Vertical Distribution
of Themisto gaudichaudii
(Amphipoda: Hyperiidea)
in Deepwater Dumpsite 106
off the Mouth of Delaware Bay
THOMAS E, BOWMAN,
ANNE C. COHEN,
and
URA McMANUS McGUINESS
SMITHSONIAN CONTRIBUTIONS TO ZOOLOGY ? NUMBER 351
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S M I T H S O N I A N C O N T R I B U T I O N S T O Z O O L O G Y ? N U M B E R 3 5 1
Vertical Distribution
of Themisto gaudichaudii
(Amphipoda: Hyperiidea)
in Deepwater Dumpsite 106
off the Mouth of Delaware Bay
Thomas E. Bowman, Anne C. Cohen,
and Maura McManus McGuiness
SMITHSONIAN INSTITUTION PRESS
City of Washington
1982
A B S T R A C T
Bowman, Thomas E., Anne C. Cohen, and Maura McManus McGuiness.
Vertical Distribution of Themisto gaudichaudii (Amphipoda: Hyperiidea) in
Deepwater Dumpsite 106 off the Mouth of Delaware Bay. Smithsonian Contri-
butions to Zoology, number 351, 24 pages, 16 figures, 1982.? Themisto gaudichaudii,
a bipolar hyperiid amphipod, was collected in discrete-depth samples taken
between the surface and 1800 m in a warm core eddy as it passed through
Deepwater Dumpsite 106, a site off the mouth of Delaware Bay where
industrial wastes are dumped in the Atlantic Ocean. The species was most
abundant in the upper strata (1-125 m) but was found in all depth intervals
sampled during the day (1-25 to 1425-1800 m) and at night in depth intervals
from 1-25 to 700-800 m. Night catches exceeded day catches, probably
because of greater net avoidance during the day. The data do not show
whether or not vertical migration occurred, probably because sampling did
not partition finely enough the upper layers, where previous studies have
shown that diel vertical migrations of Themisto occur. Abundance was greater
in the warmest, shallow, central part of the eddy than in the deeper and
peripheral parts of the eddy.
OFFICIAL PUBLICATION DATE is handstamped in a limited number of initial copies and is recorded
in the Institution's annual report, Smithsonian Year. SERIES COVER DESIGN: The coral Montastrea
cavernosa (Linnaeus).
Library of Congress Cataloging in Publication Data
Bowman, Thomas E.
Vertical distribution of Themisto gaudichaudii (Amphipoda?Hyperiidea) in Deepwater
Dumpsite 106 off the mouth of Delaware Bay.
(Smithsonian contributions to zoology ; no. 351)
Bibliography: p.
1. Themisto gaudichaudii?Vertical distribution. 2. Waste disposal in the ocean?Environ-
mental aspects?Delaware Bay (Del. and NJ.) 3. Crustacea?Vertical distribution. 4.
Crustacea?Delaware Bay (Del. and NJ.)?Vertical distribution. I. Cohen, Anne C. II.
McGuiness, Maura McM. III. Title. IV. Title: Deepwater Dumpsite 106 off the mouth of
Delaware Bay. V. Series.
QL1.S54 no. 351 [QL444.M315] 591s [595.371] 81-18442 AACR2
Contents
Page
Introduction 1
Acknowledgments 1
Nomenclature 1
Geographic Distribution 2
Previous Studies of Vertical Distribution 4
Themisto gaudichaudii in Deepwater Dumpsite 106 4
Location and Water Types 4
Sampling 4
Analysis of Data 7
Results and Discussion 8
Literature Cited 11
Vertical Distribution Graphs (Figures 6-16) 13
in

Vertical Distribution of Themisto gaudichaudii
(Amphipoda: Hyperiidea) in Deepwater
Dumpsite 106 off the Mouth of Delaware Bay
Thomas E. Bowman, Anne C. Cohen,
and Maura McManus McGuiness
Introduction
The name "Deepwater Dumpsite 106" (DWD-
106) is applied to a rectangular area 90 miles east
of the mouth of Delaware Bay (Figures 1, 2)
where acid waste, industrial chemicals, and ra-
dioactive wastes are dumped, principally in liquid
form either in solution or in suspension. Evalua-
tion of the effects of this waste disposal upon the
environment is the responsibility of NOAA,
which in 1972 established a program to examine
the physical and biological characteristics of
DWD-106. This program included the Nektonic
Sampling Program, consisting of 4 cruises that
sampled the nekton with 10-foot Isaacs-Kidd
midwater trawls (IKMT). Samples from one of
these cruises (cruise 2, 23 Jul-3 Aug 1975) offered
the opportunity of examining the depth distri-
bution of the most abundant pelagic amphipod
in DWD-106, Themisto gaudichaudiiGuerin-Mene-
ville, 1825, and the study reported herein was
therefore undertaken.
ACKNOWLEDGMENTS.?We are most grateful to
Robert H. Gibbs for the collections used in our
study, for analyzing our data with his computer
Thomas E. Bowman and Anne C. Cohen, Department of Invertebrate
Zoology, National Museum of Natural History, Smithsonian Institu-
tion, Washington, DC 20560. Maura McManus McGuiness, 2008
121st St., S.E., Bellevue, Washington, 98004.
program, for responding patiently to our inquiries
concerning data and collection methods, and for
reviewing the manuscript. Lee-Ann Hayek gave
useful advice on statistical treatment of our data.
Frank D. Ferrari and Edward J. Zillioux also
reviewed the manuscript.
Nomenclature
The amphipod genus Themisto Guerin-Mene-
ville, 1825, was replaced with Euthemisto by Bov-
allius (1887) because it was a junior homonym of
the nudibranch Themisto Oken (1815). Parathemisto
Boeck (1870) and Themisto Guerin-Meneville were
combined under Themisto by Stephensen (1924),
corrected by Barnard (1930) to Parathemisto, the
name currently used by most zoologists. Volume
3 of Okens Lehrbuch der Naturgeschichte (1815) was
placed, however, on the Official List of Rejected
Works by Opinion 417 of the International Com-
mission of Zoological Nomenclature (1956),
which ruled that no name published in Oken's
volume 3 acquired the status of availability by
reason of having been so published. Consequently
the name Themisto, first made available by
Guerin-Meneville and a senior synonym of Par-
athemisto, is the valid name of the amphipod
genus.
1
SMITHSONIAN CONTRIBUTIONS TO ZOOLOGY
72? 70?
DEEP WATER
DUMPSITE 106
38?N
74? 72?
FIGURE 1.?Location of Deepwater Dumpsite 106 (from Ingham et al., 1977).
38?N
70?W
Geographic Distribution
Themisto gaudichaudii is an abundant and widely
distributed hyperiid amphipod, inhabiting sub-
polar and temperate seas of both hemispheres. Its
worldwide distribution, especially in the Southern
Ocean, is summarized by Kane (1966). It is one
of a few truly bipolar pelagic animals (Dunbar,
1979), but is known in the North Pacific only
from the Yellow Sea, East China Sea, and Korea
Strait (Yamada, 1933; Bowman, 1960). A few
scattered records from the tropical Atlantic (Vos-
seler, 1901) are incompatible with the distribution
pattern of T. gaudichaudii and require confirma-
tion before they can be accepted. Similarly, Ev-
ans' (1961) records of Themisto species in the
tropical Atlantic at temperatures of about 25?-
27?C are questionable.
Themisto gaudichaudii is a variable species, oc-
curring in several forms previously accorded spe-
cific status. Recently Sheader and Evans (1974)
demonstrated that the characters used to separate
T. gaudichaudii and T. gracilipes (Norman) depend
on body size and undetermined conditions affect-
ing development. They therefore reduced T. gra-
cilipes to a junior synonym of T gaudichaudii. This
merging expands the global distribution of T.
gaudichaudii given by Kane (1966) to include the
Mediterranean Sea (Stephensen, 1924), Tristan
da Cunha (doubtfully ? Stephensen, 1924), the
NUMBER 351
72#50*W
99*30'N
38*90'N
72*50'W 72*00'W
FIGURE 2.?Bathymetry in vicinity of Deepwater Dumpsite 106 (from Bisagni, 1977).
SMITHSONIAN CONTRIBUTIONS TO ZOOLOGY
Juan Fernandez Islands, and the Yellow and East
China seas (Bowman, 1960).
The southern limits of T. gaudichaudii along the
North American Atlantic coast have not been
determined, but it is known to penetrate Virgin-
ian coastal water to the latitude of the mouth of
Chesapeake Bay, about 37?N (Bigelow and Sears,
1939; Grant, 1979; Short, 1980). At present the
zooplankton composition of shelf waters between
Cape Henry and Cape Hatteras is largely unex-
plored, but Virginian coastal water overlies the
shelf south to Cape Hatteras (about 35?20'N),
and T. gaudichaudii probably will be found to
reach Cape Hatteras also. Themisto gaudichaudii
does not occur south of Cape Hatteras. It was not
present in numerous plankton samples collected
between mid-Florida and Cape Hatteras and ex-
amined for calanoid copepods (Bowman, 1971).
Previous Studies of Vertical Distribution
From past studies of vertical distribution in T.
gaudichaudii, 4 general statements can be made.
1. The vertical range appears to be consider-
able, from the surface to depths sampled by nets
towed at the end of 1000 meters of wire (Stephen-
sen, 1924), or about 500 meters actual depth
(Schmidt, 1912). The latter, stations of the Dan-
ish Oceanographical Expeditions 1908-1910,
were mainly sampled by non-closing nets, Peter-
sen's young fish trawl (Schmidt, 1912). Despite
Schmidt's confidence that the catches of these
trawls reflected with reasonable accuracy the pe-
lagic biota present at depths at which the trawls
were towed horizontally, confirmation with clos-
ing net samples has been desirable. The present
study presents such confirmation for the first
time.
2. Most of the population occurs in the upper
100-300 meters.
3. Within the upper 100-200 meters a marked
diurnal vertical migration takes place, toward the
surface at night and away from the surface during
daylight (Hardy and Gunther, 1935; Bary, 1959;
Kane, 1966; Everson and Ward, 1980).
4. Juveniles tend to be more restricted than
adults to shallow depths (Bigelow, 1926; Bous-
field, 1951).
Themisto gaudichaudii in Deepwater
Dumpsite 106
LOCATION AND WATER TYPES.?DWD-106 is
located at 38?40'N to 39?00'N and 72?OO'W to
72?30'W, over the continental slope and rise,
where water depths range from 1550 m in the
northwest corner to 2750 m in the southeast
corner (Figures 1, 2). It lies within "one of the
most variable and complex oceanographic regions
of the entire western North Atlantic" (Ingham et
al., 1977). The surface layer is normally occupied
by slope water, which lies between fresher shelf
water to the west and more saline gulf water to
the east. Occasional intrusions of the cool, low
salinity shelf water occur into the upper layers of
the dumpsite, and anticyclonic warm core eddies
spun off from the Gulf Stream pass through the
dumpsite at irregular intervals, about 3 times a
year, at least partly covering the dumpsite about
20% of the time. Such an eddy occupied the
dumpsite throughout cruise 2, with its center just
northeast of the dumpsite (about 39?45'N,
71?55'W). Its presence was clearly shown by the
depression of the 15?C isotherm to depths of up
to more than 500 m, resulting in a nearly iso-
thermal pool of 15?-16?C North Atlantic central
water below about 50 m, surrounded by slope
water (Figure 3). Above 50 m the temperature
rose sharply to 24?-26?C at the surface (Figure
4). Eddy water in the dumpsite was arbitrarily
divided into an eastern area in the core of the
eddy and a western area in the periphery of the
eddy (Figure 5).
We have not been able to obtain from our
sample analyses any correlation between the
depth distribution of Themisto gaudichaudii and
physical oceanographic conditions. For further
details of the latter consult Goulet and Haus-
knecht (1977).
SAMPLING.?Samples were collected with a 10-
foot IKMT lined with 3/8 inch (0.95 cm) stretch
mesh. At the cod end was a 1-m plankton net of
49 48 50 47 79 78 77 76 75 71 70
DISTANCE (NAUTICAL MILES)
FIGURE 3.?East-west vertical section through Deepwater Dumpsite 106 in July 1975 showing
temperature structure (?C) in upper 2000 m. (from Goulet and Hausknecht, 1977).
49 48 50
100-
150 H
STATION NUMBER
47 79 78 77 76 75 71,72 70 38 16,17
FIGURE 4.?East-west vertical section through Deepwater Dumpsite 106 in July 1975 showing
temperature structure in upper 200 m (from Goulet and Hausknecht, 1977).
SMITHSONIAN CONTRIBUTIONS TO ZOOLOGY
72#W
39#N -
FIGURE 5.?Trawl positions, cruise 2 (symbols indicate beginning position of tow; Stars = trawls
made in slope water; squares = transitional water; open circles = western eddy; closed circles
= eastern eddy; dashed line = DWD-106; from Krueger, et al., 1977).
no. 00 (0.752 mm) mesh equipped with a 4-
chambered, cod-end sampler designed after Aron
et al. (1964). Four samples, 3 closed, 1 open, were
taken during each fully successful haul. Fishing
depth was monitored electronically aboard ship,
allowing selection of depths at which chambers
were closed. A flow meter was not used and
specimens will be reported as numbers per hour
of towing time rather than as numbers per volume
of water filtered.
The main trawling program consisted of
oblique tows. In each tow the net was sent to the
desired depth with all chambers and the cod end
open; then the net was retrieved slowly and the
chambers were closed at selected intervals of de-
creasing depth. These discrete-depth samples
were taken from the surface to 800 m, with a few
samples as deep as 1800 m. Sampling intervals at
night in the upper 200 m were narrow in order to
determine more precisely the upper levels reached
by vertical migrators. Because this stratum nor-
mally is sparsely inhabited during the day by
midwater nekton, the principal target of the pro-
gram, sampling of shallow strata was often cur-
tailed in the daytime in favor of sampling the
deeper strata more effectively.
Because a flow meter was not used, quantita-
tive comparisons of samples are not reliable. The
quantity of water strained per unit of time may
have varied considerably because of fluctuations
in ship speed and in the amount of clogging of
the net. The most serious offenders in net-clogging
NUMBER 351
were salps, which were abundant at DWD-106.
All 3 of the 25-1 m night tows and 2 of the 3 50-
26 m night tows in the western eddy collected
large numbers of salps. One to 4 quarts of salps
were discarded from each of these samples at the
time of collection. Unfortunately some Themisto
may have been thrown out with the salps, since
juvenile Themisto gaudichaudii have been observed
clinging to salps in large numbers (Madin and
Harbison, 1977).
Another factor affecting the quantitative reli-
ability of the cruise 2 samples is the patchy
distribution of T. gaudichaudii due to the formation
of swarms (Hardy and Gunther, 1935; Bary,
1959; Nemoto, 1959; Gray and McHardy, 1967;
Everson and Ward, 1980), which appear to be
limited to near-surface waters; at greater depths
swarms disperse (Kane, 1966). Thus patchiness
may not be a significant factor except in shallow
strata.
Some samples, collected by oblique tows that
extended over 2 or more strata, were not used
because samples of the included smaller strata
were available.
Night samples are those taken from after sunset
to before sunrise; day samples are those from after
sunrise to before sunset. Only 2 samples were
collected less than 2 hours from sunrise or sunset:
1 hour 59 minutes, and 1 hour 54 minutes. Thus,
in effect day began 2 hours after sunrise and
ended 2 hours before sunset; night began 2 hours
after sunset and ended 2 hours before sunrise.
ANALYSIS OF DATA.?Both "compressa" and
"bispinosa" forms (Stephensen, 1924; Hurley,
1955) of T. gaudichaudii were present in the cruise
2 samples, but of the total of 937 specimens
caught, only 4 (0.43%) were "bispinosa." This
agrees with reports that "compressa" is more
abundant in inshore waters and at lower lati-
tudes, whereas "bispinosa" predominates in off-
shore waters and at higher latitudes (Bigelow,
1926; Barnard, 1932; Kane, 1966). Our analysis
is concerned with the "compressa" form only.
The average number of specimens/hour for
each stratum in both eddy regions was derived
by calculating the number/hour for each sample
in that stratum, summing the numbers/hour for
all samples of the stratum, and dividing by the
number of samples collected in the stratum, in-
cluding samples that did not contain Themisto:
2 -
LTs
Is
? Av. no. of specimens
/hour (for any stratum)
where Ns = number of specimens in a sample, Ts
= number of hours of tow in collecting that
sample, and s = an individual sample from the
same stratum. This method gives equal weight to
all the individual samples and was chosen in
preference to calculating the sum of all specimens
in a stratum divided by the number of hours
towed in the stratum:
E Ts
because the former is an average of the number/
hour of the replicate samples rather than an
average of the total catch in the stratum. Aver-
aging the replicates weights for patchiness. Unless
all replicate numbers/hour are equal, averaging
the total catch discards the information derived
from replicates.
The sex, length, and state of maturity were
determined for all specimens of Themisto. The
stages, listed below, are based on the state of
development of the oostegites in females, and of
the second antenna in males, as in Kane (1963).
Stage Oostegites (2)
1 Absent
2 Up to 1/2 length of gills
3 Subequal to gills
4 (adult) Longer than gills
Segment of ant. 2
flagellum (6)
1-4, short
~8, short
~13, short
~21, elongate
Conclusions offered herein concerning the ver-
tical distribution of T. gaudichaudii are drawn from
inspection of graphs that summarize virtually all
of our data (Figures 6-15). Each graph has right
and left components. The left component shows
the numbers of amphipods/hour of towing. The
base of each triangle, on the depth ordinate,
encompasses the depth interval sampled; the apex
8 SMITHSONIAN CONTRIBUTIONS TO ZOOLOGY
of the triangle indicates the mean number of
amphipods/hour for all tows sampling that depth
interval and is read from the abscissa (in Figures
8-15 solid lines ? 9, dashed lines = 6*). The
number/hour for individual samples is indicated
by a circle in Figures 6 and 7; and by a circle
(9) or a triangle (6*) in Figures 8-15; samples that
caught 0 amphipods are not shown, but are in-
cluded in the mean numbers/hour.
In the right component each tow is represented
by a triangle. The base of the triangle encom-
passes the depth interval sampled; the apex shows
the duration of the tow and is read from the
abscissa. The numerals, displayed sequentially
beyond the apices, indicate the numbers of Them-
isto collected during each tow.
Comparisons based on these graphs should be
tempered by the lack of reliable information on
volumes filtered, amount of net clogging, and
patchiness. These factors must have contributed
to the considerable variation in numbers of Them-
isto collected by replicate tows at the same depth
interval. For example, the 3 replicate night tows
at 0-25 m in the eastern eddy caught 7, 24, and
99 specimens (84, 720, and 1188 specimens/hour).
The 5 replicate night tows in the western eddy at
101-125 m caught 0, 0, 0, 3, and 48 specimens
(0, 0, 0, 36, and 720 specimens/hour).
RESULTS AND DISCUSSION.?A condensed sum-
mary of the sampling and results is given below.
No. of samples
Total hrs. sampled
Av. and range of sam-
pling time (min.)
Eastern eddy
Day
29
7.9
16.4
(3-92)
Night
27
4.4
9.85
(2-55)
Western eddy
Day
23
9.8
24.6
(6-76)
Night
39
6.6
10.2
(1.2-38)
Total Themisto caught 155 442 140 182
Mean Themisto/hr. 40.28 211.7 17.28 52.44
Greater numbers of Themisto were caught at
night than during the day, especially in the east-
ern eddy. The mean catch/hour was about 5
times greater at night in the eastern eddy and 3
times greater in the western eddy. If almost the
entire vertical range of the populations was sam-
pled, as seems probable, it must be concluded
that net avoidance was greater during the day.
Like most hyperiid amphipods of the infraorder
Physocephalata, Themisto has very large com-
pound eyes and may be able to detect daylight
and moving objects at depths of several hundred
meters, especially if the moving object stimulates
bioluminescent flashes. The most sensitive known
eyes, those of certain deep-sea fishes (Denton and
Warren, 1957), may be able to detect some day-
light down to about 1000 m (Clarke and Denton,
1962). While dip-netting with a night light from
shipboard at night, it was found that very rapid
sweeps were required to capture the hyperiid
amphipod Parapronoe crustulum, a rapid-swimming
species comparable in size to T. gaudichaudn (un-
published observations, T. E. Bowman). If T.
gaudichaudii approaches P. crustulum in swimming
speed, significant numbers would be expected to
evade the IKMT in depths where the latter was
visible.
The greatest numbers of Themisto were taken in
shallow tows, with the majority of specimens
being collected above 125 m. Thus at night 86%
and 92% of the specimens in the eastern and
western eddies, respectively, were caught between
125 m and the surface. (Percentages are based
upon numbers/hour.) Only a small fraction of
the population occurred below the 15?-16?C
isothermal layer; in the eastern eddy 98% of the
night specimens were collected between 300 m
and the surface. About 1/3 of the eastern eddy
night specimens (34%) occurred in the 0-25 m
layer, above the isothermal layer.
Even though the population was concentrated
in the upper layers, significant numbers were
caught at the greatest depths sampled, e.g., a
sample of 53 specimens (265/hr) in a 800-1000
m day tow in the eastern eddy. Stages 2, 3, and
adults (both sexes) occurred from the shallowest
(1-25) to the deepest (1205-1337 or 1425-1839
m) strata sampled. Stage 1 occurred from 1-25 m
to 600-700 or 600-800.
Only 1 haul was taken deeper than 1400 m, a
day trawl between 1839-1425 m, which caught 1
stage 2; this single specimen could easily have
been a contaminant. Whether Themisto was pre-
NUMBER 351
sent deeper than 1426 m cannot be determined
from the DWD-106 data.
Life history stages and sexes were not segre-
gated by tows. In all tows catching at least 8
specimens, 3-8 of the 8 stages (4 male, 4 female)
were caught per tow; in the 5 tows catching more
than 30 specimens, 6-8 of the 8 stages were caught
per tow.
In general Themisto was more abundant in the
eastern eddy than in the western eddy. This
difference in abundance was also reported for
mesopelagic fishes by Krueger et al. (1977), who
discussed 3 possible explanations. (1) Pollutants
were dumped mainly at the western boundary of
DWD-106, this being nearest to the coast. (2)
Eddies have a stagnant core in which particles
tend to be trapped and accumulate. Outside the
core, which lies in the east eddy, the anticyclonic
movement of the water tends to spin off particles.
A contrary view, however, is that water is en-
trained at the outer edges of the eddy and is lost
at the surface. (3) There is an inshore-offshore
gradient in abundance, increasing from slope wa-
ter to the east eddy.
It cannot be determined whether or not any of
the above explanations were operative for Them-
isto abundance. Concerning pollutants, all 9 day-
time tows beween 151 and 600 m in the western
eddy caught 0 Themisto, and 6 of these 9 tows
were made near or even in the wakes of ships
dumping industrial waste. But Themisto was
caught in the other 4 tows taken near such dump-
ing, i.e., the only 2 tows taken between 0-151 m
and 2 of the 4 600-800 m tows.
The graphs (Figures 6-15) show differences in
the details of vertical distribution between males
and females and among the 4 growth stages.
These differences are not related to known aspects
of macro- or micro-behavioral patterns reported
for pelagic amphipods, nor are they distinctive
enough to suggest new interpretations.
Considering the total catch (Figure 16), the
percentage of females, 47.5-63.2, was higher than
that of males, 36.0-52.5. A possible explanation
for the lower catch of males is that they are
stronger swimmers and therefore more successful
in evading the net; however, we have no support-
ing evidence for this explanation. In some am-
phipods the apparent greater swimming ability
of the males is reflected in the development of
their pleopodal protopods, which are thicker than
those of the females and contain larger muscles.
Such differences are not evident in Themisto. The
low numbers of stage 1 juveniles could be ex-
plained at least 2 ways: (1) the duration of this
stage is short; (2) juvenile Themisto are much more
likely than adults to be associated with salps or
hydromedusae (Sheader and Evans, 1975; Madin
and Harbison, 1977).
The small numbers of stage 3 females is puz-
zling. Our designation of growth stages to aid in
analysis does not imply, however, that these stages
are of equal duration. Figure 16 suggests that the
duration of stage 3 may be shorter than that of
stage 2.
The well-documented migration of Themisto
upward at night and downward during the day
in the upper 100-200 m is not apparent in the
DWD-106 samples, perhaps because the day tows
did not partition the upper layers finely enough,
especially in the western eddy, where only 2
discrete-depth, day tows were made between the
surface and 150 m.
The shallowest eddy samples may have been
collected at temperatures close to the maximum
normally experienced by T. gaudichaudii, 24?-
26?C. Whether or not the amphipods were living
at such temperatures is not known, since we do
not know at what depths in the 0-25 m interval
they were collected. If they were captured at 24-
25 m their ambient temperature probably did not
exceed 18?C. In any case, not only did substantial
numbers of T. gaudichaudii survive and breed for
about 1 month in the warm-core eddy, but the
species was most abundant in the shallowest,
warmest parts of the eddy.
Krueger et al. (1977) found that the presence
of the eddy during cruise 2 increased markedly
the proportion of the northern Sargasso Sea fish
species over that normally present in DWD-106.
Eddy-free samples of amphipods from DWD-106
have not been available for comparison, but the
10 SMITHSONIAN CONTRIBUTIONS TO ZOOLOGY
composition of the amphipod fauna of the cruise
2 samples gives little evidence of an influx of the
tropical species characteristic of the Gulf Stream
or Sargasso Sea. Themisto gaudichaudii was by far
the dominant species, followed by Phronima seden-
taria, a widespread species that ranges in the
Atlantic from about 50?S to about 45?N in the
western Atlantic and the west coast of Ireland in
the eastern Atlantic (Shih, 1969). Less common
were species of Vibilia and Scina. In general the
number of amphipod species was low, and there
was a virtual absence of species known to be
limited to tropical waters.
A possible explanation for the near absence of
tropical amphipod species is suggested by Goulet
and Hausknecht's (1977) statement that the eddy
was "old and degenerate. It had lost much of its
original warm core character in the upper 50 m
and its original North Atlantic Central Water
below 500 m had been mixed into Deep Slope
Water."
An admirable review by Laval (1980) summa-
rizes in detail the evidence that all hyperiids are
associates of gelatinous zooplankters, at least in
their early stages, and not "planktonic" in the
sense that this term applies to calanoid copepods
and euphausiids. Some hyperiids remain closely
associated with gelatinous hosts throughout their
lives; others, including Themisto, remain associ-
ated while juvenile but later become essentially
free-living and pelagic, returning occasionally to
rest or feed upon a host. We do not know how a
post-juvenile Themisto apportions its time between
a truly pelagic life and "a benthic-like existence
on the pelagic substratum provided by gelatinous
animals of the zooplankton" (Laval, 1980). Nor
do we know whether the vertical migration of
Themisto within the upper 200 m is performed
actively by the amphipod or is due only to the
migration of its hosts. Such questions cannot be
answered by examination of plankton samples as
in the present study, but require direct observa-
tions in the field or in laboratory aquaria.
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Vertical Distribution Graphs
(for explanation of graphs see pp. 7, 8)
14
DAY NIGHT
SMITHSONIAN CONTRIBUTIONS TO ZOOLOGY
DAY . NIGHT
300 200 100
amiphipodsx
100 200 300\ ^800 1300 75 50 25 25 50
hour minutes of sampling
FIGURE 6.?Vertical distribution of Themisto gaudichaudii, eastern eddy, all stages and both
sexes combined.
NUMBER 351 15
25.
50.
75.
100.
125.
150.
m
300.
400.
500.
60Q
70Q
800.
DAY NIGHT
0,2,17,5,9,0,2,0
200 100
DAY NIGHT
K),2,2
79
2 0
?
 3 0
?
 400 500 75 50 25 25
minutes of sampling
FIGURE 7.?Vertical distribution of Themisto gaudichaudii, western eddy, all stages and both
sexes combined.
16 SMITHSONIAN CONTRIBUTIONS TO ZOOLOGY
DAY NIGHT
25.
5Q
75
100
125.
D150.
E
^200
H
300.
400.
500.
600.
80Q
900-
1400
m
50
j A
|A
9 0.0.0
cr 0.0.0
50 100 150
DAY . NIGHT
,0,0 9
.i.o d
minutes of sampling
50
F"IGURE 8.?Vertical distribution of Themisto gaudichaudii, eastern eddy, stage 1.
NUMBER 351 17
25.
50.
75.
100.
125.
D15Q
E
^200,
H
300.
400.
500.
600.
70Q
800.
900-
1300
m
DAY NIGHT DAY . NIGHT
minutes of sampling
20
 0 20
a m p h i p o c ^
FIGURE 9.?Vertical distribution of Themisto gaudichaudii, western eddy, stage 1.
18 SMITHSONIAN CONTRIBUTIONS TO ZOOLOGY
DAY NIGHT
900-
140Q
DAY . NIGHT
,0.1.19 ?
',1,10 d
?2/|\l,0.5
Cl\|/0,0.2 d
9 0/|\24,2 9
0-0X1/22.2 d
minutes of sampling
FIGURE 10.?Vertical distribution of Themisto gaudichandii, eastern eddy, stage 2.
NUMBER 351 19
m
25.
50.
75.
100.
125.
150.
200.
300.
400.
500.
600.
70Q
800
900-
1300
DAY NIGHT
i
DAY .
L A
n
i i
V
I /
h /
f 917/
r \
r >L 9 0 < x ^
? oroV.
L ^O0e^-^^%P dO^^^QSD
o'oS^
9q^b
rfO^P
90/0
L 9 0 , ^
I tfl^^
NIGHT
W>,2,1 9
rOAOtf
^0,1,0 9
K'lAO d
pi
W 3 3 ?
/2A12 d
W0M0P d
hfiPX2P 9
W0,0,0,0,0 d
WoAJ.OP 9
Wmpp d
2^SP9
0/^3 cr
oS^>o9
0\^^^09
d^-^09
9/^-^0 rf
0^^.0 9
V i ^ ^ l cr
oV^>,o9
J 9 0 , 0 , 5 , 1 ^ 0 , 0 , 0 ^ - ^ ^ ^ ^ ^ ^
1 inIII iiinrin s ^ H
100
 (J 100 200 300
Xteyf
75 50 25
 0
 2
'
5
minutes of sampling
FIGURE 11.?Vertical distribution of Themisto gandichaudii, western eddy, stage 2.
20 SMITHSONIAN CONTRIBUTIONS TO ZOOLOGY
25.
5Q
75L
100.
125.
D15Q
E
^200
H
300.
400.
500.
600.
800.
900-
1400
m
50
9 0.0.0.
C0.0.0
50 100 150
DAY
9o/
o-2\
9 0/
COl
9 0/
0T3\
90 /
9 0/
NIGHT
K2.17 9
tfO.0,10 or
Vo.0 9
#0,4 tf
ko.0.1 9
^0,0,2 cf
\0,0,l 9
/0.0,10o-
\p.0 9
Wo 9
p.O9
9 0.0.0.1,
coppp
90.0,0.
0-1.0.0
9 0,0,0.0.
0-0.0,0.1
09
>09
9 P<
rfO.O
90.0,0,1
cr 0,0.0,0
.o 9
3,0,0 or
75 50 25
minutes of sampling
ir
FIGURE 12.?Vertical distribution of Themisto gaudichaudii, eastern eddy, stage 3.
25 50
NUMBER 351
25.
50.
75.
100.
125.
D150.
E
P
T
H
300J
m
400.
500.
600.
700]
800.
900-
1300
21
DAY NIGHT
50
DAY . NIGHT
,0,00 9
0,0,3 or
PA1 9
1,Q0o-
50
minutes of samplingamphipocyhour
FIGURE 13.?Vertical distribution of Themisto gaudichaudii, western eddy, stage 3.
22 SMITHSONIAN CONTRIBUTIONS TO ZOOLOGY
25.
50.
75L
10Q
125.
D
E
!j!2oa
H
m
3oa
400.
50Q
60Q
800.
900-
1400
DAY NIGHT DAY . NIGHT
> *
90,0,0,
tfO.0,0,
100 o 10? 200 300 400 500 | 75
amphip(xte/hour
50 25
 0 25
minutes of sampling
50
FIGURE 14.?Vertical distribution of Themisto gaudtckaudii, eastern eddy, stage 4 (adults).
NUMBER 351 23
800.
900-
1300
DAY . NIGHT DAY . NIGHT
100 200 I 75 50 25
 0 25
minutes of sampling
100 o
FIGURE 15.?Vertical distribution of Themisto gaudichaudii, western eddy, stage 4 (adults).
24 SMITHSONIAN CONTRIBUTIONS TO ZOOLOGY
100%
75
50
25
EAST EDDY WEST EDDY
FIGURE 16.?Themisto gaudichandii, growth stages in eastern and western eddies of DWD-106,
in percentages of total specimens caught in cruise 2 (juveniles = stages 1-3, adults = stage 4).
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