Joumlllof EXPERIMENTAL MARINE BIOLOGY Journal of Experimental Marine Biology and Ecology AND ECOLOGY ELSEVIER 288 (2003) 181 - 20 1 www.elscvicr.com/locatc/jcmbc Effects of pressure on swimming behavior in planula larvae of the coral Porites astreoides (Cnidaria, Scleractinia) Joel L. Stake *, Paul W. Sammarco Department of B;%g,J: University ofLouisiana lit LlIli,yelle. Lafayette, LA 7115114. USA Louisiana Universities Marine Consortium (LUMCON). 8/24 Ilwv. 56. Chauvin, LA 711344. USA Received 8 August 2002 ; received in revised form 20 November 2002 ; accepted 15 December 2002 Abstract Mechanisms governing the behavior of coral planulae are not well understood. particularly those manifesting themselves between the time when the larvae are released and when they settle. Larvae from the hermatypic coral Porites astreoides Lamarck were exposed to different levels of hydrostatic pressure- J03.4, 206.9. 310.3, 413.8. and 517.1 kPa (including ambient pressure). Data were collected at stops of the above pressures for 15 min each, respectively. This was done in both an increasing sequence and a decreasing one. When exposed to increases in pressure from 103.4 kl'a, larvae swam upward (negative barotaxis) in a spiraling motion. Upon exposure to decreasing pressure [rom 517.1 k l'a, larvae moved downward (positive barotaxis). but the magnitude of the vertical movement was much less than in the case of increasing pressure. This suggests that these larvae are more sensitive to increased pressure than decreasing pressure. High variance was also observed in the responses of these larvae at both the intra- and inter-colony levels. Thus. this behavioral trait is variable within the population. The trait may be genetically based. and thus may be susceptible to alteration by natural selection. although this remains to be demonstrated. This study is the first to document these behavioral mechanisms in coral larvae. :D 2003 Elsevier Science S.Y. All rights reserved. Keywords: Coral; Planula; Pressure; Larval behavior: Porites astreoides; Barotaxis * Corresponding author. Department of Biology, University of Louisiana at Lafayette, Lafay ette, LA 70504, USA. Tel.: + 1-337-482-5246; t~IX: + 1-337-482-5834. E-lJIlii/liddres.'"s : jstake(i])yahoo.eom (LL. Stake). psammat'eo@lullleon.edu (P.W. Sammarco). 0022-09811031$ - see front matter ('; 2003 Elsevier Science B.V. All rights reserved. doi:10.10 16/S0022-098 I(03 )000 I8-2 182 ./L Slake. P. II: SlIlIIlIIlI l'm / ./. Exp. Min: BioI. Ecol. 288 (2003) /8 / - 20/ 1. Introduction One of the mea ns by which scleractinian cora ls maintain their popul ations is throug h the release of swimming larvae that are car ried by currents to settle and estab lish nearby reefs, as we ll as to help ma intain the natal reefs fro m whi ch they we re released (Sammarco, I994b). Sw imming larvae can act as a means of dispersal in many marine benthic orga nisms (Thorson, 196 1, 1964; Doy le, 1975), particularly sede ntary or sessi le orga nisms (Harrison and Wallace, 1990). Such dispersal can prov ide a means by which to co lonize new habitats, reco lonize old habitats, and promote gene flow (Thorson, 1961; Scheltema, 1977; Gerrodette, 1981; Ayre et al., 1997). The impact of plankton ic dispersal, however, has been the subjec t of controve rsy. One view is that reefs are dependent upon eac h othe r for larval recruits (Harrison et aI., 1983, 1984; Williams et al., 19R4; Babcock and Heyward , 1986), and the other is that many larvae are retained on their natal reef (Done, 1982; Sam marc o and Andrews, 198R). This appare nt controversy has been review ed in detail by Sammarco ( 1994a, b); he has demonstrated, in fact, that both short- and long-distance dispersal can be simultaneo usly occ urring. The many qu est ions regarding what happ ens to these larvae between the time they are released and the time that they are cued to se ttle remains open. Much research has been performed on adult co ral reproduction, part icularly ove r the past 20 years (e.g. Rinkevich and Loya , 1979; Koj is and Quinn, 1980; Fadlalla h and Pearse, 1982; Fadlallah , 1983; Chornesky and Peters, 19R7; Harrison and Wallace, 1990; Richm ond and Hunter, 1990; McGu ire, 1997 ; Shlesinger et aI., 1998), and larval dispersal mechan isms (e.g. Richm ond , 1987; Sammarco and Andrews, 1988, 1989; Harrison and Wallace, 1990; Sammarco , I994a ,b, 1996). Research on physical oceanographic mecha? nisms has provided for prediction of larval dispersal (Samm arco, 1994b). There are many modes of reprodu ction in the Scleractinia (see Sammarco, 1982; Harrison and Wallace , 1990), but two are generally used in this order. The first is the release of gametes followed by ex terna l fertilization, generally term ed " broadcas ting " . The second is the development of planula larvae wi thin the polyp followed by release of fully developed larvae , termed " brood ing". Once formed, either by broadcast spawning or broodin g, the planu la larva is simple in struc ture. Similar to the adults, they consist of only two ce ll layers, an ectoderm and an endo derm (Barnes et a I., 1993). T he composi tion and interna l structures of the larvae have been identifi ed by Pennata et al. (2000) . The majority of studies conducted on planulae have been perform ed on brood ing species (Atoda, 1953; Harrigan , )972; Fadlallah, 1983). The pla nula 's biochemical compos ition has been found to con tain large amounts of lipids (70% by dry weight ), protein ( 17%), and carbohydrates ( 13%; Richm ond , 19R7). Developm ent periods differ between broadcast and brooded larvae. For broadcas t larvae, a min imum of 48-72 h is required before they are competent to settle (Hodgson, 1985; Babcock and Heyward, 1986). Brooded larvae are fully developed at release, and a minimum of on ly -4 h is required until they are competent to sett le (Harriga n, 1972). Larvae derived from either form of reproduction, however, may have a larval period of up to 90 days (Richmond, 1987) (although some inves tiga tors bel ieve 90 days is an ove restimate; Mund y and Babcock, 1998). 1.L. Slake. PlY Sammarco / 1. Exp. Mar . Bioi. Em/. 288 (2003) 181-201 IR3 After release, the ciliated planulae can swim through the water column in any direction, and a number of swimming patterns have been described (Harrigan , 1972; Rinkevich and Loya, 1979). Planular swimming rates average from I to 5 mm s" I (Atoda, 1951 a.b,c; Harrigan, 1972; Tranter et al ., 1982; Fadlallah, 1983). Direction of movement can be horizontal or vertical. At a horizontal swimming rate of 5 mm s" I, however, this velocity would be exceeded by several orders of magnitude by the current velocity of the water in which it is swimming; for example, current velocities of 8-1 a cm/s have been commonly measured in the central region of the Great Barrier Reef. Thus, a planula is under the primary inf1uence of currents in the horizontal plane, as are many planktonic larvae (Sammarco, 1994b). Many planktonic larvae have the ability to regulate their vertical position (Mileikovsky, 1973). Mileikovsky concludes that because some pelagic larvae can swim at reasonable speeds (> I em min - I), they are able to control, to some degree, their vertical distribution in the water column. This allows them to position themselves vertically even in areas with strong tidal currents such as estuarine and marine nearshore areas. Mass mortality of corals has been documented at an increasing rate in recent decades (Antonius, 1985; Munro, 1983 ; Bythell and Sheppard, 1993; Ohman et al., 1983; Miller, 1996 ; Sammarco, 1996; Santavy and Peters, 1997 ; Green and Bruckner, 2000; White et aI., 2000; Porter et al., 200 I; Riegl , 200 I). In some cases, this has been attributed to increased sea surface temperatures (SSTs) and the resultant bleaching of corals (Brown, 1990; Goreau and Hayes, 1994; Huppert and Stone, 1998). Because higher seawater temperatures generally occur in shallow depths, an understanding of any behavior that assists a coral larva to regulate its depth becomes important. Recruitment of coral planulae to a disturbed area is critical to the reestablishment and recovery of reef ecosystems. Thus, understanding the factors that influence these larvae as they are dispersed is also critical to predicting reef regeneration processes. 1.1. Sensitivity to pressure Some larvae are known to be sensitive to changes in hydrostatic pressure at certain developmental stages (Rice, 1964; Knight-Jones and Morgan, 1966; Morgan, 1984). In general, larvae move up when pressure is increased and move down when pressure is decreased (Forward, 1989). It is possible that coral planulae have similar sensitivities and behaviors, although to date, this has not been investigated. Some researchers suggest that, because a coral species is found generally at 6 -8 m depth on inshore fringing reefs and rarely found below 20 m on the outer continental shelf, biological factors-such as settlement behavior-rather than physical factors may be limiting their distribution (Mundy and Babcock, (998). Information on the effects of hydrostatic pressure on vertical swimming behavior in coral planulae would provide valuable information on the sensory abilities of coral planulae, i.e. , whether they are able to perceive and react to pressure in their surrounding environment. The purpose of this investigation was to determine whether planulae from the scleractinian coral Porites astreoides Lamarck are sensitive to changes in hydrostatic pressure, and to determine the effects of variation in pressure on the vertical swimming behavior of the larvae. The results of thi s study provide insight into the evolution and 184 ./.L. Slake. P 11' Snmmarco / 1. Exp. 111m: etot. Em!. 288 (2003) 181- 2111 adapti ve significa nce of this behavioral trai t and should open ques tions regarding associated sensory mech ani sm s. Th is study may also provide insight into physiological mechan ism s co ntributing to dep th-d epend ent se ttlemen t of cora l planul a larvae. 2. Materials and methods A total o f 20 adult colonies of P. astreoides we re co llected from a depth of 1- 4 m on bridge supports and sea wa lls in the lower Flor ida Keys. P. astreoides is known to occ ur at depths rang ing from I to 50 rn, but is most frequently found at 1- 10 m. The channe ls and areas used as sample sites were Little Duck -Missou ri Channe l, Spanish Harbour Channe l, Bahi a Honda C hanne l, Moser Chan ne l, and Ohio - M isso uri C hanne l. All P astreo ides co lonies we re of the " gre en " morph oCoral colon ies were co llected - 10 days before the new moon, to allow coll ect ion o f larvae from adult co lonies upon their release, which normally occurs at or nea r the new moon (M cGuire, 1998). Co lon ies were maintained at the Mote Marine Laborator y-Center for Tropical Research , Summerland Key, FL, in 10- and 20-gal aqua ria outdoors on wet tables. Max i-je t')') pow erheads were used to provide wat er flow and aera tion in each of the aq uaria . Colonies were kept outside to ex pose them to natural lunar reproductive cues. Water tab les we re covered with 60% shade cloth to redu ce the amo unt of so lar irradia tion. Aquaria temperatures were ma intain ed at a mean o f 26 ? 2?C. Tem pera tu re was main ? tained by using ch illed seawater to coo l the tank s and heated seawater to heat them , when necessary . Sa linity was kept betwee n 34 and 37 ppt and mainta ined by add ition of freshwa ter to decre ase sa linity. Ny lon mesh derived from stock ings wa s used to co llec t coral larvae. Th e mesh wa s placed on pyramidal PYC frames for support. Adult co lonies were co vered with the nets each evening for 7 days before and after the new moon and were checked eac h morning for larvae. Larvae we re rem oved by pipet ting, pla ced in se parate holding chambers, one for eac h co lony, and co unted. Lar vae we re held in g lass via ls and placed in a water bath to help maintain a con stant temperature. Eac h via l was ae rated from a sing le ai r pump. Larvae were labeled accord ing to parent co lony and date o f release. The pressure chamber (Fig . I) was co ns tructed from transparent PYC pipe, 0.80 m in len gth (0.0. =7.62 em; J.D. =6.99 ern). It wa s sealed at the bott om using a PYC end cap sec ured with Oatey '" C lea r PV C ce ment. Th e top wa s sea led using PVC bushings, again glued with Oat ey '" Clear PY C ce ment. A 2.54-cm hole in the cap serve d as both an acc ess point for the introduction and removal of larvae, and an attachment poin t for tubing from the pressure regulator and air so urce . Th e side of the appa ratus was fitted with a metric ruler as a referen ce guide for record ing the vert ical position of eac h larv a. Co mpressed air from a SCUBA tank was used to regul ate pressure in the chamber. A Victorw SR 250 s ing le-st age regulator was used to co ntro l chamber pressure. Th e first stage of the regul ator was used to reduce the air pressure from 19,30 0 to < 1379 kPa. A second stage was a lso used as a pressure-release va lve . The ex perime nt followed a one-way ANOYA design using repeated me asures. Th e ex perime nta lly varied factor was pressu re. The response variable was the verti cal position of the larvae in the tank, The pressures used were 103.4, 206.9, 310.3, 41 3.8, and 5 17.1.1 .l.L. Slake, P. HI Sammarco / J. ?1'". Mill: Bial . Em!. 288 (2003) /8/ - 20/ IRS C B (+'1 (" E ~ F=, /"G .:?\....J' ...-- ? A DF Fig. I. Experimental pressure chamber. (A) Compressed air cylinder at 19,300 kPa. (B) SCUBA first stage, reducing pressure to 1379 kPa. (C) Second stage regulator, for tine control of air pressure (103.4 -517.1 kPa). (D) Observation chamber. (E) SCUBA second stage, used as release valve. (F) Metric ruler, used as reference for vertical position of larvae, kPa (total pressure, including ambient). The pressures were applied stepwise in ascending and descending order. The experiment was repeated 10 times, using planulae from separate colonies each time (II; = 10). Larvae < 24 h old were used for the experiment, to ensure that they were all at approximately the same stage of development. Ten randomly selected larvae from one adult colony were placed inside the pressure apparatus, which was filled with seawater. The larvae were permitted 15 min to acclimate . At the end of this IS-min period, a timer was started and observations were taken. The vertical position of each larva was recorded every 5 min starting at 0 min and ending at 130 min. Qualitative observations were also made regarding planular swimming behavior. During this time , the pressure was increased by 103.4 kPa every 15 min until a maximum of 5 J 7. J kPa (the equivalent of 40 m depth) was attained. After this pressure had been reached and the 15 min was allowed for a response to that pressure, the pressure was reduced by 103.4 kPa every 15 min until it once again reached ambient. Larvae were only used once, and no colony was used as a source of larvae more than once during the experiment. The data were analyzed using repeated-measures ANOYA for both individual trials and pooled data. Data were log-transformed before analysis for purposes of normalization (Sokal and Rohlf, 1995). Statistical details may be found in figure legends. Only significant results will be discussed. 186 .J.L. Stake, I ~ W SIII/II / lim'O / .1. Exp. ;\1/1/: Bioi. ?1'01. 288 (2003) 181 - 201 3. Results 3.1. Effects I?I' increasing pressure Larvae derived from all colonies exhibited a significant response to increasing pressure (significant movement of the larvae either upward or downward was considered a response). On ave rage, larvae derived fro m all colonies exhibited an upward response as pressure was increased . Although the response was variable , no larvae from any single colony exhibited an average downward response to increasing pressure. Larvae from a number of colonies exhibited similar response trends to the stepped treatments. In particular, larv ae from colonies 2, 3, 4, and 5 all exhibited a highly significant response to pressure (Fig. 2). As pressure increased, larvae generally moved upwards in the chamber steadily until - 3 10 kPa was reached, after which they remained at a depth or 3-4 ern . Post hoc comparisons showed that each treatment was highly significa ntly different from the base treatment (103.4 kPa; 0 kPa plus ambient pressure). Planulae derived from colonies 6, 9, and 10 exh ibited a slightly delayed response to pressure increases. They exhibited upward swimming in response to increasing pressure (a) Pressure (kPa) (b) Pressure (kPa) 103 207 310 414 517 103 207 310 414 517 oo Ul Ul E E ~ ~ .g S?.~ 5 ? .~ 10.~ 10 D. D. 'iii iii ~ 25't: lJ 25 ., ., > 45> 45 7070 ' (c) o Ul E ~ g 5 ., 'iii 10 s 'iii lJ 25 1: ~ 45 70 Pressure (kPa) (d) Pressure (kPa) 103 207 310 414 517 103 207 3 10 4 14 517 o Ul1//I ?????????j?? ?? 1 E ~ l: 5 .g 'iii 10 g, ~ 25 ~ 45 70 Fig. 2. Mean depth (em) of planulae in experimental apparatus. under conditions of increas ing pressure from 103.4 to 517 .1 kPa. Exposure duration = 15 min for each step. Mean shown with 95% con fidence limits . Data log? transformed for purposes of normali zation (Sokal and Roh lf, 1995) . Data show n for colonies (a) 2 (b) 3 (c) 4 and (d) 5. Sig nificant diffe rences between press ures (H - r: adj . I' value < 0.00 I. one-way ANOVA with repeated meas ures). Post hoc contrasts significa nt between the tirst treatm ent and all others ( 103.4 vs. 206 .9. 3 10.3.4 13.8. and 5 17. 1 kPa. respec tively: p 0.05). All other post hoc comparisons between treatment I and the others were highly significantly different (p < 0.00 I) . Larvae from colony 7 also showed a significant response to increasing pressure by swimming upward (Fig. 5), but post hoc contrasts indicated no significant change in vertical position until the onset of treatment 4. This difference was significant, but less pronounced than in previous colonies (p < 0.05). Treatments I and 5 were also highly significantly different (p < 0.00 I) . An analysis of larvae from all 10 colonies combined demonstrated that, as a group, all larvae responded significantly to increases in pressure by swimming upward (Fig. 6). There was also a highly significant difference in response between larvae derived from different colonies (p < 0.00 I). Post hoc contrasts showed that the baseline treatment was significantly different from all other treatments (p < 0.00 I) . (a) Pressure (kPa) (b) Pressure (kPa) 103 207 310 414 517 103 207 310 414 517 Ui Ui E E ~ !<: <: 2 5? 5.g t' 'iii f .~ 10 ? 0 10 e,e, ! I I ~ iii iii u ~ 25 ~ 25 ~ 45 70? > 45? 70 (c) Pressure (kPa) 103 207 310 414 517 c Ui f ??? E ~ <: 5 .Q ~ 10 t t 0 n, iii 25? o i 45 > 70 Fig. 3. Mean depth (em) of planulae in experimental apparatus. under conditions of increasing pressure from I03.4 to 517.1 kPa. Data shown for colonies (a) (, (b) 9 and (c) 10. Mean shown with 95'X, confidence limits. Significant differences between pressures (H-F adj. p 25 f ! ! I !45 70 Fig. 5. Mean depth (em) ofplanulac in experimental apparatus, under conditions of increasing pressure from 103.4 to 517.1 kPa . Data shown for colony 7. Mean shown with 95% confidence limits. Significant differences between pressures (H - F adj. I' < 0.00 I, one-way ANOYA with repeated measures). Post hoc contrasts were significant between the tirst treatment and all others (I' < 0.00 I), except between treatment I vs. treatments 2 and 3. Pressure (kPa) 103 207 310 414 517 o 45 I---+--- All colonies I70 Fig. 6. Mean depth (em) of planulae in experimental apparatus. under conditions of increasing pressure from 103.4 to 517.1 kPa. Data shown for all coral colonies exposed to this set of treatments. Mean shown with 95% confidence limits. Significant differences between pressures (1-1 - F adj. I' < 0.00 I , one-way ANOYA with repeated measures), Post hoc contrasts were significant between the tirst treatment and all others (I' < 0.00 I) . 190 1.L. Slake. P. W Sammarco / 1. EIp. M(//: Bioi. Ecol. 2811 (2003) 1111 - 201 Larvae derived from colony 2 showed an ove rall s ignificant down ward response when expo sed to decreasing pressure (Fig. 7). Th is trial was the only one to show a significant difference between all treatments and the base treatment (5 17.1 kPa) when examined via post hoc comparisons (p < 0.00 I) . Decreasing pressure elicited a signifi cant downward response from planulae deriv ed from colonie s 5 and 8 (Fig. 8). Movement in respons e to the first treatm ent (4 13.8 kPa) was, howe ver, non-si gnifi cant for both co lonies (p>0.05). Post hoc comparisons between treatment I vs. treatm ent s 3, 4 , and 5 reveal ed sig nifica nt downward movem ent (p < 0.001 ). Planulae derived from co lonies 9 and 10 also respond ed significantly to decreasin g pressure by sw imming downward , but with a slightly wea ker response in co lony 10 (Fig. 9). A post hoc comparison indicated that the response to the first two treatm ents was not significantly different from the base pressu re in either of the colonies (p>O.05). Similar post hoc comparisons betw een the base treatment and treatment 3, however, were signi ficant , but at a slightly lower level of significance than the colonies con sidered above (p < 0.05). There were highl y significant differenc es , how ever, in post hoc comparisons between treatm ents I and 4 (p < 0.00 I for co lony 9, and p < 0.0 I for co lony 10, respectively), and betw een treatments I and 5 (p < O.OO I and p 25 45 70 (b) 517 414 Pressure (psi) 310 207 103 o 45 70 Fig. X. Mean depth (em) of planulae in experimental apparatus. under conditions of decreasing pressure from 517.1 to 103.4 kPa. Data shown for colonies (a) 5 and (b) X. Mean shown with 95% confidence limits. Significant differences between pressures (H -- F adj. jJ < 0.00 I, one-way ANOVA with repeated measures). Post hoc contrasts were significant between the first treatment and all others (jJ < 0.00 I), except between treatment I vs. treatment 2. (p>0.05). Post hoc comparisons did demonstrate a significant difference between the base treatment and treatment 4 for both colonies, although the level of significance was different for each colony (p < 0.05 for colony I and p < 0.00 I for colony 7). This was also true for comparisons between treatments I and 5 (p < 0.0 I and p < 0.00 I, respectively). Planulae from colony 4 showed an overall significant downward swimming response to decreasing pressure, but less pronounced than that exhibited by larvae from the other colonies (Fig. II). Post hoc contrasts only revealed significant differences between treatments I and 2 (p < 0.0 I); all other comparisons were non-significant (p>0 .05). 192 J.L. Slake, Po W Sammarco / J. Exp Mar; Bioi. Em /. 288 (200]) 181- 2111 Pressure (psi) (a) 517 414 310 207 103 o Ul E ~ ~5 'iii o 0.10 iU o 'f: Gl > 25 45 70 Pressure (psi) (b) 517 4 14 310 207 103 o 45 70 Fig. 9. Mean depth (em) ofplanulae in experi me ntal appara tus, unde r conditio ns ofdecreasing press ure from 5 17. 1 to 103.4 kl'a . Data shown for colonies (a) 9 and (b) 10. Mean shown wi th 95%. confide nce limits. Sign ifican t differences between pressures (H- F adj . I' < 0.00 I, one-way ANOVA with repeated measures). Post hoc con trasts were significan t between the first treatment and all others ( I' < n,ooI), exce pt between treatment I vs, treatment 2. Post hoc con trast betwee n treatment I vs. treatment 3 is less sign ific ant (1' <0.05) than other comparisons. When data from the larvae of all colonies were pooled, the repeated-measures ANOYA revealed a highly significant downward swimming response to decreasing pressure (Fig. 12). There was a highly significant difference between the responses of larv ae derived from different colonies. In addition, a significant two-way interaction was detected between pressure and parent colony (p < 0.00 I). That is, planulae responded significantly to pressure changes, but the nature of that response varied highly significantly between colonies. Post hoc comparisons revealed a significant difference between treatment I and all other treatments (p < 0.001). Larvae were also demonstrated to exhibit a significant downward 193 J.L. Slak e. t: HI Sammarco / J. Exp. AIm: Bin!. Eco!. 288 (2003j /8/ -20/ Pressure (psi)(a) 517 414 310 207 103 0 "' E ~ c: 50 :;: 'iii 0 0.10 f ??? ???? ! ...... ...... .... ! f ??? ???? ?f (ij 0 t: Ql >25 45 70 ? Pressure (psi) (b) 517 414 310 207 103 0 E "' ~ B5 'iii 0 0.10 (ij 0 t: Ql >25 f f..... .??? ?? f 45? f f 70? Fig. 10. Mean depth (em) of planulae in experimental apparatus, under conditions of decreasing pressure limn 5/ 7.1 to 103,4 kPa. Data shown for col onies (a) I and (b) 7. Mean shown with 95 '1., confidence limit s. Signifi cant differences between pressures (H- F adj.!, < 0.00 I. one-way ANOVA with repeated measures). Post hoc contrasts were significant betw een the firs: treatment and all oth ers (I' .: 0 .00 I ), except for treatment I vs. treatments 2 and 3. swimming response (vs . simple random movement) in association with decreasing pressure (Kendall's coefficient of rank correlation, T, P < 0.00 I , for all treatments). 3.3. Planular swimming behavior When exposed to increases in pressure, planulae began to actively swim toward the surface in a spiraling motion . Although not all larvae were observed swimming in this pattern, for those traveling longer distances, a spiraling pattern was most frequently observed. Larva e were clearly not moving toward the surface passively, but actively 194 J.L. Stake, P. W Samma rco / J. Exp. Mill: Bio/. Eco!. 288 (21103) / 8 / - 211/ Pressure (kPa) 517 414 310 207 103 o t----Jr-----Lr----t----jrE ~ c: o 5 :.;:: 'iii o 0.. 10 Oi u 'E CI> > 25 45 70 Fig . II . Mea n dep th (cm) of planul ae in experimental apparatus, under co nd itions of decreas ing pressure from 5 17.1 to 103.4 kPa . Data shown for co lony 4. Mea n shown wi th 95% confi dence limits. Significa nt di fferences between pressures (H - F adj . P < 0.00 I , one-way ANOVA with repea ted measures). Post hoc con trasts were s igni fi cant between treatmen ts I and 2 i p < 0.00 J); all other contrasts non-significant. sw imming upward under co ndi tions of increasing pressure . Whe n exposed to decreasing pressure, planul ae actively moved down ward. No larvae were observed to sink passively under conditions of decreasing pressure. Th is suggests that larvae ac tively regulate their position in the water co lumn dependin g upon whi ch stimulus is being applied. Pressure (kPa) 517 4 14 310 207 103 o 45 70 ~----f---_+ f? ---- -.I ------y " 1......- All colon ies I Fig. 12. Mean depth (em ) of planulae in experimental apparatus. under conditions of decreasing pressure from 51 7.1 to (OJA kPa. Data shown lor all coral colonies exposed to decreasing pressure. Mea n shown with 95% confi de nce limi ts. Significant differences between pressures (H - F adj . J1 < 0.00 I. one-w ay ANOVA wi th repea ted meas ures) . Post hoc co ntrasts were sig ni ficant betwee n the first treatment and all o thers (p