 1996; Seitz et al. 2003a; King et al. 2005; Lipcius et al. 2005; Seitz et al. 2005). Juvenile blue crabs may be abundant in oligohaline salinities and tidal freshwater marshes, where they appear to obtain osmotic advantage for larger molt increments (Haefner and Shuster 1964; Ettinger and Blye 1981; deFur et al. 1988) as well as refuge from predators (Rozas and Odum 1987). Activities vary with salinity, and movement of mouth parts, antennules, and the abdomen increases at lower salinity (McGaw et al. 1999), which are associated with blue crab behavior and use of vari- ous habitats along the estuarine gradient. In higher latitude estuaries like Delaware and Chesapeake bays, mature females become concentrated over the winter in deep waters of the polyhaline zone, where they migrate before brood production the following spring or summer (Schaffner and Diaz 1988; Lipcius et al. 2001, 2003; Sharov et al. 2003; Aguilar et al. 2005). At lower latitudes, mature and ovigerous females also aggregate in high salinity zones, because they incubate and hatch eggs near the mouths of estuaries (Tagatz 1968a;Tankersley et al. 1998). In the Caribbean basin, species of Callinectes par- tition the habitat by salinity along the estuarine gra- 578 T HE B LUE CRAB dient (Norse 1978a, b). Callinectes maracaiboensis , C. bocourti, and C. sapidus are most tolerant of low salin- ities and extend into oligohaline salinities (0-101), with all three of these species also extending into mesohaline and polyhaline salinities. The mesoha- line zone includes (in order of euryhalinity) C. exas- peratus, C. danae , and C. marginatus , and these three species extend into polyhaline and near-marine waters. Again in order of tolerance to low salinities, C. exasperatus, C. marginatus , and C. ornatus occur in near marine and marine salinities (>30). Callinectes similis also occurs in polyhaline and near-marine salinities. Along estuaries of the Pacific Coast of Central America and Columbia, C. toxotes dominates in low salinities whereas C. arcuatus is dominant in the mesohaline to polyhaline zones. Hypoxic Habitats Blue crab densities are zero in anoxic waters of eutrophic estuaries, such as areas deeper than 10 m in major tributaries and the central channel of Chesapeake Bay (Pihl et al. 1991) and in the Neuse River in North Carolina (Eby and Crow- der 2002), as well as delta areas of the Mississippi plume (Rabalais et al. 2001). Crab densities are diminished in hypoxic areas because crabs may move into very shallow waters during hypoxic events, such as nocturnal hypoxia in summer or during plankton blooms (e.g., Loesch 1960; Pihl et al. 1991). Juveniles of both C. sapidus and C. similis readily detect and avoid hypoxic waters (Das and Stickle 1994). Similarly, adults avoid hypoxic habitats <4 mg dissolved O2 L -1 (Bell et al. 2003a). As hypoxic waters have extended inshore from deeper waters, suitable blue crab habitat has been ?compressed? into shallower areas, as seen in the Neuse River estuary and Pamlico Sound (Selberg et al. 2001; Eby and Crowder 2002) and modeled for the Patuxent River tributary of Chesapeake Bay (Mistiaen et al. 2003). Spatial and Temporal Variation in Habitat Use In estuaries like Chesapeake Bay, significant sea- sonal and long-term variation in area and composi- tion of structured habitats has occurred, including long-term declines in submerged aquatic vegetation (Orth and Moore 1984), oyster reefs (Mann et al. 1991; Rothschild et al. 1994), and coarse woody debris (Everett and Ruiz 1993). The evidence for the crucial nursery function of these habitats indi- cates that their loss imposes significant negative effects on survival and growth for blue crab popula- tions. However, the combined effects of multiple habitat loss and fragmentation creating a mosaic of refuges are complex, because juvenile blue crabs, their prey, and their predators may respond differen- tially and with fluctuating densities to such habitat changes (Irlandi 1997; Eggleston et al. 1998a, b; Micheli and Peterson 1999;Hovel and Lipcius 2001; Hovel et al. 2002; Hovel 2003; Hovel and Fonseca 2005). The interaction of transport processes, move- ment, and habitat value at multiple scales indicates that landscape-level factors should be considered in analyses of habitat use (e.g., Stockhausen and Lipcius 2003; Hovel 2003). Habitat value for blue crabs also depends inter- actively on patch size and complexity (i.e., density of structural elements within the patch) (Heck and Orth 1980a; Irlandi 1997; Hovel et al. 2002; Hovel and Fonseca 2005). Seagrass patches as small as 0.25 m2 support higher densities, higher growth, and higher survival of juvenile blue crabs than do bare sediments adjacent to seagrass patches (Perkins- Visser et al. 1996; Eggleston et al. 1998a, b; Hovel and Lipcius 2001). Smaller patches also afford greater access to bivalve prey, such as the hard clam, Mercenaria mercenaria (Irlandi 1997). Survival rates of tethered juvenile crabs vary interactively with patch size and landscape configuration, such that survival is higher in small (1-3 m2) than in large (>100 m2) patches, and is higher in patchy than in continuous seagrass beds (Hovel and Fonseca 2005). Effects of patch size of seagrass habitat vary temporally with seasonal changes in predator or cannibal use of patches (Hovel and Lipcius 2001; Hovel et al. 2002). E COLOGY OF J UVENILE AND A DULT B LUE CRABS 579 1 Salinity is presented as a pure ratio with no dimensions or units, according to the Practical Salinity Scale (UNESCO 1985). These studies indicate that patchy seagrass landscapes are valuable refuge habitat for juveniles, and that effects on survival can only be understood when larger scales of habitat structure are considered. Habitat fragmentation may favor ecological interactions at the edges of structural habitats because smaller patches have greater ratios of edge to interior. Higher ratios may increase probability of key attributes of patch use, such as habitat encounter by recruiting crabs, supply of food items that are moving past, and predators moving around the patch edges (Eggleston et al. 1998a, b; Blackmon and Eggleston 2001). Greater relative habitat edge may allow large mobile predators (such as large blue crabs) to gain access to smaller prey (such as small blue crabs) that receive refuge from interiors of larger patches (e.g., Eggleston et al. 1998a, b, 1999; Davis and Hines, unpubl. data). The complex dynamics of benefits and disadvantages of edges and interior zones of patchy habitats like seagrass may result in marked short-term (seasonal) changes in abundances of blue crabs and their prey (Bologna and Heck 2002). Habitat configuration and connectivity also interact to affect habitat use and value for blue crabs. Vegetated habitats (both seagrass and salt marsh patches) act as corridors for blue crabs to gain access to oyster reefs for food, with both sea- grass and fringes of emergent marsh plants provid- ing refuge from predation upon the crabs by mobile predators such as birds (Micheli and Peterson 1999). Features that increase juvenile use of Gulf Coast salt marshes are reticulated marsh geomor- phology, low tidal amplitude, and long periods of tidal inundation (Thomas et al. 1990). In subestuar- ies of Chesapeake Bay, blue crab abundance is related to the interaction of salinity zone, presence of adjacent salt marsh habitat, and watershed land use (King et al. 2005). Juveniles are most abundant in higher salinities and in areas adjacent to salt marshes of subestuaries with watersheds that are predominantly forested or in agriculture, whereas suburban and urbanized watersheds have lower juvenile densities. Thus, sites with connection to marsh habitats providing detritus sources for blue crab food, especially deposit feeding bivalves like the Baltic macoma Macoma balthica , favor blue crab abundance, whereas human development of water- sheds appears to reduce blue crab abundance, albeit through indirect ways (Seitz et al. 2003a; King et al. 2005). At higher latitudes, blue crabs undergo marked seasonal shifts in habitat use with the onset of cold winter temperatures and cessation of feeding, move- ment, and molting. Juvenile crabs in New Jersey appear to shift their habitat over winter from unveg- etated habitat to eelgrass roots and debris in marsh creeks (Wilson et al. 1990b). In Delaware Bay and Chesapeake Bay, crabs move into deeper water in fall (late September, October, and November), with inseminated females migrating from shallow nursery habitats to higher salinity zones in the lower bay. Migrating females move down deeper (13-25 m) water along the Bay?s main channel and not along the shallow shoulders of the mainstem (Turner et al. 2003; Aguilar et al. 2005.). Over-wintering blue crabs in lower Chesapeake Bay (90-98% female) were most abundant in water deeper than 9 m where sediments are composed of 40 to 80% fine silty sand (Schaffner and Diaz 1988). Winter dredge surveys throughout Chesapeake Bay confirm the concentration of mature females in the lower bay, with large males in deeper water of lower salinities and small juveniles in depths of 2 to 13 m (Sharov et al. 2003). Much less is known about temporal variation in habitat use of blue crabs in estuaries at lower lati- tudes, probably because seasonal temperature changes are much less pronounced in these systems. Even so, these systems are subject to seasonal changes, and general patterns of habitat use along estuarine salinity gradients appear to be similar among estuaries along the East Coast of North America and also in Gulf Coast systems (e.g.,Van Engel 1958; Darnell 1959; Tagatz 1968a; Archam- bault et al. 1990; Steele and Bert 1994; Guillory and Perret 1998; Kahn et al. 1998). However, many of the assessments for lower latitude estuaries tend to cite literature from the mid-Atlantic region, and in the absence of comparably detailed studies for a particular region, such inferences may not be valid. For example, mature females along the Gulf Coast 580 T HE B LUE CRAB of Florida often exhibit seasonal shifts in habitat as they migrate northwest alongshore for long dis- tances in a pattern that appears distinctly different from East Coast estuaries (see Adult Movement sec- tion below). In addition, predation rates upon post- settlement juveniles are much higher in estuaries of the central Gulf of Mexico than in mid-Atlantic estuaries, which severely regulate blue crab abun- dance and distribution among habitats (Heck and Cohen 1995). MOVEMENT Approaches and Methods for Movement Studies Blue crab movement varies with life stage and molt stage, and depends on habitat and geographic region, as well as on tidal and seasonal cycles (Gillan- ders et al. 2003). Movement patterns may be deduced indirectly from spatially disjunct distribu- tions of life stages, as when settling larvae occur mainly in the lower estuary but larger juveniles are distributed throughout the estuary. Movement is also inferred by temporal variation in the distribu- tion of a life stage, as when immature females molt to maturity and mate in upper estuarine zones but ovigerous females later occur primarily near the mouth of estuaries (e.g.,Van Engel 1958). Movement between points (without knowing the route traveled) can be estimated directly by mark-recapture studies involving large numbers (thousands) of crabs marked with inexpensive exter- nal or internal tags (e.g., Cronin 1954; Judy and Dudley 1970; van Montfrans et al. 1986; Fitz and Wiegert 1991a; Steele 1991; Davis et al. 2004b; Aguilar et al. 2005) (Fig. 4). External tags are readily visible to fishers and typically cause little harm to crabs, but they are lost during molting. As a result, external tags usually have been applied to large mature crabs that do not molt (females) or only molt infrequently (males). Internal tags that are retained during molting also have been used, but these may require expensive equipment to insert the tags (e.g., micro-wire tags; van Montfrans et al. 1986; Fitz and Wiegert 1991a; Davis et al. 2004b) or to detect them (e.g., ?pit tags?;Wolcott and Hines 1996). Internal tags are often not seen by fishers, and also may cause significant mortality (e.g., dart tags; Fannaly 1978; Souza et al. 1980) or may induce limb autotomy (e.g., elastomer injection; Davis et al. 2004b). How- ever, some forms of internal tags (especially micowire or coded wire tags and elastomer injec- tion) work well for juveniles as small as 10 mm CW (van Montfrans et al. 1986; Fitz and Wiegert 1991a; Davis et al. 2004b). External ultrasonic telemetry tags, which are expensive and usually are applied to small numbers of crabs >60 mm CW, allow acquisition of detailed data on the path of movement and other selected aspects of behavior and physiological functions (Nye 1989;Wolcott and Hines 1989a, b, 1990, 1996;Hines et al. 1995; Clark et al. 1999a, b, 2000;Terwin 1999; Bell et al. 2003a, b) (Fig. 5). Ultrasonic tags may also be used to transmit data on environmental variables (temperature, conductivity, depth, light) that crabs encounter, and they have been developed to signal physiological variables (muscle action potentials, posture, suture breaks of the exoskeleton) that are integral components of crab behaviors (locomotion, feeding, fighting, mating, molting) in relation to location and time (Wolcott 1995;Wolcott and Hines 1996) (Fig. 6). Electronic tags that record data about environmental variables to micro-chips that can be down-loaded to a computer when the tag is recap- tured have been applied recently to blue crabs to deduce movement and behavior (Wolcott et al. 2004). Post-Settlement Dispersal of Juveniles Dispersal of juveniles after settlement increases the array and extent of nursery habitats. Our knowledge of the characteristics of secondary, post- settlement dispersal varies among estuarine systems, with some important contrasts among Mobile Bay, Pamlico Sound, and Chesapeake Bay, for example. In Mobile Bay, Mississippi, post-settlement dispersal is largely limited by intense predation on juveniles (J1 to J5), to the extent that few dispersing juveniles are collected (Heck et al. 2001). However, densities E COLOGY OF J UVENILE AND A DULT B LUE CRABS 581 582 T HE B LUE CRAB 582 of larger juveniles (J5 to J9) in various nursery habi- tats (seagrasses, marshes, and adjacent bare sediment) in various Gulf Coast estuaries occur at levels similar to similar habitats in East Coast estuaries (Heck and Coen 1995; Heck et al. 1995; Morgan et al. 1996; Heck et al. 2001; Rakocinski et al. 2003). In North Carolina sounds, field collections of dispersing juveniles provide contradictory results. Some studies indicate dispersal occurs at the earli- est instars (J1, and well before J5) (Etherington and Eggleston 2000, 2003; Blackmon and Eggleston 2001; Etherington et al. 2003; Reyns and Eggleston 2004), whereas other studies indicate that dispersal occurs at later stages (J4-5) (Forward et al. 2004). Juvenile crabs emigrate rapidly after settlement in a density-dependent process that redistributes them from high-density settlement sites to areas with low larval supply. Dispersal from seagrass settle- ment habitats occurs pelagically by a circadian rhythm of swimming on nighttime flood tides (Forward et al. 2003, 2004). Although secondary dispersal is a consistent process across a broad range Figure 4. Tagging methods for blue crabs. (A) Juveniles with red elastomer injected into swim paddle legs. (B) Micro-wire injection of crabs by a specialized machine that inserts a tiny segment of magnetized wire into the mus- cle at the base of a swimming leg; the tag persists through the molt cycle. (C) Close up of micro-wire (arrow) that can be detected by magnetic sensor. (D) Highly visible plastic tag attached to dorsal carapace of a mature female, which will not molt. Photos by Alicia Young-Williams, SERC. A B D C of wind conditions (Etherington and Eggleston 2003), hurricanes and storm events may modify the dispersal, moving a range of life stages at wider spa- tial scales throughout the sounds, but effects depend on seasonal timing, amount of rainwater runoff causing flushing, salinity declines, and low dissolved oxygen (Etherington and Eggleston 2000, 2003; Eby and Crowder 2002; Mallin et al. 2002; Posey et al. 2005). In Chesapeake Bay, dispersal of juveniles from the settlement habitat appears to reflect ontogenic change in activity and behavior at about the 5th to 7th crab instar (Hines et al. 1987; Pile et al. 1996; Etherington and Eggleston 2000, 2003; Diaz et al. 2001). It is clear that, following some lag interval after settlement, juveniles typically disperse. This dis- persal may occur over several months in lower lati- tudes and during more limited seasons at higher lati- tudes, but timing details of the dispersal are not clear. In northern estuaries, juveniles do not undergo migratory dispersal in winter when water tempera- tures fall below 10oC (Hines et al. 1987). In Chesa- E COLOGY OF J UVENILE AND A DULT B LUE CRABS 583 Figure 5. Dorsal views of blue crabs equipped with ultrasonic biotelemetry tags that signal location and selected behaviors and physiological functions. (A) Commercial transmitter (arrow) for location. (B) Customized transmitter for location, feeding, and fighting. 1 = transmitter; 2 = reed switch on merus of right chela; 3 = magnet on carpus of right chela; 4 = electrode wires for detecting contraction of the right mandibular muscle. (C) Customized transmit- ter for location and molting; 1 = transmitter; 2 = reed switch and adjacent tube for magnet; 3 = magnet in post-molt position on spring connection to ventral carapace. (D) Customized transmitter for location and sensing of pre-copu- latory mate guarding during mating. 1 = wires running to transmitter located on ventral carapace; 2 = reed switches; 3 = magnet on spring bar that will be depressed when mature male grasps this pre-pubertal female. Tags designed by Thomas G.Wolcott. Photos by Anson H. Hines and Thomas G.Wolcott. See Wolcott and Hines (1989a, b, 1990); Clark et al. (1999b); Carver (2001). A B 4 1 2 3 D 1 2 3 C 1 2 3 584 A 1 2 B 1 2 4 3 C 12 3 4 E COLOGY OF J UVENILE AND A DULT B LUE CRABS 585 peake Bay, juveniles disperse from lower Bay settle- ment habitats to arrive in upper Bay subestuaries during fall and late in the following spring (Hines et al. 1987, 1990). Although the behavioral and physi- cal mechanisms have not been studied explicitly for the post-settlement dispersal, juvenile movement up the estuary may involve selective tidal-stream trans- port, as exhibited by megalops and small juveniles entering lower estuaries (Lipcius et al. 1990; Little and Epifanio 1991; DeVries et al. 1994; Tankersley and Forward 1994; Forward et al. 2003a, b; see also Tankersley and Forward, Chapter 10) or by oviger- ous females moving out of estuaries to hatch their eggs (Tankersley et al. 1998). Alternatively, juveniles may ride the salinity wedge up-estuary like mud crab larvae (Cronin 1982), some fish species (McCleave and Whipplehouser 1987), invertebrate larvae (de Wolf 1974), and some phytoplankton species (Tyler and Seliger 1978); or, small crabs may simply swim up-estuary under their own naviga- tional mechanism and power. Post-settlement dis- persal also may be driven by storms in North Car- olina sounds (Etherington and Eggleston 2000). Although the seasonal timing of juvenile dispersal appears to be consistent among years, the abundance of juveniles dispersing up the estuary varies greatly among years, with fluctuations in numbers of recruits settling into the estuary (Hines et al. 1987, 1990; Lipcius and Van Engel 1990; Pile et al. 1996; Rakocinski et al. 2003). Figure 6 (Opposite page and above). Side views of biotelemetry tags that signal location and selected behaviors and physiological functions of blue crabs. (A) Feeding tag, which signals location and contraction of the mandibular muscle. 1 = transmitter; 2 = waterproof patch covering insertion of electrode through the anterior-lateral carapace into the right mandibular muscle and the haemocoel, to detect the myopotential of muscle contraction. (B) Feeding and threat display tag, which signals location, contraction of the mandibular muscle, and meral spread of the chelae. 1 = transmitter; 2 = waterproof patch covering the insertion of electrodes through the carapace into the left mandibu- lar muscle; 3 = magnet on carpus of right chela; 4 = reed switch on merus of right chela. Threat displays are signaled when the magnets are positioned close to the reed switches. (C) and (D) Molting tags. 1 = transmitter; 2 = reed switch and parallel insertion tube for magnet; 3 = magnet; 4 = spring-like attachment of magnet to ventral carapace. As the crab swells during ecdysis (C) and backs out of the old carapace, the magnet is pulled from the tube and springs into post-molt position (D) away from the reed switch, changing the telemetry signal. Photos by Thomas G. Wolcott and Anson H. Hines. See Wolcott and Hines (1989a, b, 1990); Clark et al. (1999b). D 12 3 4 Juvenile Movement among Habitats within Subestuaries Once dispersed, juveniles >20 cm CW tend to remain within subestuaries. Juvenile movement reflects their shifting habitat use within subestuaries and may involve active locomotion over distances from 10 m to a few km (Hines et al. 1995, unpubl. data;Terwin 1999). In subestuaries of upper Chesa- peake Bay, telemetry studies showed that juveniles (60-80 mm CW) move along shorelines within subestuaries, with a typical pattern of periods of meandering slowly (2 m h-1) in shallow (<1 m) water interspersed with rapid (>50 m h-1) direc- tional movement to a new meandering area (Hines et al. 1995; Terwin 1999). Juveniles tend to move rather directly and rapidly across channels, and not to meander in deeper water where they are most vulnerable to cannibalism by large crabs; however, small juveniles do not seem to move in a net direc- tion along the axis of the subestuary as do larger pre-pubertal and adult crabs (Hines and Ruiz 1995; Hines et al. 1995;Terwin 1999; Hines and Wolcott, unpubl. data).Tagged juveniles released experimen- tally into small (1-10 ha) coves of Chesapeake Bay remained within the release sites and did not move away until molting to maturity (Davis et al. 2005b). Juveniles often exhibit short-term movement into salt marsh habitats, depending on tidal fluctua- tions. Tagging studies in Georgia showed that juve- niles move with the tide into salt creeks and onto salt marsh surfaces, but they do not move far (<100 m) into the interior of the marsh habitat (Fitz and Wiegert 1991b, 1992). In Texas and North Car- olina, juveniles also move onto salt marshes with tidal flooding (Zimmerman and Minello 1984; Het- tler 1989). In lower Chesapeake Bay, daily exchange of tagged juveniles was minimal (5-8% d-1) among adjacent types of habitats, including small (<100 m) tidal marsh creeks, seagrass beds, and unvegetated habitats (van Montfrans et al. 1991). Crabs buried on low tides to remain in a creek for 8 to 12 d, resulting in a population turnover time (exchange with new juveniles from surrounding areas) of >65 d (van Montfrans et al. 1991). Premolt juveniles typically move in advance of ecdysis to refuge habitats for molting. Juvenile are thought to move cyclically into and out of seagrass beds in many areas as the primary site for molting (Van Engel 1958). In subestuaries of Chesapeake Bay that have lost submerged aquatic vegetation, telemetry showed that juveniles moved up to hun- dreds of meters in shallow water along shore to molt in woody debris (Hines et al., in prep.). In other telemetry studies in the same subestuary, pre-puber- tal males moved 1 to 3 km into salt creeks during the few days before the molt to maturity (Hines et al. 1987; Shirley et al. 1990; Wolcott and Hines 1990). Movement patterns were highly variable, but distance traveled per day diminished from averages of about 200 m d-1 during 3 to 6 d before molting to about 50 m during 1 d before ecdysis, when movement ceased (Wolcott and Hines 1990). In subestuaries of North Carolina Sounds, pre-pubertal males and females similarly moved an average total distance of about 2 km (net distance about 1 km) over 7 d (Shirley and Wolcott 1991). Distance trav- eled per day similarly diminished markedly at 3 d before ecdysis, but movement did not differ signifi- cantly by sex and males did not appear to orient to salt creek habitats as in upper Chesapeake Bay (Shirley and Wolcott 1991). Laboratory experiments demonstrate that juve- niles of both C. sapidus and C. similis readily detect hypoxic water and move to avoid low oxygen levels (Das and Stickle 1994). However, responses to hypoxia may depend on severity and timing of low oxygen events, as it does for adults (Bell et al. 2003a, b). Juvenile movement among estuaries and even among adjacent subestuaries appears to be minimal (e.g., van Montfrans et al. 1991; Hines et al. 1995; Terwin 1999). However, with declining water tem- peratures in fall in higher latitude systems like Chesapeake Bay, juveniles move from shallow (<1 m) water to deeper (>2 m deep) channels where they spend the winter months (pers. obs.). Movement of Adults Adult blue crab movement can be considered in two categories. The first is small-scale short-term 586 T HE B LUE CRAB movement that occurs within estuaries over dis- tances typically <10 km over days to weeks for both males and females. The second is large-scale, sea- sonal migratory movement, which may involve dis- tances of 10 to 800 km, especially for females mov- ing from mating areas to areas of brood incubation and egg hatching. Adult Short-term, Small-scale Movement Short-term, tidal movement and orientation have been studied along beaches of the Gulf Coast by observation and tracking experiments (Nishi- moto and Herrnkind 1978, 1982). Using blind- folded crabs and experimental beaches of different orientation, these studies indicated that blue crabs orient well to surge direction and that they use a sun-compass for orientation with tidal fluctuations and irregular shorelines. Short-term and daily movement within estuar- ies has been estimated with tagging-recapture stud- ies. In North Carolina, Judy and Dudley (1970) esti- mated that short-term (42 d) movement averaged about 6 km (range 3.5-11.6 km) d-1, but this included a large proportion of migrating females, and the report does not provide the ability to sepa- rate the data for males and non-migrating females. Tagging studies in Florida showed that mature males rarely moved more than 10 km from the tagging site (Oesterling and Adams 1982). In a study near a power plant in Chesapeake Bay, Souza et al. (1980) estimated that blue crabs moved 400 to 900 m per day, although the study crabs endured a high level of mortality (19-39%) from handling and dart tags. However, these mark-recapture studies only indicate the net distance traveled and provide little informa- tion about the path taken by the crab, which is key to understanding small-scale, non-migratory movement. Using ultrasonic telemetry, the path and detailed characteristics of non-migratory movement of adult and juvenile (>60 mm) blue crabs have been stud- ied in the Rhode River, a subestuary of upper Chesapeake Bay (Wolcott and Hines 1989a, 1990; Hines et al. 1995;Terwin 1999;Turner et al. 2003) (Fig. 7). Intermolt crabs moved at an average speed of about 10 m h-1 during the warm season, but speed varied by month from a high average speed of about 15 m h-1 in July to about 5 m h-1 in May or late September, and no movement of males by late November to March. Speed also varied by size and life stage, with large males (>140 mm CW) moving faster (15 m h-1) than similar sized females (8 m h-1) or large juveniles (100-120 mm CW) (5 m h-1). As indicated above for molting juveniles, movement of mature males decreased markedly at premolt stage D2-D3 about 3 d before ecdysis (Wolcott and Hines 1990). Although movement was highly variable among intermolt individuals, male and female crabs exhibited a char- acteristic pattern of slow meandering (0-10 m h-1) within a radius of 50 to 200 m over a period of 1 to 5 d interspersed by sudden rapid (50 to 700 m h-1) movement for a distance of 0.5 to 4 km that was directionally oriented along the axis of the subestuary. Unlike juveniles, this intermittent rapid, directional movement by adults resulted in their moving from the subestuary into the Chesa- peake mainstem in about 2 to 3 weeks. As dis- cussed under the section on Foraging below, slow meandering is associated with foraging in patches of higher density prey, whereas fast directional move- ment is apparently triggered by agonism among crabs competing for food. Short-term mass shoreward migrations of blue crabs, described as ?jubilees,? occur in small-scale areas within subestuaries in response to hypoxic or anoxic conditions during early morning hours of summer plankton blooms, or possibly in response to seiches of deeper anoxic water (Loesch 1960). Ultrasonic telemetry showed that movement which is less extreme than jubilee events may also result in less visible shifts in depth distribution toward shore (Bell et al. 2003a, b). This movement appears to be an avoidance response that puts crabs into more oxygenated shallow water. However, blue crabs exposed to hypoxia during sudden upwelling events were not successful at moving to normoxic water, sometimes resulting in their remaining in severely hypoxic waters (<2 mg L-1) for several hours (Bell et al. 2003a). E COLOGY OF J UVENILE AND A DULT B LUE CRABS 587 Adult Large-scale and Seasonal Migratory Movement Tagging studies of large, mostly mature crabs have been conducted over the past 80 y in many estuaries along the East and Gulf coasts of North America, both to understand seasonal patterns of movement that explain cyclical and spatial variation in catch of males and females within estuaries and to determine whether blue crab movement between estuaries is a key factor in defining stocks for fishery management. Such tagging studies typically achieve only low recapture rates. Initial studies in Chesa- peake Bay included Fiedler?s (1930) tagging of nearly 1,800 crabs with a recapture of 10.8%,Truitt?s (1939) tagging of 4,600 crabs, and Cronin?s (1949) test of various tags to obtain up to 22.6% return. These early researchers determined the basic annual cyclical pattern that mature female crabs move directionally to the mouth of the Bay after mating to overwinter near the spawning area. By contrast, males showed a non-directional, random movement within the estuary. Initial tagging studies for Delaware Bay (Cronin 1954; Porter 1956) indicated more variable movement by mature females, with- out clear directional migration seaward, though much of the movement occurred near the already wide mouth of the bay. Most tagging studies have found a general pat- tern of retention of males and females within a home estuary, with females typically moving direc- tionally down estuary over greater distances 588 T HE B LUE CRAB Figure 7. Example of a track of a large male blue crab fitted with an ultrasonic biotelemetry tag (as shown in Fig. 5A) in the Rhode River subestuary of Chesapeake Bay. The crab was released at the open circle (left end of track line); dots are locations about 12-h intervals over several days. Crab was recaptured at ?X? (right end of track). Note meandering locations alternating with directional movement along channel of the subestuary. (depending on where they were released), males showing little net directional movement, with only a small fraction of individuals of either sex moving into neighboring estuaries, and no large-scale migra- tion along the coast. For example, Judy and Dudley (1970) estimated movements of tagged blue crabs in North Carolina sounds, with a 30% return of all released crabs comprised of 17,237 females and 5,691 males. For males, 88 to 100% of returns occurred within 24 km of their release site, whereas females had 64 to 90% recovery in the area of release. Few recaptured crabs (2.2% of males and 6.2% of females) moved >24 km from release sites, with no males moving substantial distances along- shore and about 1% of females moving 65 to 258 km along inland waters from Oregon Inlet to Chesapeake Bay and about 1% moving alongshore 25 to 210 km from southern North Carolina estuar- ies to South Carolina. Similarly, in Chincoteague Bay,Virginia, Cargo (1958) tagged nearly 400 crabs in late summer and showed that mature females move southward within the bay to more saline waters over the subsequent months. In coastal South Carolina, Fischler and Walburg (1962) tagged 4,353 crabs >125 mm CW and showed no migra- tion between estuaries. Tagatz (1968a) tagged >11,500 blue crabs in the St. John?s River estuary, Florida. Most of his 35% recaptures, especially mature females, showed downstream movement, with only about 5% of the recaptures occurring outside the home estuary ? mostly as movement within 50 km along the intra-coastal waterway, with two individuals being recaptured about 500 km away after a year. Along the west coast of Florida, Oesterling (1976) tagged 6,287 crabs and found that 95% of recaptured males were caught within 18 km of their release site. In Lake Borgne, Louisiana, and Mississippi Sound, Perry (1975) tagged and released 1,023 adults (155 males, 868 females) with a 30% return that showed crabs traveled 3 to 60 km over 4 to 261 d (mean = 40 d) at large, and that females moved from low salinities in Louisiana to over-win- ter in high salinities of Mississippi Sound. Within the Sound, movements appeared random, with little movement between adjacent estuaries. In bay and lagoonal systems of Texas, adult male crabs with tags moved <8 km and females moved <20 km (More 1969; Benefield and Linton 1990). For estuaries along the Atlantic East Coast as well as the central and western regions of the Gulf Coast, tagging studies indicate that, sometime after mating, females typically migrate along the axis of the estuary to spawn near the mouth or just outside the starting estuary, but migratory movement among estuarine systems is minimal (see tagging studies for Delaware Bay [Cronin 1954; Porter 1956], Chin- coteague Bay [Cargo 1958], Chesapeake Bay [Churchill 1919; Fielder 1930; Truitt 1939; Van Engel l958; McConnagha 1993], North Carolina sounds and estuaries [Fischler 1965; Judy and Dud- ley 1970; Schwartz 1997], South Carolina [Fischler and Walburg 1962], St. John?s River estuary of northeast Florida [Tagatz 1968a], Mississippi Sound [Perry 1975), Louisiana [Darnell 1959], and Texas [Daugherty 1952a; More 1969]). In studies that tagged large numbers of crabs, a small percentage were occasionally recaptured long distances from the release estuary. For example, Schwartz (1997) tagged 22,781 crabs in the Cape Fear River estuary, North Carolina, over 2 y. Of the 13.4% crabs (1,338 males, 1,723 females) that he recaptured, 2,985 were caught locally, and only 10 (0.3%) moved north to other estuarine systems while 66 crabs (2%) moved south. One female moved as far south as Key West, Florida (1,256 km), and one male was recaptured as far north as Hoopersville, Chesapeake Bay, Maryland (617 km). One crab was caught in an adjacent North Carolina sound as late as 6.5 y later. In contrast to the pattern of females remaining in association with their mating estuaries in most regions, tagging studies along the west coast of Florida showed that a significant proportion of mature females move long distances from their mat- ing estuary, although many reproduce near their home estuary as well (Oesterling 1976; Oesterling and Adams 1982; Steele 1991). Oesterling (1976) tagged and released 6,287 crabs during warm months along the west coast of Florida with a 10.7% return rate. Many females were recaptured substan- tial distances from release sites, often well away from their home estuary: 43% were recovered >16 km away, 25% were caught >48 km away, and 4% trav- E COLOGY OF J UVENILE AND A DULT B LUE CRABS 589 eled >322 km away. Mature females that left their home estuary moved in a northerly direction along the peninsular coast and westerly along the panhan- dle; however, there appeared to be a westward bar- r ier to further migration in the vicinity of Apalachicola Bay, which was deduced to be a pri- mary spawning ground for the region. Similarly, Steele (1991) tagged more than 13,000 blue crabs during 2 y in Tampa Bay, with a recapture rate of nearly 25%. Mature females tended to move out of estuaries alongshore in a northerly direction, with 29% of recoveries occurring >765 km up the coast, and with several individuals traveling >800 km in approximately 100 d. In further tagging studies, Steele (1991) showed that females released along the southwest coast from Key Largo to Sarasota Bay contributed to the northwestward migration and moved as far as Apalachee Bay. In contrast, of 2,767 tagged crabs released in Apalachee Bay, 38% were retained in the Bay and only 5% were recaptured west of the release Bay, suggesting that the low salin- ity flow from the Apalachicola River impedes fur- ther westward migration of mature females (Steele 1991). Crabs tagged near Key Largo that moved in the other direction along the east coast of the penin- sula moved as far as Biscayne Bay (Steele 1991). With seasonal declines in water temperature at higher latitude (e.g., Chesapeake Bay, Delaware Bay), mature males, like juveniles, move into deeper channels of subestuaries and main estuarine basins, where they cease moving for the winter months. Males do not appear to move long distances during this seasonal shift (Hines and Wolcott, unpubl. data), but movement into deeper water in winter reduces exposure to lethal combinations of severely cold, low salinity surface waters (Rome et al. 2005). Mature female blue crabs typically exhibit two phases of migratory movement after mating (Tankersley et al. 1998). The first phase (phase I) involves movement from mating locations to the lower estuary before brood production. The second phase (phase II) occurs during brood incubation just before egg hatching and involves movement to the mouth of, or off-shore from, the estuary. In large estuaries like Chesapeake and Delaware bays, phase I migration may occur over distances of 200 km or more, whereas in smaller estuaries and for females mating in the lower portions of large systems, crabs may only migrate fewer than tens of km during phase I migration. In Chesapeake Bay, this initial phase of migration occurs during late September through November and not earlier in the season (Turner et al. 2003;Aguilar et al. 2005). Both over- the-back tags and ultrasonic telemetry show that after mating during July through September, mature females forage and move about in subestuaries in the typical small-scale pattern of alternating meander and short directional movement (Turner et al. 2003; Aguilar et al. 2005). Although this period of forag- ing before migration allows a female to recover from her molt to maturity, the migration appears to occur in seasonal synchrony rather than being triggered by completing a non-synchronized period of physio- logical preparation after molting (Aguilar et al. 2005). In Chesapeake Bay, migration is manifested by relatively fast seaward movement that involves walking on the bottom, or swimming in the water column, or both (Wolcott et al. 2004), typically along the deeper channel of the Bay?s mainstem (Aguilar et al. 2005) (Fig. 8). In Chesapeake Bay, females cease migrating for the winter and settle into bottom sediments of the mainstem as water temperatures drop below about 9? to 10?C, with some females remaining in the mesohaline zone and others arriving in the polyhaline zone for winter. As water temperature rises in spring and females become active, those that over-wintered in the mesohaline zone complete phase I seaward migra- tion. Although fishers in Chesapeake Bay report a ?wave? of mature females moving up the estuary in spring, this movement reflects increases in female activity and feeding as temperatures increase north- ward, thus increasing their vulnerability to fishing rather than reflecting actual movement of females up-estuary. In phase II of migration, ovigerous females exhibit selective tidal-stream transport (see Tankers- ley and Forward, Chapter 10) by a tidal rhythm of swimming at the surface on nocturnal ebbing tides, thus moving near to, or out of, the mouth of estuar- ies, where they hatch their eggs (Tankersley et al. 1998, in review; Forward et al. 2003a, b; Carr et al. 590 T HE B LUE CRAB 2004; Forward and Cohen 2004; Ziegler et al. in review). After their eggs hatch, some females reverse their tidal-stream transport on flooding tides to move back into the lower estuary, where they may produce subsequent broods (R.A. Tankersley, Biol- ogy, Florida Institute of Technology, pers. comm.; Tankersley et al., in review). Other females may remain outside the estuary (D. Ritschoff, Duke Uni- versity Marine Laboratory, pers. comm.), which may account for some of them moving to neighboring estuaries. However, mature females do not move back to lower salinity zones of estuaries (Fischler 1965;Hines et al. 1987, 1990). For very large estuar- ies like Chesapeake Bay and Delaware Bay, female spawning migration out of the mouth of the bays is not well documented or understood. For example, Prager (1996) assumed that females had a mean resi- dence time of 4 to 21 d in the spawning area of Chesapeake Bay, but there are no empirical measures of this, and other studies indicate that mature females remain in the lower Bay spawning sanctuary throughout the summer (Hoenig et al. 2003; Lipcius et al. 2003). For Chesapeake Bay during the sum- mer, peak abundances of egg-bearing females move E COLOGY OF J UVENILE AND A DULT B LUE CRABS 591 Figure 8. Locations of tagged female crabs recaptured by fishers. Crabs were tagged and released in summer and fall off the mouth of the Rhode River subestuary. Recapture sites indicate that migration occurs during fall and that the migration route follows the eastern side of the deep channel of the mainstem of the estuary. From Aguilar et al. (2005). CB = Chesapeake Bay; MD = Maryland; SERC = Smithsonian Environmental Research Center; VA = Virginia. progresssively from northern to southern portions of the lower bay spawning area, but there is little evi- dence of females migrating out of the mouth of the bay proper (Lipcius et al. 2003). In Delaware Bay, which has a large, progressively widening mouth, spawning females occurring over a broad area of the mouth may move out onto the shelf as well (C. Epi- fanio, University of Delaware College of Marine Studies, Lewes, pers. comm.). DIET AND FORAGING Diet General Blue crabs are epibenthic generalist predators that forage on a diversity of sessile infaunal and epibenthic invertebrates and on motile fish and crustaceans, as well as feeding omnivorously on plant material, detritus and scavenged carrion (Darnell 1958;Tagatz 1968a; Odum and Heald 1972; Laugh- lin 1982; Alexander 1986; Ryer 1987; Hines et al. 1990; Hsueh et al. 1992a; Mansour 1992; Meise and Stehlik 2003; Stehlik et al. 2004) (Fig. 9). The diet of blue crabs can be assessed by observations of feeding and behavior (e.g., Hughes and Seed 1981; Moody 1994), but the murky water of estuarine habitats often makes this impossible. Although food items are typically disassembled by the chelae, maxillipeds, and mandibles during feeding, identification of recently consumed stomach contents is readily pos- sible if foreguts are preserved in the field before the gastric mill pulverizes identifiable chunks of food (e.g., Laughlin 1982; Hines et al. 1990; Mansour 1992). However, variation among food items in digestibility, gut clearance time, and regurgitation (e.g., of shell fragments) may affect this approach (e.g., see Custer 1985; Haefner 1990b). Tests of foregut clearance rates for three types of prey (mus- sel, fish, shrimp) indicate that contents should be sampled within 2 h of feeding (Custer 1985). Some researchers advocate adjusting diet composition to give more weight to crabs with fuller stomachs than nearly empty stomachs, and some studies quantify diet by presence-absence of items to quantify the frequency of individuals in a sample that includes food items (e.g., Hines et al. 1990; Stoner and Buchanan 1990; Mansour 1992). Others studies do not make such adjustments, or they use weights of stomach contents to assess dietary importance (e.g., Laughlin 1982). In any case, analysis of stomach contents allows reasonably accurate quantification of diet for crabs from a full array of habitats, with or without direct observation of feeding. As reflected by stomach contents and other feeding observations (Table 1), blue crab diet includes at least 99 species from several phyla, espe- cially molluscs (typically 20-40% of stomach content weight or volume), arthropods (10-26%), chordates (fishes; 5-12%) and annelids (polychaetes; 1-7%). Stomach contents also often include detritus and unidentified, partially digested matter, as well as sedi- ment that may be ingested incidentally. Juvenile blue crabs have the digestive enzymes to utilize plant detritus, but the importance of such low quality food is not evident for blue crabs except, perhaps, when restricted to certain refuge habitats (McClin- tock et al. 1991). Xanthid crabs, blue crabs them- selves, and fish are important secondary components of the diet. In a trophic web analysis of Chesapeake Bay (Baird and Ulanowicz 1989), the diet of blue crabs was described as consisting of about 60% bivalve molluscs, with the remainder comprising polychaetes, amphipods, dead fish, and juvenile blue crabs (Darnell 1958; Tagatz 1968a;Virnstein 1977; Nelson 1981; Paul 1981). Although blue crabs cer- tainly exhibit a broad diet, quantitative studies show that bivalve molluscs are dominant prey that consis- tently comprise the largest volume or weight of juvenile and adult diet in many habitats (Laughlin 1982; Hines et al. 1990; Eggleston et al. 1992; Man- sour 1992; Meise and Stehlik 2003). Importantly, however, the diet of blue crabs exhibits significant ontogenetic, temporal, and spatial variation. Because they are readily attracted by baits that release oily chemical plumes (e.g., Atlantic men- haden Brevoortia tyrannus and other alosid fish,Amer- ican eel Anguilla rostrata , and various shellfish), blue crabs are often considered to be primarily scavengers and predators on the species used by fisheries in traps or trotlines. However, stomach contents of crabs sampled by methods independent of baits (e.g., 592 T HE B LUE CRAB 593 Figure 9. Diet of blue crabs and three species of common demersal fish ( Leistomus xanthurus , Micropogonias undulatus , Trinectes maculatus ) that comprise the guild of epibenthic predators in upper Chesapeake Bay. Composition of stom- achs weighted by stomach fullness for nine food categories are shown for crabs and fish collected with otter trawls on muddy and sandy sediments in early (June) and late (September) season. Note that blue crab diet is more diverse in June than September, with an increasing specialization from June to September on clams, which comprise 60% of their diet. From Hines et al. (1990). 594 Table 1. Prey items fed on by, or found in stomachs of, juvenile and adult blue crabs. Prey Species References Foraminifera Fitz & Wiegert (1991b), Laughlin (1982), Ropes (1989) Porifera Mansour (1992) Cnidarians hydroids Mansour (1992) Actinaria Mansour (1992) Polychaeta Fitz & Wiegert (1991b), Laughlin (1982), Ropes (1989) Glycera sp. Mansour (1992) Laeonereis culveri Hines et al. (1990), Laughlin (1982) Nereis spp. Hines et al. (1990), Mansour (1992), Ropes (1989) Nereis succinea Hines et al. (1990), Laughlin (1982) Pectinaria sp. Mansour (1992) Mollusca Darnell (1958, 1961), Eggleston (1990a,b,c), Menzel & Hopkins (1956), Tagatz (1968a) Bivalvia Laughlin (1982), Ropes (1989) Anadara sp. Mansour (1992), Orth et al. (1984) Argopecten irradians Irlandi et al. (1995) Brachidontes exustus Custer (1985) Brachidontes sp. Laughlin (1982) Crassostrea virginica Abbe & Breitburg (1992), Carriker (1951), Eggleston (1990a,b,c), Krantz & Chamberlin (1978), Laughlin (1982), Lunz (1947), Mansour (1992), Marshall (1954), Menzel & Nichy (1958), Micheli & Peterson (1999) Dreissena polymorpha Boles & Lipcius (1997) Gemma gemma Ropes (1989) Geukensia demissa Hughes & Seed (1981), Irlandi et al. (1995), Laughlin (1982), Lin (1989, 1991), Seed (1982) Ischadium recurvum Ebersole & Kennedy (1995) Macoma balthica Hines et al. (1990), Mansour (1992), Mansour & Lipcius (1991, 1993) Macoma mitchelli Hines et al. (1990), Mansour (1992), Mansour & Lipcius (1991, 1993) Macoma spp. Laughlin (1982), Mansour (1992), Mansour & Lipcius (1991, 1993) Mactra sp. Laughlin (1982) Mercenaria mercenaria Arnold (1984), Carriker (1951), Irlandi (1994), Micheli (1997a,b), Micheli & Peterson (1999), Ropes (1989), van Engel (1958) Mulinia lateralis Mansour (1992), Orth et al. (1984) mussels Lin (1991), Mansour (1992) Mya arenaria Ebersole & Kennedy (1995), Hines et al. (1990), Mansour (1992), Mansour & Lipcius (1991, 1993), Ropes (1989) Mytilus edulis Ropes (1989) Ostrea sp. Lunz (1947) Rangia cuneata Ebersole & Kennedy (1995), Laughlin (1982) Tellina sp. Laughlin (1982) Gastropoda Eggleston (1990a,b), Laughlin (1982) Astyris lunata Cote et al. (2001) Bittium sp. Laughlin (1982) Bittium varium; Billiolum varium Cote et al. (2001), Mansour (1992), Wright et al. (1996) 595 Table 1, continued. Prey Species References Gastropoda, continued Cephalaspidea Mansour (1992) Hydrobia sp. Ropes (1989) Ilyanassa (Nassarius) obsoleta Tagatz (1968a) Littoraria irrorata Hamilton (1976), Heard (1982), Seed (1982) Littoraria littorea Heard (1982), Laughlin (1982) Melampus coffeus Darnell (1958, 1961) Mitrella lunata Mansour (1992), Martin et al. (1989) Neritina virginea Darnell (1958, 1961) Neritina reclivata Laughlin (1982) Odostomia sp. Krantz & Chamberlin (1978) Pyramidellidae Mansour (1992) Crustacea Ampelisca sp. Martin et al. (1989) Amphipoda Hines et al. (1990), Laughlin (1982), Mansour (1992), Ropes (1989) brachyuran larvae Fitz & Wiegert (1991b) Callinectes sapidus Heck & Spitzer (2001), Hines & Ruiz (1995), Laughlin (1982), Moksnes et al. (1997), Orth et al. (1984), Peery (1989), Ryer et al. (1997) Cirripedia Laughlin (1982), Mansour (1992), Ropes (1989) Clibinarius sp. Laughlin (1982) Copepoda Fitz & Wiegert (1991b) Corophium sp. Laughlin (1982) crabs Ropes(1989) Gammarus sp. Laughlin (1982) hermit crabs Laughlin (1982) Isopoda Hines et al. (1990), Laughlin (1982), Mansour (1992) majid crabs Mansour (1992) Mysidacea Laughlin (1982) Mysidopsis sp. Laughlin (1982) Neopanope sp. Laughlin (1982), Mansour & Lipcius (1993) Ostracoda Fitz & Wiegert (1991b), Laughlin (1982) Palaemonetes sp. Laughlin (1982) Palaemonidae Fitz & Wiegert (1991b) Penaeidae Fitz & Wiegert (1991b) Penaeus duorarum Custer (1985) Penaeus sp. Laughlin (1982) Peracaridea Fitz & Wiegert (1991b) Rhithropanopeus harrisii Hines et al. (1990) shrimp Laughlin (1982), Mansour & Lipcius (1993) Uca spp. Fitz & Wiegert (1991b), Heard (1982), Hughes & Seed (1995) Xanthidae Fitz & Wiegert (1991b), Heard (1982), Hines et al. (1990), Hughes & Seed (1995), Laughlin (1982), Mansour (1992) Insecta Chironomidae (larvae) Hines et al. (1990), Laughlin (1982), Mansour (1992) Coleoptera Tagatz (1968a) Dicrontendipes Tagatz (1968a) Diptera Tagatz (1968a) Hemiptera Tagatz (1968a) by trawling) reflect their natural diet (e.g., Laughlin 1982; Hines et al. 1990; Mansour 1992), which does not frequently include these bait items. Blue crabs scavenge many species in the by-catch of near-shore fisheries, including many species of fish and decapod crustaceans, but these food resources are clearly enhanced by human activities. Ontogenetic Variation in Diet Blue crab diet changes during ontogeny, with juveniles feeding on smaller, more diverse epibiota and infauna of shallow sediments, and large adults feeding on larger, less diverse, epifauna and often on more deeply buried infauna (Laughlin 1982; Stoner and Buchanan 1990; Mansour 1992). Although Tagatz (1968a) reported that all sizes of crabs ate the same food types, the preponderance of evidence indicates that as crabs grow larger than about 70 mm CW, their diet becomes more focused on bivalve molluscs, particularly infaunal clams. For example, Laughlin (1982) found that bivalves increased in stomach contents from about 24% in small juveniles 596 T HE B LUE CRAB Table 1, continued. Prey Species References Insecta, continued Hymenoptera Tagatz (1968a) Odonata Tagatz (1968a) Bryozoa Mansour (1992) Echinodermata Ophiuroidea Mansour (1992) Ascidiacea Mansour (1992) Molgula manhattensis Beaven (1956) Pisces Fitz & Wiegert (1991b), Hines et al. (1990), Laughlin (1982), Mansour (1992), Ropes (1989) Anchoa mitchelli Laughlin (1982), Mansour (1992) Bairdiella chrysoura Custer (1985) Etropus sp. Laughlin (1982) Fundulus heteroclitus Kneib (1982), Martin et al. (1989) Leiostomus xanthurus Mansour (1992) Microgobius sp. Laughlin (1982) Micropogonias undulatus Laughlin (1982) Trinectes maculatus Laughlin (1982), Mansour (1992) Aves duck Milne (1965) Plant material Hines et al. (1990), Laughlin (1982), Ropes (1989) algae Ropes (1989) Spartina sp. Fitz & Wiegert (1991b), Ropes (1989) submerged aquatic vegetation Laughlin (1982) Detritus Hines et al. (1990), Laughlin (1982) (<31 mm) to 39% in large crabs (>60 mm) in Apalachicola Bay, Florida. In upper Chesapeake Bay, small infaunal Baltic macoma comprised more than 60% of the diet of large crabs (Hines et al. 1990). Where or when bivalve prey are not common, fish, shrimp, gastropods, and crabs become important for larger blue crabs. Plant material is common in the stomach contents (10-12%) of small (<60 mm) blue crabs in seagrass habitat, but much less so for large (>60 mm) crabs. The diet of small blue crabs rarely includes other blue crabs, whereas they are a signifi- cant dietary component (10-15%) of larger blue crabs (Laughlin 1982; Hines et al. 1990). Diets of all sizes of juvenile to adult blue crabs commonly include xanthid crabs (Laughlin 1982; Mansour 1992). Large blue crabs evidently are also capable of capturing several species of estuarine fishes (5-15% of stomach contents), although small juveniles typi- cally do not (Laughlin 1982; Hines et al 1990; Man- sour 1992). In Puerto Rico at a site where bivalves were uncommon, variation in the diet of C. sapidus clustered into four crab size classes (10-20 mm, 21- 30 mm, 31-80 mm, 81-150 mm CW) (Stoner and Buchanan 1990). Stomachs of small crabs included mainly amphipods, foraminiferans, polychaetes, and detritus, whereas larger crabs ate more fish, crabs, and bivalves (Stoner and Buchanan 1990). Thus, the diet diversity of blue crabs tends to decrease during ontogeny, with stomach contents of small blue crabs including more species of smaller prey and larger crab stomachs including fewer species but with increasing composition of bivalves, juvenile blue crabs, and fish. Temporal Variation in Diet Blue crab diet shows temporal variation on sev- eral time scales. Although blue crabs feed through- out the diel cycle (see Foraging Behavior and Activ- ity below), diet composition generally does not exhibit significant diurnal-nocturnal variation (Laughlin 1982; Ryer 1987; Hines, unpubl. data) Diet may vary temporally with tidal fluctuations that provide blue crabs with access to prey species of intertidal habitats like salt marshes, mangroves, ribbed mussel Geukensia demissa beds, and sand flats (Ryer 1987; Lin 1989; Fitz and Wiegert 1991b; pers. obs.). Blue crabs readily move into intertidal habi- tats to forage on ribbed mussels, and their foraging success may be limited by duration of high tide and distance needed to travel to reach prey from the sub- tidal zone (Lin 1989). Conversely, motile species that also forage or seek refuge in the intertidal habi- tats may become more important in the diet of blue crabs on low tide as both prey and blue crabs become concentrated in shallow subtidal habitat. In areas like upper Chesapeake Bay where tidal ampli- tudes are small and intertidal habitats are not used by blue crabs, tidal variation in diet may be concomi- tantly small (pers. obs.). Over longer time periods, blue crab diet shows significant seasonal variation, probably reflecting changing prey availability through seasonal recruit- ment and depletion (Laughlin 1982; Hines et al. 1990; Meise and Stehlik 2003). In Apalachicola Bay, Florida, molluscs comprised a greater portion of blue crab diet in winter-spring than in summer-fall, especially in small crabs that fed on small, shallowly buried bivalves (Laughlin 1982). In upper Chesa- peake Bay, infaunal bivalves and juvenile blue crabs formed a greater portion of the diet of large blue crabs in late summer to early fall, as the abundance of surface-dwelling infauna declined markedly with increasing predator activity in the early season (Hines et al. 1990). Mansour (1992) found signifi- cant annual variation in blue crab diet in Chesa- peake Bay (two-fold difference among years in some prey items such as cannibalized crabs), reflecting variation in prey availability because ontogenetic shifts in diet were similar among years. Similar large annual variation in diet was found in New Jersey salt marsh systems (Meise and Stehlik 2003). Spatial Variation in Diet Because blue crabs forage opportunistically over large spatial extent, diet may vary along with prey species available in diverse habitats. Diet varied sig- nificantly among sites within Apalachicola Bay (Laughlin 1982) and Chesapeake Bay (Mansour 1992), and among estuarine systems (e.g., Tagatz 1968a; Heard 1982; Laughlin 1982; Hines et al. E COLOGY OF J UVENILE AND A DULT B LUE CRABS 597 1990; Stoner and Buchanan 1990; Mansour 1992; Meise and Stehlik 2003; Stehlik et al. 2004). For example, whereas molluscs form the main part of the diet in most estuaries, xanthid crabs formed a large part of the diet in the Hudson-Ruritan estuary (Stehlik et al. 2004) and in some locations and times in Chesapeake Bay (Mansour 1992). On the other hand, broad categories of the diet of blue crabs remain similar across the full latitudinal range of the species (e.g.,Tagatz 1968a; Laughlin 1982; Hines et al. 1990; Stoner and Buchanan 1990; Mansour; 1992; Cesar et al. 2003; Meise and Stehlik 2003; Stehlik et al. 2004). Although bivalves appear to be important in the diet in most areas (Menzel and Hopkins 1956; Darnell 1958;Tagatz 1968a;Alexan- der 1986; Hines et al. 1990; Mansour 1992; Meise and Stehlik 2003; Stehlik et al. 2004), the species of bivalves shift among habitats to include eastern oys- ters in oyster reefs (Eggleston 1990a, b, c), ribbed mussels in salt marshes (Lin 1989), and infaunal Baltic macoma and soft clam Mya arenaria in subtidal soft-bottom habitats (Hines et al. 1990; Mansour 1992). Many food items are taken opportunistically as they are encountered by crabs moving among habitats. For example, in salt marshes fiddler crabs (Uca spp.) and periwinkles (Littoraria spp.) are impor- tant components of the diet (Heard 1982). Plant material is common in stomach contents of blue crabs associated with seagrass beds (Halodule wrightii, Ruppia maritima ) (Darnell 1958; Tagatz 1968a; Laughlin 1982; Alexander 1986) and salt marshes ( Spartina spp . ) (Alexander 1986), whereas plant material in stomachs is almost absent in estuarine habitats lacking plants (Hines et al. 1990). Juvenile blue crabs may use structured habitats like seagrasses because those habitats provide diverse and abundant food resources, as well as refuge from predation (Laughlin 1982; Thomas et al. 1990; Perkins-Visser et al. 1996). Seagrass habitats provide abundant infaunal and epifaunal prey in the root-rhizome and canopy regions, as well as detritus (Heck and Orth 1980a; Orth et al. 1984; Mansour 1992). Interspecific Variation in Diet The diets of other species of Callinectes appear to be similar to C. sapidus and to show similar ontoge- netic and spatial variation (Paul 1981; Haefner 1990a; Stoner and Buchanan 1990; Hsueh et al. 1992a). In diets of C. arcuatus and C. toxotes on the Pacific coast of Central America, plant material and detritus decreased with increasing crab size whereas shrimp, fish, and bivalves increased (Paul 1981). The diet of C. toxotes appears to be more carnivorous and less varied than that of C. arcuatus (Paul 1981). In Mobile Bay,Alabama, fish, bivalves, brachyuran crabs, and gastropods comprised 85% of the diet of C. sim- ilis and 91% of the diet of C. sapidus (Hsueh et al. 1992a). In a comparative study of diet ontogeny in four species of Callinectes in Puerto Rico (Stoner and Buchanan 1990), C. danae showed the least var- ied diet, which was relatively low in detritus and higher in mostly large motile prey. For C. danae , crabs (mostly hermit crabs) were most important in the diet of small individuals up to 125 mm CW and less important for larger individuals. Fish increased with significant seasonal variation in the stomachs of smaller to larger sizes of C. danae , and shrimp impor- tance increased with crab size up to 125 mm but was unimportant in large crabs. In C. bocourti , crab remains and detritus decreased as crab size increased, polychaetes were common in smaller crabs, and bivalves were variable but present in all sizes of crabs (Stoner and Buchanan 1990). In C. ornatus , detritus and shrimp decreased with crab size, bivalves and gastropods increased with crab size, amphipods were proportionately high in small crabs, and fish and crabs were common in mid-sized crabs (Stoner and Buchanan 1990). By comparison, in C. ornatus in Bermuda, stomach contents were dominated (~40%) by cerithiacean gastropod mollusc species (including Modulus modulus and two others), with carbonate substrate, plant material, crustaceans, nereid poly- chaetes, fish, and bivalve molluscs comprising other important components (Haefner 1990b). Of the Callinectes species studied, the diets of C. bocourti (Stoner and Buchanan 1990) and C. similis (Hsueh et al. 1992a) appear to be most similar to that of C. sapidus . This diet similarity may reflect morphological and behavioral similarities, as these species are thought to be most closely related to each other (Norse and Fox-Norse 1982; Williams 1984). 598 T HE B LUE CRAB Blue crab diet is similar to that of portunid crabs other than Callinectes spp., many of which are preda- tors of soft-bottom habitats and often take molluscs and decapod crustaceans. This similarity is evident in European green crabs Carcinus maenas (Ropes 1968, 1989), Ovalipes spp. (Caine 1974; Haddon et al. 1987; Sponaugle and Lawton 1990), P ortunus pelagicus (Williams 1981; deLestang et al. 2000), and Scylla serrata (Hill 1976). Foraging Behavior and Activity Blue crabs are capable of using visual cues to track and catch prey, particularly faster-moving prey like fishes, fiddler crabs, and other blue crabs. On occasion they have been observed climbing out of water for prey (Abbott 1967), or reaching out of water to capture Littoraria spp. snails on emergent salt marsh vegetation (Hamilton 1976). There are sev- eral records of crabs cruising along the water?s edge of salt mashes with their eyestalks extended out of water like periscopes to track fiddler crabs (Uca spp.), and then making quick dashes out of the water to grab their quarry and return to the water to eat (Hughes and Seed 1995;W. Herrnkind, Florida State University, pers. comm.;T.Wolcott, North Carolina State Universtiy, pers. comm.). Prey movement seems to be of paramount importance in attracting the initial attention of the predator (Hughes and Seed 1995; Mascaro et al. 2003), and crabs appear strongly attracted to even slowly moving prey such as snails (Hughes 1989), sometimes resulting in dis- traction of the crab from items already being han- dled (Hughes and Seed 1995). The reverse case where visual behavior is modified by chemical cues also may occur (Diaz et al. 1999, 2001). Although the ability to distinguish colors has been attributed to blue crabs, in part based on mating and threat dis- plays, the physiological (optic pigment) basis for this is not established (Bursey 1984), and there is no record of blue crabs using prey color in foraging behavior. Foraging in blue crabs is typically mediated by chemotactile cues, as their chemo-sensory abilities are very sensitive to trace dilutions of chemicals (Pearson and Olla 1977). Chemical cues modulate feeding behavior in already active crabs rather than initiating foraging from a quiescent state (Zimmer- Faust et al. 1996). Small juveniles (4th-5th instar) exhibit a hierarchy of responses to flow, odors, and visual cues during orientation (D?az et al. 2003). Larger blue crabs orient well in slow currents and move in a zig-zag pattern back and forth across odor plumes to locate food (Weissburg and Zim- mer-Faust 1993, 1994; Zimmer-Faust et al. 1995; 1996; Powers and Kittinger 2002). This orientation to odor plumes is very sensitive to turbulence and current speed (Weissburg and Zimmer-Faust 1993, 1994; Zimmer-Faust et al. 1995; Finelli et al. 1999; Powers and Kittinger 2002;Weissburg et al. 2003). Flow properties and turbulence affect the ability of crabs to orient to odor plumes emanating from actively pumping bivalves (hard clams, bay scallop Argopecten irradians ), and high flow speed or large sediment particle size increase boundary layer tur- bulence, thereby decreasing crab success at chemo- orientation. In addition to these boundary layer effects, high flow speed also lessens the probability that crabs contact odor plumes, so that crabs orient best in very slow (<3 cm s-1), smoothly flowing currents and perform poorly in fast currents or no flow. When they are orienting in odor plumes from distant food, blue crabs use both cephalic and tho- racic appendages for olfactory-mediated foraging, and the combination of these may provide elements of redundancy that are valuable when chemical signals are weak or intermittent and also may pro- vide a more three-dimensional perception of the chemical plumes (Keller et al. 2003). For orienting to prey at a distance and in currents, crabs typically use chemo-sensory structures on their antennules, involving antennule flicking and increased pumping of the scaphognathite (Hazlett 1971; Pearson and Olla 1977; Eggleston 1990a; Keller et al. 2003). As they reach the location of prey they use chemotac- tile receptors on the dactyls of their walking legs (Lipcius and Hines 1986; Eggleston 1990a; Keller et al. 2003). They may probe the sediment with their chelae pointed down and then proceed to excavate prey to sediment depths of 10 to 15 cm (Blundon and Kennedy 1982a, b;Alexander 1986; Lipcius and E COLOGY OF J UVENILE AND A DULT B LUE CRABS 599 Hines 1986). Prey are manipulated with their claws and maxillipeds to allow the mandibles to take bites for ingestion. Once prey are obtained, blue crab claws are capable of exerting large forces to crack and open shells of most bivalves (Blundon and Kennedy 1982a). Although some bivalves (e.g., large Atlantic rangia clams Rangia cuneata and hard clams) are armored heavily enough to prevent cracking (Blundon and Kennedy 1982a; Seitz et al. 2001a), some crabs develop handling techniques to chip the edges of shells (a ?can-opener? technique) and gain entry to soft tissues of prey (Blundon and Kennedy 1982a; Eggleston 1990a, b; Hughes and Seed 1995). Repeated application of relatively moderate force to an area of molluscan shell creates micro-fractures that eventually cause fatigue failure (Elner 1978; Boulding and LaBarbera 1986). Wolcott and Hines (1989a) used ultrasonic biotelemetry equipped with electrodes inserted into the origin of blue crab mandibular muscles to trans- mit muscle action potential during contraction; this allowed the recording of the number of bites used to consume prey items (Fig. 10). The number of bites was positively correlated with prey size, with ~200 bites typically required to consume the small clam Macoma balthica . In general, feeding bouts for a vari- ety of prey involved ten to several hundred bites (Nye 1989;Wolcott and Hines 1989a). When con- suming prey with a hard exterior (e.g., bivalves), crabs typically had pauses in mandibular bites at the start of a feeding bout as prey were manipulated by the chelae, for example, to open shells; for prey with a hard interior structures (e.g., fish), pauses in biting occurred at the end of the feeding bout when food was most manipulated to tear apart the skull and vertebral column (Nye 1989). Foraging is markedly affected by the molt cycle. Feeding ceases during late post-molt stages (D2-D4), ecdysis (E), and soft post-molt stages (A, early B), but crabs are voracious feeders from late post-molt through early pre-molt stages (late B, C, D0, D1). Cessation of feeding around the time of molting allows the soft-crab fishery to hold high densities of premolt crabs without fear of cannibalism while waiting for the crabs to molt to the commercial soft crab stage. Feeding activity exhibits significant variation on diel, tidal, and seasonal cycles. Although nocturnal feeding activity is often attributed to blue crabs (Warner 1976), sampling crabs on a 24-h cycle for analysis of stomach contents showed only a weak trend for increased feeding at dusk in lower Chesa- peake seagrass beds (Ryer 1987). Feeding was related to the tidal cycle in adjacent salt marsh creeks, with crab guts being fullest on high tides. Ultrasonic biotelemetry transmitters equipped with electrodes to record mandibular muscle contractions of free-ranging crabs in upper Chesapeake Bay showed that blue crabs feed in distinct feeding bouts 4 to 7 times throughout the diel cycle, but with sig- nificant peaks of feeding during crepuscular times of morning and evening (Nye 1989;Wolcott and Hines 1989a). Similarly, a congener, C. arcuatus , fed most actively at night in the laboratory, with a prominent peak at dusk and a lesser peak at dawn (Paul 1981). Insofar as foraging success of blue crabs is related to their use of currents to orient to odor plumes of prey (Weissburg and Zimmer-Faust 1993, 1994; Zimmer-Faust et al. 1995; 1996; Weissburg et al. 2003), tidal currents may produce cycles of foraging. Seasonal cycles of feeding activity are clearly related to temperature because feeding rates decline with low temperatures and appear to cease below 7? to 8?C (pers. obs.). Predator-Prey Population Interactions, Community Effects, and Food Web Dynamics Direct Effects of Blue Crab Predation Covariation of Predator-prey Populations. Foraging of blue crabs has major effects on prey populations and benthic community structure. Negative correlations of predator and prey abun- dances often reflect these effects. Densities of infau- nal prey increase seasonally during winter-spring recruitment and decline markedly as blue crabs and other epibenthic predators become active during warm summer-fall months, especially in high lati- tude estuaries like Chesapeake Bay (Virnstein 1977, 1979; Holland et al. 1980; Hines et al. 1990). In 600 T HE B LUE CRAB estuaries at lower latitudes, seasonal effects are less marked but still evident (Livingston 1976; Laughlin 1982). The abundance and habitat use of prey such as eastern oysters may be limited by blue crab preda- tion that can lead to local extinction of oyster spat in Louisiana (Menzel and Hopkins 1956), South Car- olina (Lunz 1947), Florida (Marshall 1954), and Chesapeake Bay (Eggleston 1990a, b, c) over a wide range of salinities (Menzel and Hopkins 1956; Car- riker 1967). Similarly, blue crab foraging exerts great negative influence on abundance of clams such as wild hard clams (Van Engel 1958; Sponaugle and Lawton 1990), cultured hard clams (Gibbons and Castagna 1985; Kraeuter and Castagna 1985a, b), Atlantic rangia (Darnell 1958), soft clams (Blundon and Kennedy 1982a, b; Eggleston et al. 1992), ribbed mussels (Seed 1980), Baltic macoma (Eggleston et al. 1992; Hines et al. 1990), and bay scallops (Bologna and Heck 1999; Bishop et al. 2005). Blue crab and bivalve prey densities (e.g., Baltic macoma) also may exhibit inverse covariation over longer intervals of several years (Hines et al. 1990). Blue crabs appear to limit abundances of certain invasive species. Experiments indicate that predation by large blue crabs may limit abundances of young rapa whelks Rapana venosa , a species which has been recently introduced in lower Chesapeake Bay (Harding 2003), although the rapa whelk appears to E COLOGY OF J UVENILE AND A DULT B LUE CRABS 601 Figure 10. Biotelemetry record of movement and feeding of a large male blue crab in the Rhode River subestuary of Chesapeake Bay in 1986. The crab was equipped with a telemetry tag like that shown in Fig. 6A that transmits mandibular muscle contraction and location. Map at top shows the track of the crab, which was released at position 1 and moved along the numbered sequences over 96 h, spending most of the time feeding at position ?6? before being recaptured in a pot at ?X.? The 4-d feeding record is shown below the map, with circled numbers corresponding to the numbered positions along the track. Lines extending upward from the axis show feeding during feeding bouts at 10-min intervals, with the number of bites presented on a log scale. Lines below the axis represent single, non-feeding bites in each 10-min interval. be well-established as an invasive species within areas of abundant crabs (Mann and Harding 2000). Blue crabs also appear to limit the geographic distribution of the long-established invasive European green crab along the East Coast of North America (deRivera et al. 2005). Although green crabs extend to lower, warmer latitudes within their native range in the eastern Atlantic, the southern limit of the invasive green crab population along the northwestern Atlantic occurs between Delaware and Chesapeake Bays, where blue crab abundance increases markedly. Moreover, the abundance of green crabs is inversely related to blue crab abundance within bays in the region of overlap of the two species between Cape Cod and Chesapeake Bay. Mortality of green crabs tethered at sites spanning the geographic overlap increased in correlation with abundance of blue crabs, and carapace remains of tethered crabs were indicative of blue crab predation. Tidal amplitude drops dramatically in the vicinity of Chesapeake Bay, which greatly diminishes the intertidal zone that appears to afford green crabs refuge from subtidal blue crab predation. Blue crab predation is pre- dicted to limit the down-stream spread of invasive zebra mussels Dreissena polymorpha in the Hudson River estuary (Boles and Lipcius 1997). Community Structure: Predator Exclusion Studies. Direct effects of predation by blue crabs are readily evident from predator exclusion experiments in soft-bottom habitats, such that infaunal species diversity and density increase markedly within cages placed over patches of the bottom to exclude the suite of blue crabs and other large epibenthic preda- tors (Virnstein 1977, 1979; Holland et al. 1980; Woodin 1981; Hines et al. 1990; Silliman and Bert- ness 2002). Although unarmored prey species dwelling on or near the surface of sediments are fed on by blue crabs, other decapods, and fishes, blue crabs have a major effect on deeper burying or armored prey (especially bivalves) (Virnstein 1979; Hines et al. 1990; Eggelston et al. 1992; Micheli 1997a, b; Seitz et al. 2001). Note that the horseshoe crab Li mulus polyphemus and the cow-nose ray Rhinoptera bonasus may also feed on the same deeply buried prey in some habitats (Orth 1975; Woodin 1981). Survivorship of infaunal bivalves in habitats dominated by blue crabs increased markedly when these prey were placed in sediment patches pro- tected by cages (Hines et al. 1990; Eggleston et al. 1992; Seitz et al. 2001a; Kuhlmann and Hines 2005). Similarly, experiments excluding wading birds and blue crabs on North Carolina salt marshes showed that predation by blue crabs, not birds, caused major mortality of mummichog Fundulus heteroclitus , espe- cially for the largest (>70 mm) fish, which suffered 90% mortality in 24 d (Kneib 1982). These experi- mental studies show clearly that when crabs and other motile epibenthic predators are excluded in Chesapeake Bay and other locations, the diversity and densities of infaunal species remain high; con- versely, predation by blue crabs and other epibenthic predators drastically reduces infaunal prey abun- dance and diversity. Blue crab predation on infauna thus exerts ?top-down? control of benthic commu- nities, although food availability as patches of infau- nal prey support higher densities of blue crabs than areas with low food resources (e.g., Seitz and Lipcius 2001).The balance of relative effects of bottom-up control of community structure by nutrients and food resources versus top-down control by blue crab predation may vary in space and time (Posey et al. 1999b; Seitz and Lipcius 2001). Indirect Effects of Blue Crab Predation In some soft-bottom communities, responses to experimental manipulation of blue crabs and other large epibenthic predators are more complex, reflecting indirect effects of predation. For example, in Georgia salt marshes experimental manipulation of the dominant grazer the marsh periwinkle Lit- toraria irrorata and its consumers (blue crabs and ter- rapins Malaclemys terrapin ) demonstrated that plant biomass and production were largely controlled by the trophic cascade of grazers and their predators, rather than by nutrient supply (Silliman and Bert- ness 2002). In the absence of the crab and terrapin predators, the periwinkles were able to convert one of the most productive marshes in the world into a barren mudflat within 8 months. In some ecosystems, however, trophic interac- 602 T HE B LUE CRAB tions are more complex and top-down effects of blue crabs and other large predators may be modu- lated by other species. Predator exclusion experi- ments in the Indian River Lagoon, Florida, showed that infaunal prey abundance and diversity did not increase in the absence of large blue crabs and fishes (Virnstein 1978). Instead, exclusion of the large predators removed predation on smaller decapods and fishes that function as an intermediate guild of predators on infauna. The small, intermediate predators increased markedly within the cages and continued to maintain the abundance and diversity of infaunal prey at low levels. The relative impor- tance of predation by large blue crabs and fishes compared to that by smaller, intermediate epiben- thic and infaunal predators has been sometimes debated with no clear conclusion (e.g., see Virnstein 1978;Ambrose 1984, 1986; Commito and Ambrose 1985a, b;Wilson 1986). However, the regions where the smaller predators appear to have greatest effects compared to blue crab effects appear to be at higher salinities or lower latitude where these smaller predators have high species diversity compared to their diversity in lower salinities and northern estuaries. Oyster toadfish Opsanus tau may limit mud crab and blue crab predation on juvenile hard clams (Flagg and Malouf 1983; Gibbons and Castagna 1985; Bisker and Castagna 1989; Bisker et al. 1989). For example, hard clams in experimental trays with oyster toadfish had nearly 70% survival and the trays contained significantly fewer blue crabs, whereas clams in trays without toadfish had only 2% survival and more blue crabs were present (Bisker and Castagna 1989). However, other experimental tests in cages indicated that oyster toadfish caused only slightly lower blue crab predation upon oysters set on cultch and had no effect on blue crab predation on cultchless oyster spat (Abbe and Breitburg 1992). Blue crabs may facilitate predation by other species in the community. Using enclosures in a North Carolina Sound, Martin et al. (1989) showed that blue crabs enhanced survival of spot Leiostomus xanthurus by removing the algae Entero m orpha intestinalis, providing access for the fish to feed upon the invertebrates in bottom sediment. In Mississippi waters, blue crabs have been reported to carry many (1-17) oyster drills Thais haemastoma on their backs during their movement within subestuaries and their longer-distance migration along estuaries (Cake 1983). Thus, the crabs transport these impor- tant predators of bivalves, snails, and barnacles to various salinity zones and among prey patches. Blue crabs also cause extensive indirect effects on soft-bottom communities through bioturbation (sedimentary disturbance by animals) by their bur- rowing activities (Woodin 1981; Hines et al. 1990). Experiments using exclusion cages placed in upper Chesapeake Bay in summer showed that blue crab digging activity thoroughly re-worked patches of dyed surface sediments down to depths of 10 cm (Hines et al. 1990) (Fig. 11). Such bioturbation has major effects on sediment stability and biogeochem- istry, which in turn regulates many aspects of infau- nal community structure, such as composition of deposit- versus suspension-feeders (e.g., Rhoads and Young 1970; Rhoads 1974). Blue crab foraging is affected markedly by indi- rect effects of other predator species in soft-bottom communities. Browsing by epibenthic fishes (espe- cially flatfish and sciaenid species) and shrimp can cause intense siphon nipping of infaunal bivalves (deVlas 1985; Zwarts 1986; Zwarts and Wanink 1989; Kamermans and Huitema 1994;Whitlatch et al. 1997), including infaunal bivalves within the estu- arine communities of blue crabs (Hines et al. 1990; Irlandi and Mehlich 1996). Deposit-feeding bivalves with cropped siphons (particularly Baltic macoma) reside nearer to the sediment surface (Hodgson 1982; deVlas 1985; Zwarts 1986; de Goeij et al. 2001) and change their feeding activity (Lin and Hines 1994; Peterson and Skilleter 1994; Skilleter and Peterson 1994). Macoma balthica , which com- prises much of the diet of blue crabs in many parts of Chesapeake Bay (Hines et al. 1990; Mansour 1992), undergoes a seasonal cycle of burial depth as intense cropping by epibenthic fishes (spot, Atlantic croaker Micropogonias undulatus , hogchoker Trinectes maculatus ) reduces siphon size faster than siphons can regenerate during summer (Hines and Lipcius, in prep.). Clams with partially nipped siphons (<40% intact weight) still tend to reside at refuge burial E COLOGY OF J UVENILE AND A DULT B LUE CRABS 603 depths >12 cm. However, as cropping increases (>40% removal), the clams move up to sediment depths <12 cm where they become accessible to excavation and predation by blue crabs. Thus, the accumulating non-lethal siphon nipping by epiben- thic fishes causes a threshold response in the lethal predation by blue crabs feeding on the clams (Hines and Lipcius, in prep.). Spatial Variation Predator exclusion experiments show that effects of predation by blue crabs and epibenthic fishes on prey populations and infaunal community structure are qualitatively similar across several spatial scales, but with significant quantitative variation within subestuaries (Hines et al. 1990), among salin- ity zones of Chesapeake Bay (Virnstein 1977, 1979; Holland et al. 1980; Hines et al. 1990), and among estuarine systems along the east coast of North America (Virnstein 1978, 1979; Holland et al. 1980; Woodin 1981; Hines et al. 1990). In some habitats, blue crabs are the numerically dominant predators regulating benthic community structure (e.g., upper Chesapeake Bay), whereas in other locations they may be one member of a diverse guild of abundant predators (e.g., Indian River Lagoon, Florida) (Virn- stein 1978; Hines et al. 1990). Blue Crab Responses to Prey Resources Selection of Individual Prey Although blue crabs feed upon a wide range of prey, they select prey with certain preferred charac- teristics that vary by species, size, morphological armor, and passive and active modes of escape. Attributes of the habitat also affect predation success and individual prey selection by blue crabs. Blue crabs and other crab species have been used as good experimental models to test theoretical hypotheses about mechanisms of prey selection by predators. Experimental studies also show that blue crab forag- ing behavior and selection of individual prey can be conditioned by environmental variables and the crabs? experience (e.g., Seed and Hughes 1997;Ter- win 1999). Prey Species. Blue crab foraging rates in com- parative laboratory experiments vary significantly among prey species that differ substantially in their vulnerability to blue crabs, such as large eastern oys- ters and small hard clams (Bisker and Castagna 1987; Eggleston 1990b); heavy-shelled Atlantic rangia clams and similar-sized thin-shelled soft clams (Ebersole and Kennedy 1995); mobile fiddler crabs 604 T HE B LUE CRAB Figure 11. Bioturbation of sediments in the Rhode River subestuary of Chesapeake Bay, Maryland. Vertical profiles traced from representative photographs show sections through patches of dyed sand place on sediment surface in late August (Time 0) and subjected to 4 cage treatments for 2 weeks. Crabs and fish were excluded (full cage) or allowed access to patches in partial cages (2-sided, no top) and uncaged areas. Note extensive bioturbation that extends to a sediment depth of 10 cm, which is the approximate limit of blue crab digging and is much deeper than disturbance by sciaenids and flatfishes. From Hines et al. (1990). (Uca spp.) and sedentary ribbed mussels (Hughes and Seed 1981; Micheli 1997; Seed and Hughes 1997); and deeply buried bivalves (soft clam) and a surface- dwelling species (hooked mussel Ischadium recurvum ) (Ebersole and Kennedy 1995). In Alabama salt marshes, blue crabs preyed most frequently on epi- faunal periwinkles Littoraria irrorata and, to a lesser extent, killifish Fundulus similis ; they rarely selected semi-buried ribbed mussels (West and Williams 1986). However, foraging rates of blue crabs are similar among species that do not differ much in vulnerability, such as small bivalves when they are at or near the sediment surface (Atlantic rangia, hard clams, soft clams, Baltic macoma, ribbed mussels, hooked mussels) (Bisker and Castagna 1987; Sponaugle and Lawton 1990; Eggleston et al. 1992; Ebersole and Kennedy 1995; Seitz et al. 2001a). These differences and similarities suggest that blue crabs respond to various traits that contribute to dif- ferences in profitability among prey species, such as costs of capture success and handling time, as well as energetic gains (Seed and Hughes 1997). Prey Size. Although crabs consume a wide size range of prey, laboratory experiments show that blue crabs exhibit size selection of prey when feeding on various molluscs, including periwinkles (Schindler et al. 1994), hard clams (Arnold 1984; Peterson 1990; Micheli 1995), ribbed mussels (Seed 1980, 1982; Hughes and Seed 1981; Bertness and Grosholz 1985; Lin 1991),Atlantic rangia clams (Ebersole and Kennedy 1994), and eastern oysters (Bisker and Castagna 1987; Eggleston 1990b, c). When offered hard-shelled prey (molluscs), blue crabs and other decapods often select smaller sizes than the predicted optimum (see optimal foraging section below) (Juanes 1992). For example, blue crabs prefer smaller than larger individuals of the ribbed mussel (Hughes and Seed 1981) and the hard clam (Micheli 1995). Blue crabs in the laboratory readily crush very small mussels, and feed on slightly larger mussels by crushing them more slowly across the umbonal region, whereas they open larger mussels gradually by chipping the posterior edges of the shell, severing the adductor muscle, and tearing the valves apart (Hughes and Seed 1981). For each prey species, blue crab foraging is also dependent on relative predator:prey size (Schindler et al. 1994). For some prey such as penaeid shrimp, large crabs are more successful at capturing larger prey; however, prey activity also affects prey capture rates (Mascaro et al. 2003). Prey Armor. Successful attack on molluscan and other prey species by crabs often depends on the prey?s armor (evolution of thick shell with ridges or spines) (Vermeij 1987). Many infaunal prey lack effective armor and are readily attacked by blue crabs (e.g., thin-shelled bivalves such as soft clams, Macoma spp.) (Blundon and Kennedy 1982a, b) and must rely on other methods of avoiding or reducing crab predation (see prey escape section below) (Seitz et al. 2001a). Even for species that have evolved armor against predators, shell thickness and armor strength are closely correlated with size, so that juve- nile prey become less vulnerable to blue crabs as they grow and attain armor strength that is effective against even large blue crabs (e.g., hard clams, Atlantic rangia clams) (Blundon and Kennedy 1982a, b; Arnold 1984). Other prey increase the crab?s handling time through other aspects of mor- phological barriers to crab manipulation, such as attachment in groups by shell cementation (oysters) (Eggleston 1990b) or by byssal threads (mussels) (Bertness and Grosholz 1985; Lin 1991). Prey Escape. Mobile prey (fish, other decapod crustaceans) typically escape crab predation by flee- ing. Although unsuccessful attacks by blue crabs are not usually documented, autotomy (limb loss) of juveniles often leaves a record of unsuccessful canni- balistic attacks (Smith and Hines 1991a; Dittel et al. 1995; Hines and Ruiz 1995). Some prey escape blue crabs by moving into refuge habitats either where they are less accessible to blue crabs or where blue crabs are less effective at foraging. Avoiding vulnerability during tidal inun- dation, some prey (e.g., Littoraria spp.) climb emer- gent vegetation, although blue crabs will reach out of the water to take prey off Spartina alterniflora plants (Hamilton 1976; Stanhope et al. 1982; Warren 1985). Similarly, sea scallops Placopecten magellanicus E COLOGY OF J UVENILE AND A DULT B LUE CRABS 605 attach to seagrasses at levels off the bottom (e.g., Barbeau et al. 1994) where blue crabs may encoun- ter them less frequently than if they remained on the bottom. Other prey position themselves on marsh surfaces where they are less vulnerable to predation by blue crabs. For example, fiddler crabs (Uca spp.) hide in burrows on high tide and forage on marsh surfaces on low tides; however, blue crabs sometimes will emerge briefly from marsh creeks on low tide to catch fiddler crabs and take them back into the creek to feed on them (Hughes and Seed 1981;W. Herrnkind, pers. comm.; T. Wolcott, pers. com.). Ribbed mussels in the interior of salt marshes suffer less mortality from blue crab predation than those on the edge of tidal creeks, and those mussels within the interior of mussel clumps are less vulnerable to blue crabs (less accessible to manipulation by crabs) than those on the outside of clumps (Lin 1989, 1991). Escape of juvenile blue crabs from canni- balistic large crabs appears to be more effective in shallow water than in deep water (Hines and Ruiz 1995). Thin-shelled molluscs and other unarmored infauna escape predation by burying to sediment depths below detection or digging capabilities of blue crabs (Blundon and Kennedy 1982b; Lipcius and Hines 1986; Hines et al. 1990; Seitz et al. 2001; Hines and Lipcius, in prep.). Vulnerability of infau- nal bivalves (soft clams, Baltic macoma) to blue crab predation increases markedly at sediment depths shallower than 10 to 12 cm (Blundon and Kennedy 1982b; Hines and Lipcius, in prep.). Effects of Habitat Characteristics.Vulnerabil- ity of infaunal prey to predation by blue crabs is greatly reduced by structure, such as rhizomes of submerged aquatic vegetation (Heck and Thoman 1981; Blundon and Kennedy 1982b), polychaete tubes (Woodin 1981), and by shell rubble within or overlying the sediment (Virnstein 1979; Arnold 1984; Gibbons and Castagna 1985; Kraeuter and Castagna 1985a, b; Sponaugle and Lawton 1990). Rate of blue crab foraging upon infaunal bivalves also differs by sediment type (sand, mud) depending on the prey species (soft clams, Baltic macoma) (Lip- cius and Hines 1986; Eggleston et al. 1992; Seitz et al. 2001a). Added structure in the sediment appears to interfere with the crabs? ability to detect the prey with the chemotactile senses of their dactyls, or the crabs? ability to excavate the prey items, or both (Lipcius and Hines 1986; Eggleston et al. 1992). Similarly, blue crab foraging rates are reduced by fronds of submerged aquatic vegetation (Heck and Thoman 1981;Wilson et al. 1990a; Heck and Crow- der 1991) and probably by drifting algae (Wilson et al. 1990b). Hypoxia causes reductions in foraging activity in blue crabs and other predators, which generally move to avoid hypoxic water (Pihl et al. 1991; Bell et al. 2003a) and reduce feeding rates (Das and Stickle 1993; Seitz et al. 2003a). However, foraging in response to hypoxia may depend on complex predator-prey interactions modified by hypoxia. For example, hypoxia may cause infaunal soft clams to decrease their burial depth and extend their siphons, making them more vulnerable to crabs (Taylor and Eggleston 2000). However, agonistic interactions between crabs reduced foraging on these clams (Taylor and Eggleston 2000). Further, blue crab for- aging also depends on hydrodynamics and duration of episodic events. Foraging activity of telemetered blue crabs in the field declined slightly when they were exposed to mildly hypoxic water (dissolved oxygen concentrations 2-4 mg L-1) and severe hypoxia (<2 mg L-1), but they continued to feed with dissolved oxygen concentrations as low as 1 mg L-1 (Bell et al. 2003b). These crabs reduced their proportion of time spent feeding during hypoxic upwelling conditions, but feeding ceased during the most severe upwelling of very hypoxic water. Optimal Foraging on Individual Prey:Time Minimizer or Energy Maximizer? Optimal for- aging theory hypothesizes that predators select prey with characteristics that maximize their net rate of energy (or biomass) intake, optimizing a balance in energy gained in successful consumption against energy costs of searching for and handling prey (Hughes 1980). The relative importance of some prey characteristics and profitability for blue crab foraging can be tested experimentally by providing crabs with choices in the laboratory and by use of 606 T HE B LUE CRAB ?sham prey? such as dead bivalve shells of various sizes filled with artificial tissues of differing food quality (e.g., Micheli 1995). Certain crab species appear to conform to predictions of their ability to evaluate prey and to choose a diet according to encounter rates with prey of differing values, e.g., Carcinus maenas feeding on blue mussels M ytilus edulis (Elner and Hughes 1978). Blue crabs also select bivalve prey species with preferences that opti- mize prey profitability with respect to handling time, prey location, and prey refuge use (Ebersole and Kennedy 1995; Seed and Hughes 1997). Even for very small species of snail Bittiolum varium and Astyris lunata, blue crabs selected prey that were most profitable (Cote et al. 2001). Blue crabs, how- ever, appear to select individual prey with behavior that simultaneously minimizes time spent handling prey and maximizes net rate of energy intake; smaller mussels that are easily crushed are preferred over larger ones that require significantly more han- dling time and technique for the crabs to open (Hughes and Seed 1981; Seed and Hughes 1997). Minimizing foraging time has adaptive advantages for blue crabs by reducing risk of exposure to preda- tors and increasing time for other activities (e.g., hiding, searching for mates). The relative impor- tance of time minimization versus energy maxi- mization is not resolved for blue crabs. At least five models attempt to explain prey size- selection behavior of crabs feeding on hard-shelled molluscs (Hughes and Seed 1995). (1) The chelar- wear model predicts that crabs prefer smaller prey sizes because larger sizes present a risk of chelar damage or wear that would select against future for- aging capability (Juanes 1992). This model does not provide a comprehensive explanation of size-selec- tion because intermediate prey sizes are sometimes selected and because sometimes smallest prey are also most profitable. (2) The prey-evaluation model predicts that preferred prey are most profitable, such that crabs appear to evaluate prey for a period of manipulation with chelae or mouth parts before accepting or rejecting an item (Elner and Hughes 1978; Jubb et al. 1983). This model does not provide a mechanistic definition of ?evaluation,? in that whereas most large prey are actively rejected, small prey may appear to be accidentally dropped and lost, leaving mid-sized prey selected by default. (3) The relative-stimulus model predicts that retention or rejection of prey depends on the strengths of tactile or olfactory stimuli from prey held in the chelae rel- ative to stimuli simultaneously contacted by the pereopods (Jubb et al. 1983). It assumes that simul- taneous contacts accumulate, so that several small prey present stronger stimuli than a single large item. Although explaining why theoretically optimally- sized prey are rejected, this model only applies when crabs are in contact with numerous prey at once, such as a clump of mussels or an oyster reef. (4) The mechanical-selection model assumes that crabs have difficulty manipulating small prey and breaking or opening large ones, such that lack of dexterity may cause accidental loss of small items, whereas the long time needed to break large prey may exceed the crab?s motivation to persist (Hughes and Seed 1981; Lawton and Hughes 1985; ap Rheinallt 1986). According to this model the crab would select for intermediate prey sizes. (5) The key stimulus model combines features of the mechanical selection and relative-stimulus models to propose that crabs adopt an opportunistic strategy of responding to the strongest stimuli from tactile or visual cues (Hughes and Seed 1995). When using tactile stimuli to for- age, crabs attack each sequential item irrespective of size, whereas prey encountered simultaneously may lead to size selection based on competing stimuli. In each case, the attack persists until the crab loses motivation, though damage to the prey acts as a reinforcing stimulus to persist longer (Hogan and Roper 1978; Abby-Kalio 1989; Seed and Hughes 1997). Many aspects of the five models are not mutually exclusive. Prey Sequestration and ?Kleptoparasitism? In laboratory settings, blue crabs sometimes hold prey for variable periods without eating it promptly (pers. obs.). Also in laboratory conditions, blue crabs sometimes steal food (kleptoparasitism) (Brockman and Barnard 1979) from other crabs, especially when crab densities are high (pers. obs.). However, it is not clear whether prey sequestration and kleptopara- sitism occur in field conditions. E COLOGY OF J UVENILE AND A DULT B LUE CRABS 607 Conditioning Modifies Prey Selection. Components of blue crab foraging can be condi- tioned adaptively with respect to the crabs? experi- ence and traits of both the environment and prey (Seed and Hughes 1997). Blue crabs may adjust their foraging behavior through learned improve- ment of handling efficiency (e.g., Cunningham 1983). This occurs, not necessarily by modifying technique, but by developing more directed sequences of standard components of attack behavior (Abby-Kalio 1989), which shortens han- dling time and increases the probability of success within a given persistence time (the time needed to successfully overcome and handle a prey item by repeated attack, e.g., to open a bivalve) (Hughes and Seed 1995). Persistence time is adjusted through experience to local prey quality and avail- ability for both sedentary prey (ribbed mussels) and mobile prey (fiddler crabs) (Hughes and Seed 1995; Seed and Hughes 1997). When offered a choice between two prey species (juvenile hard clams and juvenile eastern oysters) in the labora- tory, blue crabs modified their foraging based on previous experience (Micheli 1997a). The crabs consumed more clams than oysters when condi- tioned on a diet of clams or a mixed diet of clams and oysters than when conditioned on oysters alone; conversely crabs conditioned on oysters ate fewer oysters or clams when tested with single prey diets. Conditioning also significantly affects size selectivity of blue crabs, such that crabs ate larger clams when conditioned with greater pro- portions of larger clams before feeding trials, whereas they ate smaller clams when conditioned with smaller proportions of small clams (Micheli 1995). Physiological status of blue crabs, as well as envi- ronmental conditions and experience, can modify prey selection by blue crabs. Hunger can broaden their diet (Micheli 1995). Conditioning responses should be considered in experiments with blue crabs. Feeding test crabs with one type of prey, or with prey supplied in a density or other particular circumstance, before starting a trial may affect the outcome of the experiment (Terwin 1999). Selection of Prey Patches Although blue crabs and other predators clearly feed on individual prey, their foraging behavior typi- cally responds to prey resources with patchy distrib- utions (Clark et al. 2000). As with most predators, interactions of blue crabs with their patchily distrib- uted prey depend on densities of prey and predators, affecting prey persistence and blue crab distribution in the environment. Other characteristics of prey patch scale (e.g., patch size, distance apart) also affect predator-prey dynamics. Prey Density: Functional Response and Aggregative Response. Density of prey in a patch affects predator-prey dynamics through the preda- tor?s functional response, which describes the preda- tor?s per capita feeding rate as a function of prey density (Holling 1959; Hassell 1978). Blue crabs feeding on bivalves in the laboratory exhibit variable functional responses that differ by prey species and habitat type (e.g., sediment, dissolved oxygen condi- tions), with responses distinguished most readily by considering prey mortality as a proportional rate (percentage) (Lipcius and Hines 1986; Eggleston 1990a, c; Sponaugle and Lawton 1990; Eggleston et al. 1992; Dittel et al. 1995; Taylor and Eggleston 2000; Seitz et al. 2001a). Blue crabs foraging on soft clams (Fig. 12) exhibited an inversely density- dependent (Type II) functional response in mud, in that the percentage of prey consumed by blue crabs increased as prey density declined.When feeding on soft clams in sandy sediments, however, the crabs exhibited a sigmoidal density-dependent (Type III) functional response, such that proportional mortality of prey declined markedly at low densities (Lipcius and Hines 1986). By contrast, blue crabs feeding on Baltic macoma in the laboratory exhibited a sig- moidal density-dependent functional response in both sand and mud (Eggleston et al. 1992). This variation in functional responses implies that blue crabs are capable of driving localized patches of soft clams to extinction in mud, whereas the declining predation at low prey densities should help soft clams to persist in sand, and Baltic macoma to persist in both sand and mud. 608 T HE B LUE CRAB Experimental field tests of varying clam densi- ties in small patches in muddy and in sandy areas confirmed the extinction of soft clams in mud and persistence at low densities of soft clams in sand, and persistence of Baltic macoma in both mud and sand (Seitz et al. 2001a). Moreover, long-term population dynamics of these two prey species in Chesapeake Bay reflect these predicted patterns of localized per- sistence and extinction (Eggleston et al. 1992; Seitz et al. 2001a). The background densities of prey also regulate the foraging response of blue crabs, as den- sity-dependent mortality of Baltic macoma varied when deployed in small experimental patches placed into areas of differing natural clam densities where crab predation dominated (Kuhlmann and Hines 2005). Density dependence of blue crabs foraging on Baltic macoma was weaker in laboratory treat- ments with two crabs than with a single crab, and foraging time at low prey densities increased when two crabs were present (Kuhlmann and Hines 2005). Exposure of soft calms to moderately low oxygen levels also caused shifts in blue crab func- tional response as the bivalves moved up in the sedi- ment and became more vulnerable to crabs (Taylor and Eggleston 2000). The blue crab functional response while feeding on juvenile eastern oysters varied by sex, with females exhibiting a positively density-dependent (Type III) response and males having an inversely density-dependent (Type II) response (Eggleston 1990a, c). The differences in functional response were due to features of both predator (males have proportionately larger chela height than females) and prey (numbers of oyster spat on the accessible edges of cultch decline at lower density). These behavioral studies indicated that juvenile oysters gradually attain a partial prey refuge at low densities and large shell size before attaining an absolute prey refuge from all blue crabs at ~50 mm shell size. Cannibalistic large crabs foraging on juvenile crabs exhibited an inversely density-dependent functional response, with juveniles suffering increas- ing proportional mortality at low densities in the laboratory and in shallow near-shore waters (Dittel et al. 1995). This study used the density-dependent foraging pattern of the crab?s functional response to test for the effectiveness of shallow water as a refuge, showing the value of even a partial refuge in reduc- ing intense cannibalism. Blue crabs adjust their foraging rate in response to previous experience and conditioning by prey densities (Terwin 1999). When provided with Baltic macoma at low prey density in laboratory experi- ments, blue crabs conditioned at low clam densities foraged more effectively than those conditioned at high clam densities. The functional response of blue crabs (or other such predators) in the context of this shift in foraging efficiency depends on the rate of crabs? learning or conditioning relative to their rate of movement over patches of differing prey density (Terwin 1999). Predator aggregation as a function of density of prey patches is termed the aggregative response (Holling 1959; Hassell and May 1974; Hassell 1978). Blue crabs clearly aggregate on prey patches (Moody 1994; Clark et al. 1999a, b, 2000; Terwin 1999; Hines and Wolcott, unpubl. data), although the E COLOGY OF J UVENILE AND A DULT B LUE CRABS 609 0 4 8 12 16 20 24 28 32 36 Density (number of clams per tank) Sand Mud P ro po rti on al m or ta lit y 1.2 1.0 0.8 0.6 0.4 0.2 0 Figure 12. Functional responses of blue crabs foraging on varying densities of soft clams Mya arenaria . Forag- ing rate of crabs depends on prey density and sediment type, as indicated by proportional mortality rates of clams at six densities exposed to crab predation in two sediments in the laboratory. Blue crabs caused increas- ing proportional mortality of clams in mud and decreasing proportional mortality in sand. Means ? one standard error are indicated. From Lipcius and Hines (1986). quantitative function of the aggregative response to prey density is not well measured. Biotelemetry of location, feeding, and agonistic displays of blue crabs in Chesapeake Bay shows that blue crabs aggregate on patches of Baltic macoma prey, causing agonistic interactions among crabs to increase markedly at feeding times (Nye 1989;Wolcott and Hines 1989a; Clark et al. 1999a, b, 2000). If agonistic interactions increase sufficiently, blue crabs distribute themselves among prey patches, thus reducing agonistic interac- tions and increasing foraging efficiency (Clark et al. 2000), perhaps similar to an ?ideal free distribution? in which predators are distributed among patches in proportion to prey resources (Kennedy and Gray 1993). The ?numerical response,? or the population reproductive response to system-wide fluctuations in prey resources (Holling 1959), is not known to hold for blue crabs, due to the complexities of many interacting and confounding factors at such a large scale. Optimal Foraging on Prey Patches. In addi- tion to tests of feeding on individual prey, blue crab foraging on prey patches has also been studied with innovative tests of optimal foraging theory. When foraging on patchy prey, predators are predicted to optimize their energy intake by selecting the most rewarding patches in which to concentrate their for- aging efforts (Hassell and May 1974; Stephens and Krebs 1986). As prey are depleted within a patch, predators foraging optimally must depart to seek a new, more profitable prey patch, based on the preda- tor?s knowledge or experience about the environ- ment, ?rules of thumb? developed from their past experience, or both combined (Hassell and May 1974; Stephens and Krebs 1986). Predators using knowledge of their surrounding are predicted to abandon a prey patch when prey density falls below the overall mean density (the ?marginal value?) of the environment (Charnov 1976). Predators that do not have complete knowledge about prey resources are predicted to depart patches according to opti- mality rules based on the predator?s experience of the quality, variability, and distribution of prey patches (Alonso et al. 1995). Blue crabs equipped with biotelemetry tags pro- vided detailed information about foraging responses to infaunal prey patches (Nye 1989; Wolcott and Hines 1989a, 1996; Bell et al. 2003b). Adult crabs spent more time meandering slowly on patches of higher densities of prey, particularly infaunal Baltic macoma, where they exhibited significantly more feeding activity than where they moved rapidly between prey patches. The crabs typically departed from a prey patch after foraging for hours to a few days, well before the patch exhibited prey depletion. The crabs above may have left an undepleted prey patch as a result of agonistic interactions interfering with feeding (Clark et al. 1999a). In an experimental test of optimal foraging behavior on patchy prey resources, Clark et al. (1999a, 2000) conducted field experiments using blue crabs foraging on small patches of Baltic macoma within large (400 m2) enclosures in upper Chesapeake Bay. The field enclosures allowed experimental manipulation of the number of blue crabs and the number of clam patches, while telemetry recorded crab location, feeding, and threat displays. The density of blue crabs and the distribution of their prey interacted to affect the foraging behavior and success of the crabs. When only a single clam patch was available, blue crabs at high density interfered with each other?s foraging success, such that clam consumption decreased as agonistic encounters increased. How- ever, when clams were provided in two patches, blue crabs at high density dispersed between patches, thus reducing agonistic interactions and increasing forag- ing rate. The crabs reduced agonistic interactions disproportionately more than the 50% attributable to halving their densities on the patches, and quickly moved off a clam patch when another crab approached. Residence time of crabs on a prey patch was shorter when there were two patches than when there was a single patch, and the crabs divided their foraging time between patches and consuming prey from each patch at a similar rate. As a result, the crabs provided with two prey patches more than doubled their consumption of clams, which is con- sistent with optimizing behavior rather than more simple opportunistic response to increased prey availability. 610 T HE B LUE CRAB Response to Prey Patch Characteristics Other Than Density. Spatial pattern of predators foraging on prey patches is determined by three components of ecological scale: ?Grain? (patch size),?Lag? (distance between patches), and ?Extent? (distance the predator-prey interaction is mani- fested). Hines et al. (2003) evaluated these compo- nents for blue crabs foraging on Baltic macoma in Chesapeake Bay by using a large grid of benthic cores for estimating spatial variation in clam density, biotelemetry of crab movement and foraging activ- ity, and field experiments testing spatial effects of crab predation effects on clams. Geostatistics of the benthic prey grid showed that the Grain of dense clam patches averaged 200 m diameter, which was similar to what was revealed by biotelemetry data that showed crabs aggregated to feed on 140-m diameter clam patches (Nye 1989; Clark et al. 1999a; Hines and Wolcott, unpubl. data). As fighting among crabs increased with aggregation, the crabs dispersed to new clam patches at a Lag of 0.5 to 5 km. Clam out-planting experiments showed that Lag affected crab foraging efficiency (Terwin 1999; Hines et al., unpubl. data). A Lag distance of 7 m resulted in highest foraging success, apparently as odor plumes from clams dismantled by crab feeding facilitated aggregation of crabs to prey patches. Prey mortality decreased both at shorter Lags as crab ago- nism increased and at longer Lags as detection of prey odor plumes diminished (Clark et al. 1999b; Hines et al., unpubl. data). The Extent of the blue crab-clam interaction was 50 to 200 km, as deduced from crab stomach contents and similar results of predator exclusion experiments within Chesapeake Bay (Virnstein 1977; Holland et al. 1980; Hines et al. 1990). The Extent of the interaction is modified by addition of competing predator species at high salin- ity and in more tropical zones (Virnstein 1977, 1978). Predicting blue crab-prey dynamics requires consideration of interactive effects of all three com- ponents of scale (Hines et al. 2003). Habitat location of prey patches affects foraging rates of blue crabs (Leber 1985; Sponaugle and Law- ton 1990; Eggleston et al. 1992; Micheli 1996; Seitz et al. 2001). Effects of habitat may vary with season, as shown with field experiments contrasting forag- ing on experimental hard clams in adjoining inter- tidal salt marshes and sand flats in North Carolina (Micheli 1996, 1997a). In winter, crabs spent more time in salt marshes than sand flats, and predation rates were highest on clam patches within the edge of salt marshes, where the emergent vegetation pro- vided the crabs some protection from gull predators. In summer when avian predators were rare and abundance of other competing blue crabs increased, crab predation intensity was similar between the two habitats. Blue crabs selected clam patches depending on conditioning for habitat location of prey (Michelli 1997b). When tested with clams in equal experi- mental patches in differing habitats, crabs condi- tioned to feed on clams in salt marsh habitat ate more clams in salt marsh habitat than in sand flat habitat, whereas crabs conditioned to feed in sand flats ate approximately equal numbers of clams in both habitats. Patch preference persisted for more than 24 h between conditioning and testing. Food Webs for Blue Crabs Analysis of food web structure using stable iso- topes of carbon and nitrogen has provided insight into trophic levels and sources of the food as it passes through the feeding paths for juvenile blue crabs in Delaware Bay ecosystems (Fantle et al. 1999; Dittel et al. 2000). Early juveniles living in the bay habitat fed primarily on zooplankton, whereas marsh- dwelling crabs, which were enriched in 13C relative to bay juveniles, used carbon derived from marsh plants (Spartina alterniflora) . Nitrogen isotope data suggested that juvenile blue crabs in marsh habitat also fed on primary consumers, such as fiddler crabs (Uca spp.) or marsh periwinkles (Littoraria spp.) Two major food web models place blue crabs at a central position of Chesapeake Bay trophic dynamics. In the first, Baird and Ulanowicz (1989) considered blue crabs as the foremost benthic scav- enger or predator in Chesapeake Bay and developed a three-season analysis of carbon biomass and flows for a food web model (Fig. 13 depicts the annual carbon standing crop). In their model, average blue crab biomass carbon varied from 500 mg C m-2 in E COLOGY OF J UVENILE AND A DULT B LUE CRABS 611 summer to 300 mg C m-2 in fall and 100 mg C m-2 in winter. They calculated indirect as well as direct trophic dependencies for each of the 36 ?model compartments? of the food web, including blue crabs. Blue crabs were ranked at 27 out of a possible 36 in trophic hierarchy, and had an average annual effective trophic level of 3.51 (only behind the trophic levels of top predatory fishes, the highest of which was bluefish Pomatomus saltatrix at 4.59), with 71% of their trophic interactions occurring at Level IV, due to essentially no feeding in winter. At this high level, blue crabs contributed carbon only in small quantities to few trophic groups; however, car- bon in blue crabs was derived (directly and indi- rectly) from a varied ?extended diet? composed of lower trophic levels, including phytoplankton, sedi- ment bacteria, polychaetes, Macoma spp., and partic- ulate organic carbon (Table 2). The indirect carbon flows indicated that the blue crab is at the hub of carbon recycling and transfers for the benthic sub- system of the food web, and accordingly blue crabs were grouped into a ?Benthic Deposit Feeder? cate- gory that is dominant in a simplified food web for the Chesapeake mesohaline ecosystem. Although rates of carbon flow in the food web model varied greatly on a seasonal basis, the overall structure of the food web did not change much seasonally. A second model of the Chesapeake food web 612 T HE B LUE CRAB Figure 13. Complex schematic food web of annual carbon standing crops and flows for 36 principal components of the Chesapeake Bay?s mesohaline zone. Carbon standing crops are indicated within the compartments in mg m-2 and the indicated carbon flows are in mg m-2 y-1. Note that blue crabs (bottom center compartment) are major ben- thic consumers with a standing stock much larger than that of any of the fish species. From Baird and Ulanowicz (1989). that emphasizes fishery species also places blue crabs in a central position (NOAA Chesapeake Bay Pro- gram, H. Townsend, pers. comm. 2004) (Fig. 14). This food web is constructed using ?EcoPath with EcoSim? modeling software. The portion of the food web model that includes the species and trophic groups with direct interactions with adult and young-of-the-year blue crabs (Fig. 14) shows hard clams, soft clams, and eastern oysters as prey species with commercial importance. It lumps all other prey species as ?other infauna and epifauna? and ?other suspension feeders,? and includes ?ben- thic algae? as a food source. This food web illustrates predation by adult blue crabs on juveniles (young- of-the-year), as well as by Atlantic croaker, migratory and resident striped bass M orone saxatilis , sandbar shark Carcharhinus plumbeus , American eel, ?littoral forage fish,? and ?piscivorous birds.? COMPETITION Competitive interactions can be difficult to prove because they require demonstration that resources (e.g., food, refuge habitat) both are used simultaneously and are limiting to at least one of the users (i.e., result in displacement or reduced growth, reproduction, or survivorship). Definitive experi- mental analysis of interspecific competition for blue crabs is limited and some experiments only provide indirect indications of competition. For example, Bisker et al. (1989) reported that oyster toadfish reduced blue crab and xanthid crab predation on juvenile hard clams in field cultures, suggesting that blue crabs and xanthids competed for bivalve prey. Nevertheless, interspecific competition in blue crabs may be inferred from niche overlap and resource partitioning among other species, especially E COLOGY OF J UVENILE AND A DULT B LUE CRABS 613 Figure 14. Food web of Chesapeake Bay developed using ?EcoPath with EcoSim? modeling software, showing the model segment that focuses on trophic interactions associated with blue crabs. SAV = submerged aquatic vegetation. From NOAA Chesapeake Bay Program. if careful attention is given to small-scale spatial and temporal comparisons that are important in niche differentiation. In upper Chesapeake Bay, blue crabs and epibenthic fishes (particularly spot, Atlantic croaker, and hogchoker) comprise a guild of preda- tors that forage on soft-bottom invertebrates during warm months (Hines et al. 1990) (Fig. 9). Diet of large blue crabs exhibited high overlap (54% to 61%) compared with other members of the guild combined, especially in early summer when most species fed on polychaetes and amphipods that were abundant at the sediment surface (Hines et al. 1990). However, diet overlap with individual fish species was only 4% to 36%, depending on type of substrate and time of season (Hines et al. 1990). In other habitats, interspecific competition with blue crabs can be deduced from observations of species with similar habitat use and feeding modes of digging infaunal bivalve prey. Such probable competitors include horseshoe crabs (Woodin 1981) and possibly some benthic-feeding rays and sharks, such as cow- nose rays (Orth 1977). In oyster and mussel aggre- gations, blue crabs and xanthid crabs may compete for food resources such as oyster spat and mussels (Galtsoff 1964; Seed 1980; Eggleston 1990a, b). Blue crabs exhibited considerable spatial, dietary, and sometimes temporal overlap with lady crab Ovalipes ocellatu s and Atlantic rock crab Cancer irroratus in the Hudson-Raritan estuary (Stehlik et al. 2004). Blue crabs and lady crabs exhibited the greatest overlap in summer. In the same study, xanthid crabs formed the major diet component of blue crabs (20% by volume), indicating that blue crabs out-compete mud crabs by preying on them (Stehlik et al. 2004). Clearly, other species of Callinectes may exhibit the greatest potential for competitive interactions, although comparative studies remain limited. Call- inectes similis and C. sapidus had high dietary overlap and similar habitat use in Mobile Bay, Alabama, although these resources were partitioned partially by size of crabs (Hsueh et al. 1992a, b). Similarly, C. sapidus, C. similis, and C. ornatus often overlap in habitat use and diet (with a major bivalve compo- nent) in lagoon systems in Florida, Caribbean islands, and Bermuda (Haefner 1990a; Stoner and Buchanan 1990; pers. obs.). In Caribbean estuaries, several species of Callinectes appear to have strong potential for competitive interactions and partition the habitat along the salinity gradient based on adult size and aggressive behavior in combination with tolerance of low salinities (Norse 1975, 1977, 1978a, b; Norse and Estevez 1977; Norse and Fox- Norse 1982; Buchanan and Stoner 1988). Callinectes toxotes and C. arcuatus also interact aggressively and partition the salinity zone of Pacific coast estuaries 614 T HE B LUE CRAB Table 2. Extended diet of blue crabs in Chesapeake Bay (Baird and Ulanowicz 1989). Because the carbon flow of the diet item passes through several compart- ments of the food web (see Fig. 13) before it is con- sumed by blue crabs (or other consumers), the sum of the diet derivation exceeds 100%. Diet Derivation (Direct and Trophic Category Indirect) Percent Phytoplankton 35.2 Bacteria attached to suspended particles 3.2 Sediment bacteria 79.5 Benthic algae 9.1 Free bacteria in water column 6.7 Heterotrophic microflagellates 5.4 Microzooplankton 8.2 Zooplankton 4.9 Ctenophores 1.3 Sea nettles 0.1 Other suspension feeders 7.2 Mya arenaria 2.9 Oysters 0.3 Other polychaetes 22.5 Nereis spp. 8.8 Macoma spp. 56.6 Meiofauna 6.3 Crustacean deposit feeders 12.4 Blue crab 3.8 Bay anchovy 0.3 Dissolved organic carbon 6.7 Suspended particulate carbon 67.4 Sediment particulate organic carbon 90.9 Sum 439.7 (Norse 1978a, b).The outcome of interspecific ago- nism also depends on body size, such that the species in lower salinities are larger than the high salinity species (Norse 1978b). However, for C. sapidus , intraspecific habitat partitioning by size is a bit more complex, with depth zonation as well as salinity zonation (Hines and Ruiz 1995). (Also see Interspe- cific Variation in Diet above.) In contrast to the ambiguities of interspecific competition, intraspecific competition seems evi- dent for blue crabs, despite the obviously extensive and fine-grained partitioning of resources by life stages throughout their life cycle. Cannibalism upon juveniles and molting crabs by larger inter- molt blue crabs is one important form of intraspe- cific competition (Ruiz et al. 1993; Dittel et al. 1995; Hines and Ruiz 1995). Laboratory experi- ments indicate agonistic interference competition among large crabs that reduces their foraging for clams (Mansour and Lipcius 1991; Moody 1994, 2001, 2003). Density-dependent foraging rates on Baltic macoma in the laboratory differed between treatments with one versus two crabs, and the pres- ence of a second crab increased foraging time at low prey densities (Kuhlmann and Hines 2005). Aggressive interactions among competing large crabs also affected their foraging rates on bivalve prey in r igorous field exper iments using biotelemetry in upper Chesapeake Bay (Clark et al. 1999a, b, 2000) (Fig. 15). These studies indicated that crabs foraging on small (0.25 m2) experimen- tal patches of clams were able to detect and respond to other crabs as potential aggressive com- petitors as far as 5 m away. In large (400 m2) field enclosures, the frequency of a crab?s meral-spread threat displays and movement increased signifi- cantly and the per capita foraging rate decreased significantly and disproportionately with increasing blue crab density. Furthermore, distributing prey between two patches instead of one resulted in dis- proportionately decreased agonistic interactions and increased clam consumption. These experi- ments all indicate clear intraspecific competition for bivalve prey resources. (See also the next section.) PREDATORS, CANNIBALISM, AND AGONISTIC INTERACTION Just as blue crabs are important predators in estuarine food webs, they are subject to predation by many estuarine species, as well as to intense canni- balism. Inter- and intra-specific predators of blue crabs change during the crabs? ontogeny, reflecting shifts in vulnerability to predators and to agonistic interactions as functions of the crabs? size and molt stage, and of their habitat use. Inter-Specific Predation upon Blue Crabs A diverse array of at least 101 species have been doc- umented to prey upon blue crabs, including fishes, reptiles, birds, small mammals, some invertebrates, and blue crabs themselves (Table 3). The effect of predation appears to vary greatly among predators (Guillory and Elliot 2001), but there have been sur- prisingly few quantitative comparisons. Although several species of invertebrates may eat blue crabs (Table 3), there are almost no quantitative estimates of this predation. Auster and DeGoursey (1994) reported that up to 81% of torpid blue crabs were preyed upon by the seastar Asterias forbesi as winter water temperatures dropped below 5?C in Connecticut. Interspecific predation among species of Callinectes may be important in regulating the dis- tribution of blue crabs zoned along the salinity gra- dient of Caribbean estuaries (Norse 1975, 1977, 1978a, b; Norse and Estevez 1977; Norse and Fox- Norse 1982; Buchanan and Stoner 1988; Haefner 1990a; Stoner and Buchanan 1990). However, effects of interspecific invertebrate predators other than Callinectes spp. seem to be small. Consumption of blue crabs by reptiles is poorly quantified, and probably does not produce major population effects. However, alligators Alligator mississippiensis may feed heavily on blue crabs in some locations (Valentine et al. 1972), and Kemp?s Ridley sea turtles Lepidochelys kempii feed exclu- sively on blue crabs in lower Chesapeake Bay (Van Engel 1987). Sub-adult loggerhead sea turtles E COLOGY OF J UVENILE AND A DULT B LUE CRABS 615 616 Figure 15. Competitive interference among foraging blue crabs: effect of prey patch distribution and crab density on foraging rate and agonistic interactions of blue crabs in large experimental enclosures. (A) Design of 20 m x 20 m fenced enclosure with a grid of bin sites for small (0.25 m2) patches of clams Macoma balthica , allowing deployment of one or two clam patches as prey for two, four, or eight large male crabs introduced into the enclosure. One of the experimental crabs was fitted with a dual channel biotelemetry tag (see Figs. 5B, 6B), allowing its movement, feed- ing, and threat displays to be tracked within the enclosure. An example of one track is shown. (B) Per capita con- sumption of clams declined with increasing crab density. (C) Clam consumption increased at low crab density and especially when clams were provided in two prey patches. (D) Agonism (threat displays) increased markedly when clams were provided in single prey patches. (E) Threat displays also increased markedly at high crab density. From Clark et al. (1999a, 2000). C Low crab density High crab density C la m s co ns um ed p er c ra b 30 25 20 15 10 5 0 Single prey patch Two prey patches D C um ul at iv e th re at d is pl ay s (s ec ) 1600 1400 1200 1000 800 600 400 200 0 Single prey patch Two prey patches Observed Expected B 0 5 10 15 2 4 8 Crab density C la m s ea te n pe r c ra b (m ea n + S E) E O cc ur en ce o f t hr ea t d is pl ay s (% o f o bs er ve d tim e) , m ea n 6 5 4 3 2 1 0 Low crab density High crab density Single prey patch Two prey patches 617 Table 3. Predator species of juvenile and adult blue crabs. Predator Species References Crustacea Callinectes sapidus (blue crab) Darnell (1958), Heck & Spitzer (2001); Hines & Ruiz (1995) Hovel & Lipcius (2001), Laughlin (1982), Moksnes et al. (1997), Moody (2001, 2003), Peery (1989), Ryer et al. (1997) Crangon septemspinosa (sand shrimp) Olmi & Lipcius (1991) Menippe adina (western gulf stone crab) Powell & Gunter (1968) Mithrax spinosissimus (Caribbean king crab) Winfree & Weinstein (1989) Palaemonetes pugio (grass shrimp) Olmi & Lipcius (1991) Echinodermata Asterias forbesi (starfish) Auster & DeGoursey (1994) Pisces Albula vulpes (bonefish) Bruger (1974) Ancylopsetta quadrocellata (ocellated flounder) Stickney et al. (1975) Anguilla rostrata (American eel) Shirley et al. (1990), Wenner & Musick (1975) Aplodinotus grunniens (freshwater drum) Darnell (1958) Archosargus probatocephalus (sheepshead) Darnell (1958), Fontenot & Rogillio (1970) in Guillory & Elliot (2001), Gunter (1945), Overstreet & Heard (1982), Overstreet (unpubl.) in Guillory & Elliot (2001) Arius bonillai (new granada sea catfish) Norse (1975) Arius felis (hardhead catfish) Darnell (1958) Bagre marinus (gafftopsail catfish) Gunter (1945) Bairdiella chrysoura (silver perch) Brooks et al. (1982), Darnell (1958), Thomas (1971) Brevoortia tyrannus (Atlantic menaden) McHugh (1967) Caranx hippos (crevalle jack) Overstreet (unpubl.) and Heard (unpubl.) in Steele & Perry (1990) Carcharhinus leucas (bull shark) Darnell (1958), Heard (unpubl.) in Steele & Perry (1990, Sadowsky (1971) Carcharhinus obscurus (dusky shark) Kemp (1949) in Guillory & Elliot (2001) Carcharhinus plumbeus (sandbar shark) Ellis (2003), Medved et al. (1985), Medved & Marshall (1981) Centropristis philadelphica (rock sea bass) Brooks et al. (1982) Centropristis striatus (black sea bass) Brooks et al. (1982) Citharichthys spilopterus (bay whiff) Stickney et al. (1975) Cynoscion arenarius (sand seatrout) Krasprzak and Guillory (1984), Overstreet (unpubl.) in Steele & Perry (1990), Overstreet & Heard (1982) Cynoscion nebulosus (spotted seatrout) Darnell (1958), Gunter (1945), Overstreet & Heard (1982), Overstreet (unpubl.) in Steele & Perry (1990) Cynoscion regalis (weakfish) Brooks et al. (1982), Lascara (1981), Merriner (1975), Thomas (1971) Dasyatis americanus (southern stingray) Dahlberg & Heard (1969) Dasyatis centroura (roughtail stingray) Hess (1961) Dasyatis sabina (Atlantic stingray) Darnell (1958) Dasyatis sayi (bluntnose stingray) Heard (unpubl.) in Steele & Perry (1990), Hess (1961) Elops saurus (ladyfish) Austin & Austin (1971) Epinephelus itajara (goliath gouper) Kemp (1949) in Guillory & Elliot (2001), Pew (1954) Fundulus diaphanus (banded killifish) Rogers (1982) 618 Table 3. continued. Predator Species References Pisces, continued Fundulus grandis (gulf killifish) Levine (1980) in Guillory & Elliot (2001) Galeocerdo cuvier (tiger shark) Kemp (1949) in Guillory & Elliot (2001) Ictalurus catus (white catfish) Heard (1973) in Guillory & Elliot (2001), Van Engel & Joseph (1968) in Guillory et al. (2001) Ictalurus furcatus (blue catfish) Darnell (1958), Lambou (1961) Ictalurus punctatus (channel catfish) Menzel (1943) Lagodon rhomboides (pinfish) Darnell (1958) Leiostomus xanthurus (spot) Brooks et al. (1982), Levine (1980) in Guillory & Elliot (2001) Lepisosteus oculatus (spotted gar) Darnell (1958), Goodyear (1967), Lambou (1961) Lepisosteus osseus (longnose gar) Stuttkus (1963) in Guillory & Elliot (2001) Lepisosteus spatula (alligator gar) Darnell (1958), Lambou (1961) Lobotes surinamensis (tripletail) Gunter (1945) Lutjanus apodus (schoolmaster) Austin & Austin (1971) Lutjanus campechanus (red snapper) Felder (1971) Lutjanus griseus (gray snapper) Starck (1971) Megalops atlanticus (tarpon) Hildebrand (1963) in Guillory & Elliot (2001) Menidia beryllina (inland silverside) Levine (1980) in Guillory (2001) Micropogonias undulatus (Atlantic croaker) Darnell (1958), Fontenot & Rogillio (1970) in Guillory & Elliot (2001), Merriner (1975), Orth et al. (1999), Overstreet & Heard (1978a), Stickney et al. (1975), Thomas (1971) in Guillory et al. (2001), van Montfrans (unpubl.) in Dybas (2002) Micropterus salmoides (largemouth bass) Darnell (1958), Lambou (1961) Morone americana (white perch) Brooks et al. (1982), Lambou (1961) Morone mississippiensis (aka M. interrupta ) Darnell (1958) (yellow bass) Morone saxatilis (striped bass/rockfish) Austin (1993), Darnell (1958), Manooch (1973), Orth et al. (1999), Truitt & Vladykov (1937), Tupper & Able (2000), van Montfrans (unpubl.) in Dybas (2002) Mustelus canis (smooth dogfish) Bigelow & Schroeder (1953) Opsanus beta (gulf toadfish) Heard (unpubl.) in Steele & Perry (1990) Opsanus tau (oyster toadfish) Abbe & Breitburg (1992), Bisker et al. (1989), Schwartz & Dutcher (1963) Paralichthys albigutta (gulf flounder) Stokes (1977) in Guillory & Elliot (2001) Paralichthys dentatus (summer flounder) Manderson et al. (2000), Moody (1994, 2001, 2003) Paralichthys lethostigma (southern flounder) Darnell (1958), Overstreet (unpubl.) in Steele & Perry (1990) Pogonias cromis (black drum) Fontenot & Rogillio (1970) in Guillory & Elliot (2001), Gunter (1945), Overstreet (unpubl.) in Steele & Perry (1990), Overstreet & Heard (1982), Van Engel & Joseph (1968) in Guillory et al. (2001) Pomatomus saltatrix (bluefish) Brooks et al. (1982), Lascara (1981) Prionitus tribulus (bighead searobin) Diener et al. (1974) Rachycentrum canadum (cobia) Arendt et al. (2001), Meyer & Franks (1996), Overstreet (unpubl.) in Steele & Perry (1990) Raja eglanteria (clearnose skate) Hildebrand & Schroeder (1928) Sciaenops ocellatus (red drum) Bass & Avault (1975), Boothby & Avault (1971), Darnell (1958), Fontenot & Rogillio (1970) in Guillory & Elliot (2001), Guillory & 619 Table 3. continued. Predator Species References Pisces, continued Prejean (2001), Gunter (1945), Orth et al. (1999), Overstreet & Heard (1978b), Scharf & Schlicht (2000), Simmons (1957) in Guillory et al. (2001), van Montfrans (unpubl.) in Dybas (2002) Scomberomorus cavalla (king mackerel) Hovel & Lipcius (2001) Sphoeroides maculatus (northern puffer) Moody (1994, 2001, 2003), Hovel & Lipcius (2001) Sphoeroides nephelus (southern puffer) Reid (1954) Sphyrna tiburo (bonnethead) Gunter (1945), Hoese & Moore (1958), Woodbury (1986) Strongylura marina (Atlantic needlefish) Brooks et al. (1982) in Guillory & Elliot (2001) Sygnathus fuscus (pipefish) Ryer (1988) Tautoga onitis (tautog) Moody (1994, 2003) Tylosurus acus (agujon) Brooks et al. (1982) in Guillory and Elliot (2001) Urophycis regius (spotted hake) Sikora & Heard (1972) Reptilia Alligator mississippiensis (American alligator) Valentine et al. (1972) Caretta caretta (loggerhead sea turtle) Van Engel (1987) Lepidochelys kempii (Atlantic or Kemp?s Ridley) Van Engel (1987) Aves Ardea alba (=Casmerodius albus ) (great egret) Bailey (1971) in Guillory & Elliot (2001) Ardea herodias (great blue heron) Steele & Perry (1990), Wolcott (pers. comm.) in Micheli (1997b); Hines (unpubl. obs.) Egretta (=Florida) caerulea (little blue heron) Rogers (1982) Eudocimus albus (=Guana alba) (white ibis) Bildstein (1993), Hammat (1981) Grus americana (whooping crane) Hedgpeth (1950), Lewis (1995) Larus atricilla (laughing gull) Barass & Kitting (1982), Bass & Avault (1975) Larus argentatus (herring gull) Micheli (1997b), Prescott (1990) Larus delawarensis (ring-billed gull) Micheli (1997b), Prescott (1990) Larus spp. (gulls) Day et al. (1973) in Guillory & Elliot (2001) Lophodytes cucullatus (hooded merganser) Steele & Perry (1990) Mergus merganser americanus Steele & Perry (1990), Stieglitz (1966) (common merganser) Nycticorax nycticorax (black-crowned night heron) Allen (1938) Nyctanassa violacea (yellow-crowned night-heron) Watts (1995) Rallus longirostris (clapper rail) Bateman (1965), Steele & Perry (1990) Somateria mollissima (common eider) Burnett & Snyder (1954) Sterna spp. (terns) Barass & Kitting (1982), Micheli (1997b) Mammalia Canis rufus (red wolf) Guillory & Elliot (2001) Lutra canadensis (river otter) Chabreck et al. (1982) Procyon loto r (raccoon) Steele & Perry (1990) 620 T HE B LUE CRAB Caretta caretta also feed on blue crabs (Van Engel 1987). Among mammals, racoons Procyon lotor and per- haps river otters Lutra canadensis and red wolf Canis rufus are capable of eating blue crabs opportunisti- cally along estuarine shorelines and marshes (Table 3). Like birds, their effects are probably low as a result of their relatively low densities. Blue crabs are an important component of the diet of at least 15 species of birds, plus two groups of birds in the genera of gulls (Larus spp.) and terns (Sterna spp.) (Table 3).The bird species include the great blue heron Ardea herodias (Steele and Perry 1990; Hines, unpubl. obs.) and the endangered whooping crane Grus americana (Hedgpeth 1950; Lewis 1995). For whooping cranes, habitat quality and food availability are very dependent on the availability of blue crabs in the birds? over-wintering habitats along the coastal marshes of the Gulf of Mexico (Lewis 1995). Some species of wading birds were very numerous historically, and may have imposed important effects on crab abundance; how- ever, recent densities of these avian predators are generally low, so that population effects on blue crabs are probably not large. Other birds (e.g., gulls, rails, mergansers) appear to take blue crabs oppor- tunistically in very shallow water. Seasonal variation in risk of predation by terns and gulls in North Car- olina affected foraging behavior and effects of blue crabs on sand flats and salt marshes, although experi- mental crabs were not consumed by the avian predators (Micheli 1997b). Predation upon juvenile or adult blue crabs has been documented for 70 species of fish (Table 3) and appears to be significant from the perspective of providing important food resources to many nearshore and estuarine species. Although some of these are records of incidental consumption of blue crabs, blue crabs serve as important prey for several sport and commercial fish species, including several sciaenids (red drum Sciaenops ocellatus , Atlantic croaker, black drum Pogonias cromis , and spotted sea trout Cynoscion nebulosus ); sheepshead Archosargus probatocephalus ; bass (striped bass or rockfish, yellow- bass Morone interrupta , largemouth bass Micropterus salmoides ); flatfish (southern flounder Paralichthys lethostigma ); cobia Rachycentron canadum ; American eel; and blue catfish Ictalurus furcatus . For cobia in Chesapeake Bay, blue crabs comprised 57%-59% of the diet (Arendt et al. 2001). Along the Gulf Coast, red drum appear to be the important piscine predator, with an average frequency of occurrence of blue crabs in their stomachs of 32% (range: 2%- 62%) and a predation index (predator diet weighted by predator abundance) that was 4.8 times greater than the next highest ranked species (Guillory and Elliot 2001; Guillory and Prejean 2001). Other predators in the Gulf of Mexico with diets that were relatively high in blue crab contents included hard- head catfish Arius felis (23%), black drum (7%), sheepshead (7%), gafftopsail catfish Bagre marinus (7%) and spotted sea trout (5%). It remains unquan- tified and unclear, however, whether this predation has important effects on blue crab populations along the Gulf Coast. Cannibalism Cannibalism by large crabs attacking small crabs, and by hard, intermolt crabs attacking soft molting crabs is a major source of mortality for blue crabs, as mentioned earlier. Analysis of stomach contents shows that crabs comprise significant portions of the diet of large blue crabs (Laughlin 1982; Hines et al. 1990; Mansour 1992). Laboratory experiments pro- vide indications of effects of size, density, and habitat on cannibalism (e.g., Peery 1989; Mansour and Lip- cius 1991). Much of the quantitative evidence for cannibalism comes from use of tethering techniques in which crabs are fitted with fishing leader tied or glued to their dorsal carapace and then staked out in particular habitats in the field (Heck and Thoman 1981;Wilson et al. 1987, 1990a, b; Ruiz et al. 1993; Dittel et al. 1995; Hines and Ruiz 1995; Pile et al. 1996). Tethered crabs are free to move within the radius of their tether, and they are checked periodi- cally for injury and survival. Although tethering can cause artifacts and biases about sources of mortality and altered behaviors (Barshaw and Able 1990; Peterson and Black 1994; Zimmer-Faust et al. 1994; Smith 1995), these problems do not appear to be serious for blue crabs as a relative measure of preda- tion rate, especially where cannibalism is the major E COLOGY OF J UVENILE AND A DULT B LUE CRABS 621 single source of mortality (Hines and Ruiz 1995). Analysis of the damaged remains of tethered inter- molt crabs and of predators caught on tethered crabs indicates that cannibalism rates are high and account for 75 to 97% of mortality in juvenile blue crabs in some estuarine habitats of Chesapeake Bay (Hines and Ruiz 1995). Molting blue crabs also suffer high mortality rates, often attributable to cannibalism, but experimental artifacts of tethering techniques may be more pronounced, because molting crabs must be held in mesh bags to secure them (Shirley et al. 1990; Ryer et al. 1997). Juvenile intermolt blue crabs tethered in non- vegetated habitats suffered high mortality rates. Rates include 40 to 90% of 30 to 70 mm crabs being killed per day mostly by cannibalism in water >70 cm deep in a central Chesapeake subestuary during summer (Hines and Ruiz 1995); 11 to 45% for 10 to 60 mm crabs in New Jersey (Wilson et al. 1987, 1990a, b; Heck and Coen 1995); 15 to 40% for 12 to 64 mm crabs in New Jersey (Wilson et al. 1990a, b); 25% for 20 mm crabs in lower Chesa- peake Bay (Pile et al. 1996); 14 to 86% for 18 to 25 mm crabs in Florida (Heck and Thoman 1981), and 85 to 91% for 5 to 20 mm crabs in Alabama (Heck and Coen 1995). Tethered juvenile blue crabs suf- fered higher rates of predation by adult blue crabs than did juvenile lady crabs because the blue crabs did not bury as deeply in sediments as did lady crabs (Barshaw and Able 1990). Relative Effects of Cannibalism versus Inter-Specific Predation on Blue Crabs Instances of predation by fish markedly reducing blue crab abundance have been quantified only in particular habitats and primarily for early post-settle- ment juveniles <10 mm CW. In seagrass habitats along the Gulf of Mexico, very intense predation on early post-settlement juvenile blue crabs (<10 mm CW) by pinfish Lagodon rhomboides , pipefish (Syg- nathus spp.), and numerous other species rapidly lim- its the abundance of juveniles entering these nursery habitats (Heck and Coen 1995; Morgan et al. 1996; Heck et al. 2001; Spitzer et al. 2003). In seagrass habitats of Mobile Bay, Alabama, for example, episodic settlement events did not result in increased juvenile abundance, because cohorts of new recruits were consumed in less than 14 d after settlement, with predation causing as much as 95% mortality d-1, such that juvenile densities rapidly returned to pre-settlement ?background? levels (Heck et al. 2001). In video-taped tethering experiments in lower Chesapeake Bay (Moody 1994, 2003), large blue crabs and several fish species (spot,Atlantic croaker, summer flounder Paralichthys dentatus , northern puffer Sphoeroides maculatus , hogchoker, and tautog Tautoga onitis ) made attempts to attack juvenile crabs (10-30 mm CW). Although 84% of attacks by summer flounder and 100% by blue crabs were successful, only 9% of attacks by spot resulted in effective predation. Northern puffers also can be effective predators on blue crabs by foraging syner- gistically in ?gangs.? Moody?s (1994, 2003) under- water remote videotaping of tethered crabs in the field and laboratory experiments showed that indi- vidual puffers distracted adult crabs from the ante- rior while others attacked from the side or rear to remove legs and then consume the crab body. Attacks by other species in his video records were not successful. In Chesapeake Bay there is a major controversy about the effect of fish predators in regulating blue crab populations because the abundances of striped bass and perhaps some sciaenid species increased markedly while blue crabs declined coincidentally during the 1990s. This correlation could indicate that fish predation contributed significantly to the decline in blue crab abundance. On the other hand, blue crabs and fish apparently coexisted in abun- dance before human populations were a factor, and fishery data indicate that much of the decline in blue crabs can be attributed to intense fishing pressure, with up to 70% of the legal population of Chesa- peake blue crabs caught each year by humans (Miller and Houde 1998; Sharov et al. 2003). Nevertheless, evidence for fish predation on blue crabs merits careful consideration. Stomach contents of fish often include blue crabs, but the percentage that blue crabs comprise of fish stomach contents is usually low and only occasionally high (Guillory and Elliot 2001; Guillory and Prejean 2001). Although anecdotal reports from fishermen note instances of striped bass stomachs full of small blue crabs (pers. obs.), Austin and Walter (1998) found that when they analyzed larger (>45 cm) striped bass (N= 2009 fish) collected from fish pro- cessing houses and by a variety of fishery-indepen- dent sampling gear, blue crabs occurred in only 9.4% of fish with stomach contents. The crabs in these stomach contents averaged 41 mm CW (range 11-150 mm). Two detailed studies of blue crab consumption by fishes in lower Chesapeake Bay seagrass beds indicate that juvenile blue crabs may be abundant in stomachs of some species, but predation by these fishes probably has relatively small effects on juvenile blue crab populations. In one study, abundance and stomach contents of fish were measured simultane- ously over two 12-h periods to estimate predator effect (Orth et al. 1999; van Montfrans cited in Dybas 2002). Gut analysis showed that 100% of red drum had eaten an average of 4.5 juvenile blue crabs, whereas 60.5% of striped bass contained 2.3 crabs and 35.7% of croaker consumed 1.4 crabs on average. The size of blue crabs in the fish stomachs averaged 23 mm for striped bass and 22 mm for red drum. Of the remaining striped bass and croaker that did not consume blue crabs, 14.5% and 39.3%, respectively, had empty stomachs, and 13.2% and 25%, respectively, contained only food items other than crabs. By integrating diet and fish abundance with seagrass aerial cover, predation effects were esti- mated for the 1.6 billion blue crabs within this habi- tat as 7.35 x 107 crabs (4.6%) by striped bass, 3.06 x 105 crabs (0.42%) for croaker, and 1.20 x 104 crabs (0.017%) by red drum. However, only one sampling period captured these large numbers of fish feeding upon blue crabs, whereas other sampling did not capture fish. Thus, in combination, fish predators could be estimated as consuming as much as 5% of the local blue crab population per 12 h period, but when averaged over several sampling periods, this percentage would be much lower. In a second, sim- ilar study (van Montfrans et al. 2005), overall fish predation on crabs was low, with striped bass having the greatest level of consumption (2.12-3.39% of total crabs available during spring and fall, respec- tively), followed by Atlantic croaker (0.33% and 0.13% of spring and fall totals, respectively), red drum (<0.00% and 0.13%, respectively) and weak- fish (0.18% of fall total only). Thus, although it is clear that blue crabs may be important in the diet of some fish and other vertebrates, and that some species are effective predators on blue crabs, there are few quantitative data demonstrating rigorously that predation by fish or other vertebrates directly regu- lates blue crab populations at life stages >20 mm CW. In contrast, in a subestuary of central Chesa- peake Bay, tethering experiments over a 16-y period (1989-2005) recorded no instances of fish predation on tethered blue crabs, whereas more than 92% of the mortality was attributed to cannibalism (Ruiz et al. 1993; Dittel et al. 1995; Hines and Ruiz 1995; Hines, unpubl. data). The long-term decline in tether mortality during the decline of crab abun- dance and the increase in abundance of potential fish predators like striped bass is consistent with the hypothesis that large blue crabs are important preda- tors whereas striped bass are not. Otherwise, increases in the fish predators would have produced higher mortality of juvenile crabs, and tethering studies would have detected frequent incidences of successful predation by fish. Although fish predators attacked juvenile crabs with fractional success at sites in the lower Bay, 100% of video-taped attacks by cannibalistic C. sapidus were successful (Moody 1994, 2003). Further, in several habitats (seagrass, salt marsh creek) of lower Chesapeake Bay, cannibalism was the only identifiable source of mortality in experimentally tethered crabs (Ryer et al. 1997), and cannibalized blue crabs in stomach contents of large crabs (>134 mm) in Virginia tributaries reached 45% (Mansour 1992). Substantial spatial and temporal variation in consumption of blue crabs results from opportunis- tic foraging by some of the key predatory fishes, as well as by blue crabs themselves (see above section on Diet). For example, the diet of striped bass var- ied spatially within tidal creeks of New Jersey salt marshes, such that blue crabs comprised a major portion of fish stomach contents, with crabs forming as much as 50% occurrence and 60% of weight of 622 T HE B LUE CRAB stomachs in one creek and zero occurrence in another creek (Tupper and Able 2000). In upper Chesapeake Bay, the diet of large blue crabs shifts over the summer from only trace occurrence of small crabs in crab stomach contents early in the sea- son to 15%-20% of stomach contents by September when infaunal prey have been depleted (Hines et al. 1990). Thus, both fish predation and crab cannibal- ism appear to be important sources of mortality for blue crabs. The scale of studies on predator effects has yet to be expanded. Cannibalism by large blue crabs upon smaller crabs and molting crabs appears to be a major source of mortality that is common through- out most estuarine habitats, which often shapes the distribution of blue crabs among microhabitats by size, sex, and molt stage (see below). Of course, human effects also may be very large, affecting abun- dance, population size structure, sex ratio, and habitat distribution (see also Fogarty and Lipcius, Chapter 16 on population dynamics). Vulnerability by Size, Molt Stage, Sex, and Morphological Variation Vulnerability of blue crabs to predation varies greatly by size and molt stage. Generally, small crabs are subject to a wider range of predators and preda- tor effects, and several studies have shown that crab vulnerability to predation decreases markedly with increasing size (Moody 1994, 2001, 2003; Dittel et al. 1995; Hines and Ruiz 1995). Tethering experi- ments and other studies in seagrass habitats along the Gulf of Mexico indicate very intense predation (averaging 85% mortality d-1 across sites, and with many sites >95% mortality d-1) on early post-settle- ment juvenile blue crabs (5-20 mm CW) (Heck et al. 2001). In seagrass habitats of Mobile Bay, Alabama, for example, episodic settlement events did not result in increased juvenile abundance because cohorts of new recruits were consumed in <14 d after settlement, with predation causing as much as 95% mortality d-1, such that juvenile densities rapidly returned to pre-settlement ?background? levels (Heck et al. 2001). Tethering experiments in central Chesapeake Bay show that vulnerability to cannibalism decreases with size, allowing large crabs to move into deeper water with relative impunity compared to small crabs, which tend to be restricted to shallow (<40 cm deep) water along the shoreline (Terwin 1999; Hines, unpubl. data). These conclu- sions are supported by telemetry studies that con- trasted movement of juveniles and adults in Chesa- peake Bay: juveniles spent much more time in shallows (<1 m deep) along the shoreline whereas adults moved mainly in deeper channels (Hines et al. 1995). Blue crabs are especially vulnerable at the time of ecdysis when they are soft and relatively immo- bile. Soft, post-molt crabs are often used for bait in sport fisheries, and soft crabs have been documented in the stomach contents of several fish species, including striped bass (Orth et al. 1999) and Ameri- can eel (Shirley et al. 1990). Two experimental stud- ies indicate that predation on soft postmolt crabs is much higher that on intermolt crabs (Shirley et al. 1990; Ryer et al. 1997). However, both studies used methods that involved tethering soft and hard crabs within mesh bags, which may affect their detection by predators. Juvenile blue crabs may exhibit variation in lat- eral spine length, color, or other morphological fea- tures (Davis et al. 2004a, 2005a). Juveniles reared in experimental hatcheries appear to have shorter spines than wild crabs, but spine length grows quickly (within 2 molts) after exposure to field con- ditions, perhaps as a result of chemical cues from fish or other predators (Davis et al. 2005). However, mortality rates in the field do not appear to differ consistently as a function of spine length and source of the juveniles (Hines et al., unpubl. data). Color of juveniles is under hormonal control of chro- matophores and varies within 1 to 2 h to match the background color and even fluctuates on tidal and diurnal rhythms (Fingerman 1955), so small crabs are able to match their background and achieve an adaptable camouflage from visual predators. No studies to date indicate that predation rates differ by sex of blue crabs, although blue crabs do partition the habitat ? and thus presumably expo- sure to predators ? by sex (e.g., Hines et al. 1990). In the soft-crab fishery, molting male crabs suffer greater mortality, yet take lesser time to molt, than E COLOGY OF J UVENILE AND A DULT B LUE CRABS 623 do females in shedding operations (Chaves and Eggleston 2003), but this is difficult to relate to nat- ural mortality rates and vulnerability to predators in the field. As a part of mating behavior, female crabs undergoing their pubertal molt pair up with mature males for a period of both pre-copulatory and post- copulatory mate guarding that provides the female with protection from potential predation and can- nibalism (Jivoff 1997a, b, 2003; Jivoff and Hines 1998a, b). Refuges from Predation and Cannibalism Blue crabs obtain refuge from predation by use of key habitats that provide structural complexity or are inaccessible to predators. Refuge habitats with structural complexity include seagrass, oyster reefs, emergent marsh vegetation, mangrove prop roots, and coarse woody debris from terrestrial trees. Habitats with structural refuges are especially impor- tant to small juveniles and molting blue crabs. Sub- merged aquatic vegetation beds have received extensive research, which found increased abun- dance and higher survival of juvenile and molting blue crabs in vegetated than in adjacent unvegetated habitat (Heck and Orth 1980a, b; Heck and Thoman 1981;Wilson et al. 1987, 1990b; Pile et al. 1996; Ryer et al. 1997) (Fig. 16). Indeed, an impor- tant component of the soft-crab fishery in Chesa- peake Bay involves dredging for premolt crabs that use seagrass as a refuge (Oesterling 1995). Refuge value of seagrass patches for juvenile blue crabs depends upon the degree of fragmentation of the seagrass beds (Hovel and Lipcius 2001, 2002), dis- tance from shoreline or other habitats such as salt marsh (Micheli and Peterson 1999), and aspects of the structure itself, such as the density and configu- ration of vegetation blades (Heck and Spitzer 2001; Hovel and Lipcius 2002; Orth and van Montfrans 2002). However, these effects of seagrass patch size and shoot density may vary seasonally because den- sities of predators and cannibalistic crabs fluctuate with recruitment into the patches, even to the point that predation rates on tethered juvenile crabs may be inversely related to shoot density as juvenile den- sities increase during fall after megalops? settlement (Hovel and Lipcius 2002). Effects of shoot density on habitat value for juvenile blue crabs may vary at different spatial scales (Worthington et al. 1992). Blue crabs move onto marsh surfaces during flood tides, which allows them to escape from many predators, as well as to gain access to food resources (Orth and van Montfrans 1987; Lin 1989; Mense and Wenner 1989; Thomas et al. 1990; Fitz and Wiegert 1991b, 1992; Micheli 1997b; Tupper and Able 2000). In Chesapeake Bay, male crabs under- going their pubertal molt move into tidal creeks in preparation for molting, which takes place along the fringing edge of emergent marsh vegetation (Hines et al. 1987; Shirley et al. 1990; Wolcott and Hines 1990). Salt marsh creeks also provide molting crabs with refuge from cannibalism (Ryer et al. 1997). Similarly, juveniles associated with other struc- tures gain refuge from predation. Juveniles in ben- thic or drift algae, e.g., Ulva lactea = U. lactuca , have reduced predation rates (Heck and Orth 1980a;Wil- son et al. 1990a). Intermolt juveniles tethered near coarse woody debris have higher survivorship than those tethered on open sediment (Hines et al., in prep.), and molting juveniles in central Chesapeake Bay seek coarse woody debris for protection when they are soft (Everett and Ruiz 1993; Hines et al., in prep). Crabs tethered near oyster shell also obtain higher survivorship than those tethered on bare sed- iment (Hines et al., unpubl. obs.). 624 T HE B LUE CRAB Figure 16. Effect of submerged aquatic vegetation on predation upon juvenile (30-70 mm CW) blue crabs tethered on bare sand and low, medium, and high den- sities of blades of eelgrass Zostera marina . Vertical bars indicate ? one standard error. From Wilson et al. (1987). Shallow water, even in the absence of structure in the environment, is a crucial refuge habitat for 0+ age class juveniles (30-70 mm CW) in Chesapeake Bay (Ruiz et al. 1993; Dittel et al. 1995; Hines and Ruiz 1995) (Fig. 17). When approached from shore, escape orientation of blue crabs in the intertidal zone was consistently towards deeper water (Wood- bury 1986). However, most predators appear to approach from deeper water, and juvenile blue crabs tethered experimentally during summer in a subestuary of central Chesapeake Bay suffered high mortality (40%-90% per day) in water deeper than 70 cm, whereas juveniles tethered in shallow water (40 cm deep) suffered less than half that mor- tality rate (15%-40% per day) (Hines and Ruiz 1995). The difference in relative morality of juve- niles tethered in shallow versus deep water (Hines and Ruiz 1995) was greater than the difference in mortality of similarly tethered juveniles in vegetated versus non-vegetated habitats (Heck and Thoman 1981; Heck and Wilson 1987; Heck et al. 1995; Pile et al. 1996). Laboratory experiments using large tanks with depth gradients showed that juveniles (30-70 mm) shifted their depth utilization from deep and medium depths to shallow water in the presence of a large crab, but not in the absence of another crab or in the presence of another small crab (Dittel et al. 1995). Survival of juveniles preyed upon by large crabs was also significantly higher in tanks with depth gradients that provided shallow refuge than in tanks without depth gradients (Dittel et al. 1995). Low salinity zones of river-dominated estuaries have fewer large crab and fish predators and afford reduced mortality (tested by tethering) than higher salinity zones (Posey et al. 2005). Similarly, upper, low-salinity ends of subestuaries of Chesapeake Bay afford lower mortality rates (also tested by tethering) to juveniles (Ruiz et al. 1993; Hines and Ruiz 1995; Seitz et al. 2003), especially in mud and sand flats associated with fringing salt marshes, including when compared to seagrass habitats at the lower end of the same tributaries (King et al. 2005; Lipcius et al. 2005; Seitz et al. 2005). Thus, low salinity zones may provide good nursery areas because of lower E COLOGY OF J UVENILE AND A DULT B LUE CRABS 625 Figure 17. Shallow water as a refuge from predation upon juvenile (30-70 mm CW) blue crabs in Chesa- peake Bay. Top panel shows mortality of juveniles tethered in shallow (15 cm) versus deep (80 cm) water on non-vegetated bottom at three shoreline stations along the Rhode River, a subestuary of Chesapeake Bay. Numbers at top of bars indicate sample sizes. Bottom panel shows annual variation in mortality of juveniles tethered in shallow (squares; 15 cm), medium (diamonds; 40 cm) and deep (circles; 80 cm) water on non-vegetated bottom of the nearshore waters of the Rhode River subestuary. From Hines and Ruiz (1995). predator abundance and reduced mortality, as well as good food resources for juvenile blue crabs. Juvenile crabs exhibit density-dependent mor- tality in muddy, non-vegetated nursery habitats of upper Chesapeake Bay, and experimental stocking in small coves resulted in higher survival at low than high juvenile densities (Davis et al. 2005). Low den- sity may afford refuge to prey (such as bivalves) attacked by predators that exhibit density-dependent foraging (such as adult blue crabs) (Lipcius and Hines 1986; Eggleston et al. 1992; Seitz et al. 2001a). Low density of juvenile crabs in combination with shallow water allows a significant but only partial refuge from predation, especially cannibalism by large crabs (Dittel et al. 1995). Although the mecha- nism by which shallow water provides refuge from predation-cannibalism is not clear, the attack success of large blue crabs appears to be lower in shallow than deep water, as indicated by the damage:mortal- ity ratios of tethered crabs (Hines and Ruiz 1995). Because large crab cannibalism on juveniles exhibits an inversely density-dependent functional response that characterizes unstable predator-prey interac- tions, large blue crabs may be able to drive juveniles locally extinct in many areas (Dittel et al. 1995).The effectiveness of the shallow-water refuge may vary with the steepness of the bottom slope, affecting the distance of forays made by large predators (Lin 1989). Human modification of the nearshore bathymetry by bulkheads, rip-rap, and dredging may have major effects on refuge value of the nearshore habitat by removing shallow depth habitat (Hines and Ruiz, unpubl. data). Increased body size also affords blue crabs effec- tive refuge from predation, which allows larger crabs to exploit certain habitats that have high risk of pre- dation for smaller crabs. Survival of tethered early post-settlement juveniles increased with increasing size through the 5th instar and was not significantly different for crabs from 5th through 9th instars, indi- cating that relative refuge in size from predation is reached by the 5th instar (Pile et al. 1996). This behavior allows juveniles to disperse from the refuge afforded by seagrass beds and to move into other shallow habitats throughout the estuary. In central Chesapeake Bay, mortality of tethered crabs de- creased markedly with increasing body size in the following categories: 30 to 50 mm >50 to 70 mm >90 to 110 mm >125+ mm (Hines and Ruiz 1995; Hines, unpubl. data). Tethered intermolt crabs >125 mm suffered no mortality in these experimental studies, indicating very low natural predation or can- nibalism in marked contrast to high estimates of mortality attributed to fishing (60%-98% per year) (Sharov et al. 2003). Vulnerability to predation or cannibalism does not appear to exhibit significant diel variation, as tethered juvenile crabs suffered similar mortality rates during day and night periods in central Chesa- peake Bay (Hines and Ruiz 1995). The lack of diel variation is consistent with the conclusion that large crabs comprise the major source of juvenile mortal- ity, because crabs are well adapted for chemotactile foraging in the absence of light and because they exhibit bimodal periods of morning and evening foraging activity that spans both day and night (Pearson and Olla 1977; Nye 1989; Clark et al. 1999b). Agonistic Displays Blue crabs may flee an attack by a predator or cannibal by walking or swimming away or digging, or they may perform aggressive behaviors that include displaying, fending, and striking (Wright 1968; Teytaud 1971; Jachowski 1974; Norse 1975, 1977). Blue crabs often exhibit a meral spread threat display (chelipeds extended laterally) during agonis- tic interactions with other crabs and potential preda- tors (Jachowski 1974). During high intensity dis- plays, the chelae may be extended to angles approaching 160?, but in lower levels of display the chelae may be angled slightly forward of the bent resting position (Wright 1968). When equipped with biotelemetry tags that transmit the crab?s loca- tion and threat displays and feeding (Clark et al. 1999a, b), free-ranging adult crabs in Chesapeake Bay exhibited a bimodal pattern in the frequency of threat displays. The peaks in display activity lagged slightly after the bimodal peaks in morning and evening feeding activity, with crabs spending 10%- 12% of their time in threat posture during peak periods (Clark et al. 1999a, b). Threat displays 626 T HE B LUE CRAB E COLOGY OF J UVENILE AND A DULT B LUE CRABS 627 occurred primarily at sites where crabs fed and increased with aggregation on prey patches and with increasing crab density (Clark et al. 1999a, b, 2000). Autotomy Autotomy is the reflex severance of an appendage, which provides adaptive advantages to limit damage and wounds (e.g., stops bleeding, avoids difficulty in future ecdysis of limb), as well as to avoid predators that are left holding the limb while the rest of the prey escapes. Most species of crabs, including blue crabs, are capable of autotomiz- ing limbs (Juanes and Smith 1995). Blue crab popu- lations often exhibit high but variable levels of limb autotomy, and the frequency of limb autotomy reflects variation in agonistic encounters, involving both other blue crabs and other potential predators (Smith 1990a). Autotomy frequency may be used as an indicator of intensity of predation and cannibal- ism within and among populations, although cau- tion in interpretation is advised because autotomy really indicates non-lethal attacks. Figure 18. Variation in limb loss by limb type in blue crabs. Top shows spatial variation among three sites in Chesa- peake Bay (UB = upper bay, PX = Patuxent River, LB = lower bay) and three sites along the East and Gulf coast (SC = South Carolina, FL = Florida,AL = Alabama) in 1989. Bottom shows temporal variation among years (1986 -1989) within the Rhode River subestuary of Chesapeake Bay, Maryland. From Smith and Hines (1991a). In the most extensive and intensive survey of limb autotomy in arthropods to date, the frequency of blue crab autotomy varied temporally both within and among years, and over broad geographic scales from Chesapeake Bay, Maryland, to Mobile Bay, Alabama (Smith and Hines 1991a) (Fig. 18). Blue crab limb loss was substantial, ranging from 17% to 39% among sites, indicating that autotomy is an important mechanism for blue crab survival. Limb loss also varied significantly spatially among sites within Chesapeake Bay (19%-39%) but did not vary significantly among sites within a small subestuary of central Chesapeake Bay. Patterns of injury among limbs were remarkably consistent among all sites, such that chelipeds had the highest autotomy rate whereas swimming (5th) legs had the least, right and left limbs were lost with equal fre- quency, and severe autotomy of several multiple (four or more) limbs was rare (Smith and Hines 1991a). Loss of single chelipeds was common (4- 17%) among sites, but loss of both chelipeds was rare (0-5%) (Smith and Hines 1991a). Frequency of limb loss was independent of sex and molt stage, but was positively correlated with crab size, perhaps because large crabs molt less often and accumulate injuries over long periods, or perhaps because ago- nistic interactions are more likely to result in death than autotomy for small crabs than for large crabs (Smith and Hines 1991a; Hines and Ruiz 1995). Within a subestuary of central Chesapeake Bay, limb loss varied significantly among 4 y, ranging from 17% to 25%, and autotomy frequency was density dependent, indicating that interactions among blue crabs may be a major cause of limb loss (Smith 1995). Consequences and costs of limb autotomy in blue crabs vary by type and number of lost limbs, because each limb is specialized for particular func- tions. Further, blue crabs are heterochelous, typically with a right crusher claw and a left cutter claw, each with distinct morphology and mechanical advantage for handling prey (Hamilton et al. 1976; Blundon and Kennedy 1982a; Seed and Hughes 1997; see also Kennedy and Cronin, Chapter 3). Although blue crabs can regenerate autotomized limbs, lost crusher claws are replaced by cutter claws and up to three molts are required for regeneration of the full size of a limb (Govind and Blundon 1985; Smith 1990b). Loss of only a single limb had no apparent effect on crab growth, although severe multiple limb loss (four or more) significantly reduced molt increment and percent weight increase in the next molt (Smith 1990b). Foraging rate on soft clams did not differ significantly between blue crabs missing a single crusher cheliped and intact crabs, but crabs missing both chelipeds consumed significantly fewer clams than did intact crabs (Smith and Hines 1991b). However, the low incidence of crabs missing both chelipeds suggests that such injury does not affect crab predation on soft clams at the population level. Effect of limb autotomy (either with a single che- liped lost or with severe multiple loss of both che- lipeds and two other legs) did not affect male crabs? abilities to mate with females, but both levels of autotomy reduced male crabs? abilities to compete for and defend females from ?take-overs? by other intact males (Smith 1992). Effects of prior limb loss on escape from predators ? especially of juveniles escaping cannibalism from large crabs ? also depended upon number and type of limbs lost (Smith 1995). Juveniles with only a single missing leg remained fully effective at escaping attacks by large crabs, whereas juveniles with severe multiple limb loss changed their escape behavior, reducing activity levels and remaining buried, which reduced their encounters with cannibalistic large crabs. OTHER SOURCES OF MORTALITY Disease is an important source of blue crab mortality in many systems, with the blood disease ( Hematodinium spp.) being particularly lethal at higher salinities, and high prevalence of the rhi- zochephalan Loxothylacus texana causing ?reproduc- tive death? by parasitic castration along the Gulf Coast (see Shields and Overstreet, Chapter 8). Analysis of disease effects on blue crabs needs much more study (Shields 2003). Estuarine conditions may exceed physiological tolerances of blue crabs, resulting in mortality of appreciable fractions of local populations. Harsh 628 T HE B LUE CRAB winter conditions at combinations of low water temperatures <3? C in low salinity (<10) areas cause mortality in upper Chesapeake Bay and Delaware Bay, especially of large adults and very small juveniles (Kahn 1998; Sharov et al. 2003;Aguilar et al. 2005). Because Callinectes sapidus is a species with tropical evolutionary origin (Williams 1984), these winter effects may limit the northward geographic distribu- tion of blue crabs along the East Coast. Although blue crabs generally avoid low oxygen conditions by moving to shallow habitats (Pihl et al. 1992; Diaz and Rosenberg 1995), mortality may result from large episodes of nocturnal anoxia or seiching of deep anoxic waters into shallow areas (see Tankersley and Forward, Chapter 10.) Further, modeling and empirical studies on the effects of low dissolved oxy- gen yield conflicting results of both increased crab catch (Eby and Crowder 2002) and reduced crab catch (Mistiaen et al. 2003), depending on the distri- bution of the fishing pressure and geomorphology of the estuary. RESEARCH PROGRESS AND PRIORITIES Research on the ecology of juvenile and adult blue crabs has progressed greatly over the past 20 y. The progress is due in part to the increased numbers of bright, energetic young scientists studying blue crabs as crucial predators in estuaries of the East Coast of North America ? numbers of their publi- cations have increased grandly, as the literature cited in this chapter shows. Progress comes, too, from new technology and tools, ranging from ultrasonic biotelemetry to global positioning systems, to remotely-sensed geographic information systems, and especially to ready access to powerful small computers. All these allow much more quantitative and sophisticated approaches to blue crab ecology. Biomolecular applications are just becoming readily accessible to marine ecologists, and these sophisti- cated tools will soon begin to allow new questions about recruitment dynamics, movement, demogra- phy, feeding interactions, and reproductive biology of blue crabs. However, we remain stymied by cer- tain problems. For example, due to the obvious problem of working with animals that shed their exoskeletons in murky water, we know very little about predation and mortality rates in the field dur- ing molting, which we believe are much higher than during intermolt periods. We now know a lot more about blue crabs in some locations like New Jersey marshes, Delaware Bay, Chesapeake Bay, the North Carolina sounds, South Carolina estuaries, Mobile Bay, and others. But there are many locations where we know little, including much of the coast of Central and South America and the Caribbean basin. Although the ecology of blue crabs is similar in many ways across sites, comparisons of East Coast and Gulf Coast sys- tems indicate important differences between tem- perate and tropical systems, with regard to recruit- ment dynamics, competition with other predators, and major habitat changes (e.g., salt marshes drop out and mangroves become important). Some aspects of habitat use may differ as a result of differ- ences in tidal regimes, such as ?micro-tidal systems? of many areas such as Indian River Lagoon, Florida, or large areas of the Gulf Coast compared to ?macro-tidal systems? of the East Coast estuaries where salt marshes regularly drain and flood on each cycle. Predation effects of blue crabs appear to vary widely with latitude, with clear dominance by blue crabs in many higher latitude estuaries but increasing complexity and diversity of the predator guild at lower latitudes. The relative roles of cannibalism and predation by fishes within and among estuaries are also not well understood. Experimental ecologists have shown major interactive effects of crab density, prey resources, and habitat characteristics, so that multi-factorial experi- ments are now an essential element in blue crab research. Improved models are needed to integrate the complexity of these interactions and to scale up their application to large systems with extensive fish- eries. It is apparent that effective protection of linked ecosystems is required for blue crab popula- tions to complete the life cycle and sustain heavy fishing pressure on top of pollution and habitat destruction. Ecologists must provide the types of data that are essential for improved large scale models. E COLOGY OF J UVENILE AND A DULT B LUE CRABS 629 Unfortunately, progress also comes from the urgency of declining blue crab populations in many estuaries, especially Chesapeake Bay, which histori- cally has been the most productive fishery. The scale of the fisheries is much greater than any of the research efforts underway, and this large-scale sam- pling by the fishery could be put to great scientific use to understand the complex ecology of the migratory life cycle of Callinectes sapidus moving large distances across many habitats. Yet use of much of this sampling effort by the fisheries is lost to sci- ence. Our knowledge of blue crab ecology could increase much more rapidly if scientists, fishery managers, and fishers could work more closely together to collect accurate, fine scale quantitative data on variation in crab abundance, distribution, and population structure. ACKNOWLEDGMENTS I am ever appreciative of Gene Cronin and Austin Williams, who generously shared their enthu- siasm and knowledge of blue crabs with me in the later years of their productive lives. I owe a great debt of gratitude to the many collaborators, post- doctoral fellows, and graduate students who have worked with me for two and a half decades on blue crab biology, especially Paul Bushmann, Glenn Davis, Jana Davis,Anna Dittel, David Eggleston, Paul Jivoff, Eric Johnson, Mark Kuhlmann, Rom Lipcius, Adina Motz, Laura Nye, Martin Posey, Greg Ruiz, Rochelle Seitz,David Smith, Jacques van Montfrans, Mike Shirley, Heather Turner,Tom and Donna Wol- cott, Oded Zmora, and Yonathan Zohar. Many technicians gave tirelessly to my crazy schemes, schedules, and long days and nights to learn the secrets of blue crab ecology, especially Rob Aguilar, Rob Andrews, Lenore Bennett, Mike Goodison, Mark and Patty Haddon, Midge Kramer, Laura Nye, Kathy Paige, Keith Porterfield, Sherry Reed, and Alicia Young-Williams. Nearly one hundred under- graduate SERC Interns and summer assistants sam- pled, counted, measured, molt-staged, and main- tained thousands of blue crabs over the years in my lab. Mike Goodison, Midge Kramer, and Alicia Young-Williams helped with the references. I thank Vic Kennedy, Ken Heck, and an anonymous reviewer for patiently improving the manuscript. My work has been supported by grants from the Smithsonian Environmental Sciences Program, NSF, Maryland Sea Grant, EPA, NOAA Essential Fish- eries Habitat Program, Disney Wildlife Conserva- tion Fund, Philip D. Reed, Jr., Blue Crab Advanced Research Consortium, and NOAA Chesapeake Bay Fisheries Program. 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