Relationships
between vertical
four species of cuphausiids’
migration
and diet in
Vernon J. H. Hu2
Department
of Oceanography,
University
of IIawaii,
IIonolulu
968.22
Abstract
Diel changes in stomach weight of euphausiids
showed a close relationship
between the
infcrrcd increase of feeding at night and strong vertical migration in four sp&es. Thysanopoda aeyualis and Thysanopoda
monncantha,
species with a small perccntagc (4 and 13%)
of depth overlap between their day and night vertical distributions
(i.e. large diel vertical
migration), exhibited a marked increase in stomach weight at night. Thysanopoda
pectin&u
(39% overlap), a moderate vertical migrator, showed a constant stomach weight throughout
day and night. Nematobruchion
sexspinosus (56% overlap), a very weak or nonmigrator,
showed increased stomach weight during the day. Increases in stomach weight at night were
accompanied by an increase in fluorescence
and the amount of phytoplankton
remains found
in the stomach, but neither was found in the species which did not show increased stomach
weight at night,
Diel vertical migration is characteristic
centage of overlap in the day and night
of many marine organisms and presum- ’ vertical distributions
for each of the varably has a number-of
important-adaptaious species. Thysanoessa raschii lives
between 150 m and the surface while E.
tional advantages. One such advantage
migrate
would be increased feeding at night by diomedeae and T. monacantha
from 500 m into the upper 100 m. If prey
predators finding a high density of prey
concentration
decreases logarithmically
at shallower depths. Studies of feeding
with depth, T. ruschii would not be expatterns of myctophid fishes (e.g. Anderson 1967; IIolton
1969) do suggest inposed to as large a change in prey density
creased feeding at night. IIowever,
prcas a stronger vertical migrator. If feeding
is related to the density of prey, this may
vious studies of euphausiids do not show
explain why T. raschii does not show as
increased feeding at night consistently.
much of a change in feeding intensity as
Mauchline
(1966) found Thysanoessa
the other species. T-his difference
also
ruschii to have the same stomach fLllncss
implies that enhanced feeding activity is
throughout the day and night. Ponomarnot the only benefit of vertical migraeva (1954,197O) found increased stomach
tion.
fullness during the night in Euphausia
Maynard et al. (1975) described
the
diomedeae; Roger (1973) found increased
composition
of the micronekton
and the
stomach fullness during late afternoon in
percentage
of each major taxonomic
Thysanopoda
monacantha
and several
group migrating into the upper 400 m at
other species of relatively
deep-living,
night in Hawaiian waters. Of the euphaumigrating euphausiids.
siids found deeper than 400 m during the
A possible reason for these divergent
day, 64% migrated into the upper 400 m
results may be differences
in the perat night. Most of their euphausiids
were
probably species of Nematohrachion
and
l This paper is part of a thesis submitted to the
Graduate Division
of the University
of Hawaii in
Thysanopoda.
partial fulfillment
of the requirements
of the Master
The objectives of my study were to inof Science degree in Oceanography.
This research
vestigate the relationships
between verwas partially
supported by National Oceanic and
diet, and feeding chrotical migration,
Atmospheric
Administration
contract
03-6-02235112.
nology in Nematohrachion
sexspinosus,
’ I thank T. A. Clarke and R. E. Young for providThysanopoda
uequulis, T. monacantha,
ing shiptime and samples. I also thank J. IIirota for
and Thysanopoda
pectinuta. A study of
help in preparation of this manuscript and for many
the diet of the three species of Thysanodiscussions.
LIMNOLOGY
AND
OCEANOGRAPHY
MARCH
1978, V. 23(2)
Vertical
migrution
poda is a way to invcstigatc trophic niche
separation or the manner in which potential competition
may bc reduced in the
planktonic
habitat. All three species arc
similar morphologically:
they have nearly llniform thoracic legs and have bladeshaped mandibles. I Iowevcr, they differ
in body length and vertical distribution.
Thysanopodu
aequulis (16 mm, median
length) has about the same depth distribution as T. monucunthu (28 mm, median
length), but is smaller, so that comparing
their diets could give insight into the rclationship
between body size and diet.
Thysunopodu monucunthu
and T. pectinutu (31 mm, median length) are of similar
size but appear, in this region, to differ
somewhat in vertical distribution;
comparing their diets could show whether or
how food rcsourccs may be separated
vertically.
The genus Nemutohruchion
is characterized by an elongated pair of third thoracic legs whose dactyl varies, according
to species, from several long spines to a
true chela, and by bilobular eyes. Comparing the diet ofN. sexspinosus (23 mm,
median length)
with that of the two
slightly
larger species, T. monucunthu
and T. pectinutu,
may show feeding differences rclatcd to the elongated thoracic
legs and bilobular eyes.
Method
Material for this study was collected off
the west coast of Oahu, IIawaii (21”1520’N, 158”15-30’W).
The annual surface
water temperature
ranged from about
24”-27”C, and the thermal structure of
the water column did not vary much during the year. There was no well defined
mixed layer present;
the temperature
from the surface to 400 m decreased by
3.8”C per 100 m (Fig. 1). The water in the
study area was about 2,500 m deep. A
chemical
description
is available
for
nearby
station
Gollum
(22”10’N,
158”OO’W) (Gordon 1971).
Vertical distributions
of euphausiids
were determined
from horizontal,
stratified tows made with a 3-m (mouth width),
opening-closing
Tucker trawl during fall
297
und diet
TEMPERATURE
IO
1
I
0:
(“Cl
20
I
w-
30
I
n
-
H
H
H
H
c--l
t---i
oo-H
II
w
L+
H
H
H
oo-H
H
H
6 oo--
c(
I
Ii
H
Ii
800
i
’
Fig. 1. Temperatures
taken by expendable bathythermographs
in sampling area. Horizontal
bars
renrcscnt maximum and rniniml~m temneraturc taken at depth on 26 March 1972,4 May 1)73, 17 June
1973, 30 July 1971, 2 October 1972, and 9 Deccmbcr 1973. (Modified
from Maynard ct al. 1975.)
and winter of several years (l970-l973),
resulting
in composite,
time-averaged
distributions.
Sampling
depths were
measured to the ncaicst 25 m with model
1170 Benthos time-depth recorders. Only
adult specimens were counted and population densities are expressed as adults
per 10’ m3. The percentage of overlap
between day and night distribution
(P)
for each species was computed as
P=
al
D
- Sd
d -
x 100,
sn
where D,, Dd = the deepest depths at
which a specimen was caught and S,,
Sd = the shallowest
depths at which a
specimen was caught during the night
and day. Differentiating
between female
T. uequulis
and Thysunopodu
ustylutu
is very difficult
(Brinton 1975); females
of both species were grouped
as T.
uequulis, since male T. uequulis outnu .mbcred T. ustylutu 23 to 3.
Specimens
used to determine
diel
298
HU
~28
1 29 AUGl
AUE
k
29 GG
F
iti
400
1 28 A”G
L3
1
(25
c -J1_ !“%,
4~--2_8AJG-4
29
AUG
(I_
30
AUG
ll-
MAY
E.A_u_G-j
30
---_--
,
1
AUG
i I
29
A”G
30
,
AUG
30
AUG
t2_6_hltY,
1100
I
0200
I
I
0600
I
I
1000
I
I
1400
I
I
1800
I
I
2200
of oblique tows used for feeding chronology
data. Tows with
Fig. 2. Depth and time distributiorrs
same type of horizontal line were grouped in same period. Numbers shown at top and bottom of brackets
at left represent intcndcd sampling depth range (trawl was open from launch to retrieval).
Date of each
tow is shown above tow. All tows wcrc taken in 1973 except those in May, which were taken in 1974.
feeding patterns were caught by oblique
tows with a 3-m Isaacs-Kidd midwater
trawl. Figure 2 shows the time and depth
of tows from which specimens were taken. Tows were grouped into four night,
three day, and dawn and dusk periods.
These tows were of shorter duration than
those used to detcrminc
vertical distributions and permitted a better picture of
diel feeding patterns. Aboard ship, catches were immediately
placed in 10% Formalin-seawater
solution to retard digcstion of stomach contents. Stomach fullness
of stomach contents
an d compaction
were determined
for each adult spccimen under a dissecting
microscope.
Where possible at least 30 individuals
were examined for each period.
Stomach weight was calculated
from
visual estimates of stomach fullness and
stomach compaction.
Stomach frrllness
was scored into five classes as was done
by Ponomareva (1970). Empty stomachs
were placed in class 0, those only onefourth full were placed in class 1, those
half-full in class 2, and so forth. This scoring system is feasible since euphausiid
stomachs retain their shape, even when
empty.
Compaction
of stomach contents was
also scored on a five point basis: l-contents immediately
fell apart into separate
particles; 2-contents
fell apart but were
associated in little clumps; 3-30%
to
60% of material remained together in a
central mass; 4-more
than 60% of the
contents held together in a clump; 5-contents were so tightly packed togcthcr
that they had to be teased apart.
Weight of stomach contents was calculated from multiple
regression equations based on data from about 45 specimens of each species. Fullness,
compaction, dry weight of stomach contents
(washed out of stomach), dry weight of
each animal,
and standard
carapace
length were mcasurcd for each specimen. These data were used in equations
for the ratio of stomach dry weight to animal dry weight and for the dry weight of
Vertical
migration
stomach content alone (dependent
variable) as functions of fullness and compaction (independent
variables) as dctcrmined by multiple linear regressions on
the raw data transformed into logarithms.
Stomachs with a fullness classification
of
0 were considered to have a stomach content weight of 0 pg.
Regressions for the dry weight of stomach content had nearly the same correlation coefficient (avg r = 0.70) as for the
ratio of stomach content dry weight to animal weight (avgr = 0.69). The equations
for stomach content dry weight alone
were used, since it was then unnecessary
to detcrminc
the dry weight for each
specimen whose stomach content weight
was needed. Stomach content weights
used to determine
feeding chronology
were calculated
and not obtained
by
weighing all stomach contents.
Because the frequency distribution
of
stomach weight for a given period was
often highly skewed, the median stomach
weight instead of the mean was used as
a measure of central tendency.
The amounts of chlorophyll
a and associated pheopigments
in the stomach
contents of eriphausiids
of all four species were determined
fluorometrically.
The animals used were caught in Novcmbcr 1974 in tows taken both day and night
at depths between 100 and 1,000 m. Approximately
20 fresh-caught euphausiids
of each species were dissected aboard
ship and the frrllncss and compaction OF
their stomachs estimated as described
above. Each stomach and its contents was
placed in 90% acetone and stored in a
freezer within 40 min after rctricval
of
the trawl. Except during transportation
to
the laboratory, samples were kept in a
freezer at all times until analysis. The entire sample was ground, transferred to a
centrifuge tube, and 90% acetone added
to bring tlic volume up to 10 ml, Chlorophyll a and associated phcopigmcnt
content wcrc determined
with a Turner
fluorometer
(Strickland
and Parsons
a and associated
1968). Chlorophyll
phcopigments
arc rcferrcd to as “total
chlorophyll.”
Fluorometric
blanks, obtained by washing out two stomachs per
and diet
299
species under a dissecting
microscope
and treating them like the other samples,
were subtracted from these values.
Stomach contents of 18 spccimcns (9
from day and 9 from night samples) of
each species were examined by light microscopy to check independently
the results of the fluorometric
analysis. A compound microscope at 400~ magnification
was used; any particle not identifiable
with this magnification
was assumed to
have come from the gut of a prey organism. Animal parts such as mandibles and
cxopodites were identified
to taxonomic
group by comparing them with mandibles and appendages from whole animals
in the plankton fraction from the same
tows, The identified prey organisms were
classified into five groups.
1. Organisms with limited locomotion.
These include phytoplankton,
foraminiferans, tintinnids,
radiolarians,
gastropod
larvae, and ctcnophores. All chain-forming phytoplankton
found in a stomach
were assumed to have been eaten as a
single intact chain.
2. Copepods. Identified
by the mandibles and thoracic legs. The number of copepods in each stomach was estimated by
dividing
the number of similar-shaped
mandibles by two. The genera most easily identified were Yleuromamma
(distinguished by mandibles and “body spot”)
and Oithona (distinguished
by characteristic knob at end of last exopodite segmcnt of thoracic legs as well as mandi-
ldcs).
3. Euphausiids
and decapod larvae.
Identified
by the crystalline
cones of
their eyes and their mandibles. The mlmber of individuals
was estimated by dividing the number of mandibles by two.
4. Chaetognaths.
Detected by their cephalic hooks. The number of chactognaths eaten was roughly estimated by
dividing the total number of hooks by 10.
5. Fish. The only fish parts found in the
stomachs were fish scales. I assumed that
all the scales within the stomach cam<:
from the same fish. Judkins and Flcminger (1972) have shown that, in the case of
sergestid shrimp, fish scales may be contaminants eaten in the net. I therefore
DENSITY
DENSITY
NO. ADULTS/IO5
m3
NO. ADULTS/IO5
m3
“f
NIGHT
--
DAY
:I
DENSITY
NO. ADULTS/IO5
m3
DENSITY
NO. ADULTS/I@
m3
Fig. 3. Vertical distribution
of four erlphausiid species. Vertical bars represent depth ranges of samples.
Maximum and minimum depths are given to nearest 25 m and density is rounded to nearest 0.5 adult per
lo5 m’. Dashed iines connect maximum densities found at each depth interval. Duplicate
tows with no
catch that had overlapping
sample depths are offset to facilitate reading.
Vertical
-N.
migration
301
nnd diet
sexsplnosus
1
0600
1200
TIME
I
#
0600
0400
1600
I
TIME
L
0800
1200
TIME
1
l
1
2000
OF DAY
1600
2000
pectinata
I--
I
0400
I
1600
OF DAY
30
‘“t,
I
1200
I
0400
I
l
0600
I
TIME
l
1200
II
1
1600
11
2000
OF DAY
OF DAY
Fig. 4. Diel changes in median dry weight of stomach contents. A-Periods
with 230 specimens; q two periods with < 1.5 specimens (Thysanopo&a pectinata at 1605 and 1913 hours). Vertical bars represent
two qllartilcs, one above and one below median. Solid lines connect those points with significantly
different medians (I’ G 0.05, U-test) and dashed lines those which are not. Vertical scale for Nematobruchion
sexspinosas is about four times larger than those for other spccics.
counted only scales that had an irregular
outline and appeared to be partially digcsted.
Results
Vertical distribution-The
percentage
of overlap between the composite day
and night vertical distributions
showed
considerable
variation (Fig. 3). Nematobrachion
sexspinosus
has a relatively
large overlap in day and night vertical
distribution
(56%). Thysanopoda aequalis and T. monacantha
show the least
overlap (4% and 13%), and T. pectinata
shows a moderate overlap (39%).
I interpreted
this to mean that T. aequalis and T. monacantha
undergo the
most extensive vertical migration, T. pectinata shows a moderate vertical migration pattern, and N. sexspinosus a very
weak vertical migration pattern. If prey
density decreased with depth, T. aeq,uaZis and T. monacantha should be exposed
to a larger change in prey than T. pectinata. Nematobrachion
sexspinosus,
in
contrast, should be exposed to a relativcly more constant prey density and a large
diel change in light intensity.
Feeding chronology--Only
in T. aequalis and T. monacantha
did stomach
weight increase during the night; T. pectinata had no obvious diel changes. Nematobrachion
sexspinosus, on the other
hand, showed increased stomach weight
during the day (Fig. 4).
302
Hu
The median
stomach weight
of N.
sexspinosus increased during the day and
early evening then decreased throughoiit
the rest of the night, showing a sevenfold
variation. If changes in stomach weight
were due to changes in feeding intensity,
this indicates
that N. sexspinosus
fed
mostly during the day and early cvenings. Four of nine pairs of adjacent periods had median
stomach
content
weights that were significantly
different
(P d 0.05, U-test).
Thysanopoda
aequalis showed only a
twofold
variation
in median stomach
weight, which decreased during the day
from 0900 to 1900 hours then increased
throughout the rest of the night, indicating that T. aequalis fed more at night.
Five of nine adjacent periods had median
stomach content weights that were significantly different (P G 0.05, U-test).
The stomach weight of T. monucantha
showed a threefold median variation, decreasing from 1200 to 1900 hours then increasing until 2350 hours and remaining
about the same until 1200 hours. This indicates that T. monucanthu
fed more at
night. Adjacent pairs of intervals
from
1200 to 2400 hours had significantly
different median weights of stomach contents (P d 0.05, U-test).
In contrast to the other species, T. pectinuta
showed no obvious pattern in
changes of median
stomach weight,
which had a twofold variation.
Median
stomach weight increased from 0400 to
0900 hours and then did not change until
1900. After a slight rise at 2200, stomach
weight decrcascd the rest of the night
until 0400 hours. This indicates that T.
pectinata
probably
fed continuously
throughout the day and night. Only two
of nine adjacent periods had median
stomach content weights that were significantly diffcrcnt (P < 0.05, U-test).
Viet-Nematobruchion
sexsginosus did
not differ in day and night pigment content (0.038 bg vs. 0.036 lg total chlorophyll/pg stomach content). The low pigment content was associated with the
small amount of phytoplankton
found in
N. sexspinosus. Phytoplankton
made up
24% of the organisms identified
in N.
sexspinosus.
Thysanopodn
aequalis and T. monacanthu both had significantly
different
(P d 0.05, t-test) day and night pigment
contents, increasing from 0.008 pg total
chlorophyll/pg
stomach content by day to
0.049 by night for T. uequalis and from
0.012 /qq total ch1orophyWp.g
stomach
content by day to 0.035 by night for T.
monucanthu.
Nemoto (196s) rcportcd
one specimen of T. monucantha
with a
half-filled stomach and 1.32 pg total chlorophyll
and one with a qiiartcr-filled
stomach and 0.53 pg total chlorophyll.
In
my study half-filled
stomachs of T. monucanthu had a range from 1.14 to 0.09 pg
total chlorophyll
and quarter-filled
stomachs had a range from 0.52 to 0.3, depending on the degree of compaction.
The change in pigment content was associated with an increase of phytoplankton remains in the stomachs. The avcragc
percentage
of organisms identified
as
phytoplankton
increased during the night
from 23 to 35% in T. aequulis and 16 to
38% in T. monacantha.
The change in day and night stomach
pigment content, as in stomach weight,
was not significant in T. pectin&a.
Pigment content remained at about 0.028 pg
total chlorophyll/~g
stomach content
throughout the diel cycle.
The diet of N. sexspinosus and T. aequulis was comprised mainly of microplankton
and copepods. Thysanopoda
moncuntha
and T. pectinatu
rclicd on
groups 3 and 4 as well as on groups 1 and
2. Figure 5 shows the pcrccntages of prey
taxa identified in each species. The diets
of N. sexspinosus and T. uequalis were
significantly
different
from those of T.
monncanthn and T. pectinatu in their cumulative
frequency
distributions
(P d
0.05, Kolmogorov-Smirnov
test).
Copepods in the stomach contents of
N. sexspinosus and T. uequalis appeared
to be diffcrcnt.
In N. sexspinosus, Pleuromummu outnumbered
Oithona by 37
to 1; in T. uequalis, Oithona
outnumbcrcd Pleuromamma
14 to 2. If there
were no drastic changes in the comgosi-
FISH
CHAETDGNATH
COPEPODS
MICROPLANKTON
CHAETOGNATHS
COPEPODS
MICROPLANKTON
S
1
1
(%I
PERCENT
PERCENT
(%I
OF
OF
DIET
1
DIET
D
;!
x
FISH
CHAETOGNATHS
EU PHAUSIIDS
8 DECAPODS
CHAETOGNATHS
EUPHAUSIIDS
8 DECAPODS
COPEPODS
MICROPLANKTON
:
1
1
t-J
’
1
]
I
PERCENT
I
OF
PERCENT
(%I
1%)
OF
DIET
DIET
304
HU
tion of prey available to these two euphausiids, N. sexspinosus and T. uequalis
may have been selecting different copepods as prey.
was found below 700 m during the day
and between 140-700 m at night. Nemcltobrachion was found in a depth range of
250-600 m (Brinton 1967) to 350-950 m
(Baker 1970) throughout
the day and
night.
Discussion
Diel changes in feeding activity (prcThe patterns in diel vertical distribusumably reflected in changes in stomach
tions of the four euphausiid species studweight)
show a close relationship
beied show that T. uequalis and T. monatwcen increased feeding activity at night
cnntha have the lcast overlap in night
and strong vertical migration. Thysunoand day range (only 4% and 13%) and podu aequalis and T. monacantha,
both
migrate the greatest distances; they are strong vertical migrators, show increased
thus strong vertical migrators. Thysnnofeeding activity at night whereas T. pecpoda pectinuta
is a wcakcr vertical mi- tinuta,
a moderate
vertical
migrator,
grator than either of these (29% depth
shows constant feeding activity throughrange overlap), Nema tobrachion
sexspiout the day and night. Nematobrachion
nosus has the smallest change in day- sexspinosus, a very weak or nonmigrator,
night vertical
distribution
(56% depth
shows increased feeding activity during
range overlap); it is a very weak or nonthe day. Roger (1973,1975) found that the
migrator. These data must be qualified by percentage of specimens whose stomachs
some limitations
in sampling:
were more than half-full increased most
rapidly in Nematobrachion
boopis beThe data for N. sexspinosus inditween 0930-1330 hours, in T. monacancate a lower abundance within the
tha between 1730-0930 hours, and in T.
water column sampled during the
pectin&a
between 1730-2130 hours (alday than at night, suggesting that this
though differences
for the last were
species may avoid the trawl during
smaller). This implies that strong vertical
the day and may not migrate at all.
migrators (T. aequulis and T. monacanThe envelope
for the range of
tha) feed more during the night while
depth-abundance
data is strongly inthey are in the upper layers; this could
fluenced by samples with high valreduce potential
competition
for food
ues; it is therefore difficult
to dewith deeper-living
euphausiids
(T. pecscribe accurately the true distribution
tin&a)
with which they may co-occur
of a species with patchy or clumped
during the day.
distribution.
If prey density (both microplankton
The vertical distribution
profiles
and net plankton) is inversely related to
are composites from samples taken at
depth (Vinogradov
1970), the increased
different
times (fall of 1971, 1972,
feeding observed here may be a response
and 1973), so that these results do not
to an increase in prey density. Thysanonecessarily reflect the true distribupoda aequalis and T. monucantha are extion of a given diel cycle.
posed to a relatively large diel change in
Despite these sampling limitations,
the prey density and show increased feeding
at night. In contrast, N. sexspinosus and
general pattern showing vertical migraT. pectin&z
presumably
do not experition in Thysanopodn
species and weaker
or no migration
in Nematobrachion
is ence as large a change in prey density as
T. aequulis and T. monacantha
and do
supported
by similar previous
results
(Brinton
1962, 1967; Baker 1970), in not show increased feeding at night. Prey
density for N. sexspinosus may even dcwhich T. uequalis was found between
950-350 m during the day and above 300 crease at night, due to migration of the
m during the night, T. monacantha
was zooplankton, while T. pectinata seems to
case in which
found below 280 m during the day and represent an intermediate
slight migration may just counterbalance
above 140 m at night, and T. pectinata
Table
1.
Summary
of vertical
Vertical
migration
distribution,
feeding
chronology,
and diet of four cuphausiids.
Body
size
(mm)
Vertical
migration
Feeding
time
Temporal
change in
stomach
fluorescence
21-25
weak
day
no change
29-33
moderate
continuous
no change
Thysanopoda
monacantha
25-32
strong
night
night>day
Thysanopoda
aequalis
13-20
strong
night
night>day
Nematobrachion
aexspinosus
Thysanopoda
peotinata
microplankton
- phytoplankton,
'bigher
crustacea
- euphausiids,
foraminiferans,
decapods.
decreasing
prey density
at daytime
depths.
Increased feeding in N. sexspinosus
during the day appears also to bc related
to increased light intensity.
In euphausiids the rhabdom appears as a spiral
(Kampa 1965). In the bilobed eyes of N.
sexspinosus, the coil in the rhabdoms of
the dorsal lobe is much tighter than in
the lower lobe and appears to be a modification for higher light sensitivity. Also,
ommatidia of the upper lobe arc directed
vertically upward in the direction of maximum intensity
of downwelling
light,
suggesting
that N. sexspinosus
relies
heavily on visual stimuli in some aspect
of its behavior. Therefore N. sexspinosus
may be better able to locate prey during
the day when light intensity is high than
during the night when light intensity is
low (i.e. it is a visual predator).
The absence of data on the vertical distributions
of prey for these euphausiids
and the simplifying
assumption that prey
density
is inversely
related to depth
make the conclusions about relationships
between vertical distributions
of euphausiids and their diet speculative
at best.
IIowever,
an explanation
is needed for
the differences
observed between species in the extent of vertical migration,
feeding chronologies,
and dietary composition.
Differences
of diet, feeding chronolo-
305
and diet
radiolarians,
Major
diet
components
microplankton*
copepods
microplankton
copepods
higher
crustaceat
chaetognaths
microplankton
copepods
higher
crustacea
chaetognaths
microplankton
copepods
tintinnids.
gy, and vertical distribution
of the four
species may serve to reduce potential interspecific competition
for food. Th ysanopoda aequnlis and T. monacantha
feed
primarily
at different
depths and, to a
varying degree, at different times from T.
pectin&a
and N. sexspinosus. Thysanopoda aequalis and T. monacantha, which
co-occur both day and night, exhibit similar temporal feeding patterns but are of
different body size and seem to eat prey
of different sizes. Thysanopoda pectinata
and N. sexspinosus differ in their feeding
chronology and daytime depth to a considerable
extent. The ways in which
these four species differ in vertical distribution,
feeding chronology,
and diet
are summarized in Table 1.
Wilson (1975) suggested that larger
predators have a competitive
advantage
because they are able to USC food of sizes
unavailable
to smaller predators. However, he also concluded that if prey-size
distribution
ascends and then descends,
a region may be generated where only
minute differences in size are needed to
permit coexistence. One prediction
that
Wilson made is that character displacement based on food size will not be conspicuous.
Comparisons of the diet of the four euphausiid species suggest that body size
is more influential
in determining
prey
type than the presence of specialized tho-
306
Hu
racic legs. Thysanopoda aequalis (13-20
mm total length), the smallest of the three
Thysanopoda
species studied, fed predominantly
on microplankton
and copepods, while the larger T. monacantha
(25-32 mm total length) and T. pectinata
(29-33 mm total length) fed on chaetognaths, other euphausiids, and dccapods
as well. Nematobrachion
sexspinosus
(21-25 mm total length) is just slightly
smaller than T. monacantha
and considerably larger than T. aequalis,
yet N.
sexspinosus and T. aequalis have similar
prey in their diet. However, the ratio of
copepods identified as prey of N. sexspinossus (37 Pleuromamma
to 1 Oithona) is
markedly
different
from that of T. aequalis (2 Pleuromamma
to 14 Oithona).
This suggests that N. sexspinosus feeds
more on the larger of these two copepods
than T. aequalis, another difference
in
euphausiid dietary niches.
Relerences
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chronology
in two
deep-sea fishes off California.
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of
euphausiids
near Fucrteventura,
Canary Islands (‘Discovery’
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BHINTON, E. 1962. The distribution
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1967. Vertical migration and avoidance capabilities of euphausiids in the California
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1975. Euphausiids
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obtained
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Am. Nat. 109: 769-784.
Submitted: 20 December
Accepted: 12 July 1977
1976