Download preliminary survey of the relationship between the feeding habit and

PRELIMINARY
SURVEY OF THE RELATIONSHIP
BETWEEN
THE
FEEDING
HABIT AND THE STRUCTURE
OF TIIE MOUTH-PARTS
OF MARINE
COPEPODS’
Masateru
Woods
Anraku
Hole
and Makoto
Oceanographic
Omori
Institution
The feeding of 6 copepods (Calanus finmarchicus,
Acartia tonsa, Cemtropages hamatrls,
C. typicus, Lahidocera
aestiua, and Tortunus cliscnudatus)
was examined with 3 diffcrcnt
foods-a)
d’lr?.t oms ( Thalassiosira fll~viatilis)
alone, b) diatoms plus Artemia nauplii, and c)
Artemia alone -and
compared with the structure of their mouth-parts.
Calanus finmarchicus
is predominantly
an herbivore,
but in absence of plant food can
definitely
cnpturc small motile animals. Acartia tonsa is a typical omnivore which can cat
cithcr plant or animal food efficiently.
Centropages hlmlatus and C. typicus are also omnivorous, but both prcfcr an animal diet. Although Labidocera
aestiva is predatory,
some
setae used for filtering food arc found at the base of the second maxillae.
Finally, Tortanus
discnudntus is a typical prcdntory copepod.
Thcrc is close rclntionship
between the structure of the mouth-parts
and the fcccling
habits of thcsc specks.
In herbivorous
spccics ( Calanus finmarchicus),
the 2nd antcnnac,
manclibular
palps, 1st mnxillnc, and maxillipcds
arc well dcvcloped
to product
a pair of
“feeding swirls.”
The 2nd maxillae arc also formed as cfficicnt filtering nets. The cutting
edges of the mandibles are provided with grinding teeth. In predatory
species (Tortanus
discaudatus),
the mouth-parts
have few setae and are on the whole much simpler. The 1st
maxillae, 2nd maxillae, and maxillipcds
arc modified as prchcnsilc appendages.
The cutting
cdgcs of the mandibles have very sharp teeth. In omnivores, thcsc appendages generally
have a structure intermediate
between those of the two previous types; the 2nd maxillae
arc used partly for filtering and are partly prehensile.
The teeth are heavier than those of
the herbivores, but they are not as stout as for predators.
2nd antennae, the mandibles, the 1st maxillae, the 2nd maxillae, and maxillipcds) when
The usual method for determining
the feeding
(Estcrly
1916; Cannon
1928;
food preferences of copepods has hitherto
Lowndes 1935). There is a correlation bebeen a direct examination of the gut. Thus,
twccn the distance between the setulcs of
Lcbour ( 1922 ) and Marshall ( 1924) rc- the 2nd maxillae and the size of the food
ported that various species feed on both
organisms filtered ( Ussing 1938; Marshall
phytopJankton and zooplankton, some feed- and Orr 1956). The movements of the
ing predominantly
on diatoms, others con- mouth-parts oE Cyclops ( Fryer 1957b ) and
sistently preying on animals. The feeding
the anatomy of the cutting edge of the manmechanisms of living copepods have also
dible of several marine copepods ( Beklcmibeen studied. Lebour (1925) observed the shev 1959) have also been examined. IIowfeeding of Anomalocera pattersoni in a cvcr, there have been few demonstrations
plunger jar and found that it even attacked
of the food taken by direct measurcmcnts
a larval fish. Many fresh-water cyclopoid
of depletion of the available diet in the
copepods are also carnivores ( Fryer 1957a).
medium.
Recently, the omnivorous nature of Acartia
The authors are indebted to Dr Mary
clausi and A. Zatisetosa has been extensively
Sears for her helpful criticism and careful
investigated ( Petipa 1959a, b ) .
reading of the manuscript. We are grateful
Copepods USC5 pairs of appendages (the
to Dr Bostwick H. Ketchurn for his valuable
Thanks
suggestions and cncouragemcnt.
L Contribution
No, 1278 from the Woods Hole
arc also due to the other members of the
Oceanographic
Institution.
The prcscnt invcstigastaff of the Mroods IIole Oceanographic Intion was made possible by grants from the Nastitution for their assistance.
tional Scicncc Foundation,
G-13010 and G-17508.
116
IN’l-l3OI~UC’lX)
N
FEEDING
AND
MOUTH-PARTS
OF MARINE
117
COPEPODS
METHODS
The copepods were caught with a small
#2 mesh nylon net in the vicinity of Woods
Hole. Specimens were then transferred to
5-L plastic containers and stored in an icebox when the air was warmer than the
water. On return to the laboratory, they
were kept in a constant temperature bath
for about a day to become acclimatized to
the temperature nearest to that of the water
where and at the time they were captured.2
Glass bottles of approximately 65 ml and
150 ml were used as experimental vessels,
the size in each case being dependent on
the size of the particular copepod under investigation. It was found subsequently that
these volumes were too small to permit the
, maximum feeding rate. However, the observations made here are concerned mainly
with qualitative information.
First, all bottles, 4 or 5 in each series,
were filled with 5 to 10 ml of filtered sea
water. Then adult females of a particular
species (one for Labidocera aestiva, and
Tortanus discaudatus, two for Centropages
typicus, five for Centropages hamatus and
Acartia tonsa, and one for stage V of Calanus finmarchicus, were transferred to the
bottles with a pipette. The food organisms
were next added in the following combinations: a) The unicellular diatom, Thalassiosira fluviatilis, E-17.5 p in diameter, was
added to the culture in concentrations of
800-1,000 cells/ml.
One bottle served as
the control. b ) In addition to the same concentrations of T. fluviatilis as in the previous series, nauplii of the brine shrimp,
Artemkq3 were added. Since a desirable
concentration of Artemia had not yet been
determined a given number was consistently
added in each experiment.
One of the
vessels served as the control. c ) The same
number of Artemia as in the preceding
series was added, but no control was used
in this series.
2 The 4 water baths were set for 2”, 8”, E”,
and 22.5% respectively.
3 The Artemia nauplii used were 1 or 2 days
old and approximately
600 ,U in body length.
In
this early developmental
stage they do not feed
on T. fLk2tiZi.s
at all.
FIG. 1. Feeding rates of Calanus finmarchicus
on diatoms ( Thalassiosira fluviatilis)
only, diatoms
and Artemia,
and Artemia alone on 19 January
1962. Ten Artemia nauplii per lSO-ml bottle were
used.
Next, the bottles in the three series were
wrapped with aluminum foil and those in
series a) and b ) set on a rotating wheel and
those of series c) were placed in the baths
without a wheel. Usually, after a 6-hr run
the contents of each bottle were preserved
in formalin, the number of diatoms were
estimated with a Sedgwick Rafter counting
chamber and all the remaining Artemia
nauplii were counted under a dissecting microscope. The copepods were then blotted
on filter paper and stored in a desiccator for
a week for dry weight determinations.
The
filtering rate (which is equal to grazing rate
but nat to feeding rate, see Gauld 1951)
and the predation rate (Artemia consumption) could thus be calculated. A value of
0.118 mg/100 individuals was used as the
standard dry weight for Artemia nauplii.
Preserved specimens of the adult female
of the 6 copepod species were dissected and
each mouth appendage
mounted on slides
-for comparison of their anatomical structure.
RESULTS
OF FEEDING
EXPERIMESTS
Calanus finmarchicus
In the ni ot experiment, only Artemia
nauplii were used as food. C;ontrary to expectation, however, a few Artemia were
eaten by stage V Calanus, but none by
those in stage IV (Table 1) . In the three
sets of experiments, it is apparent (Fig. 1)
118
MASATERU
TARLE
1.
Preclntion
ANRAKU
AND
rates of Calanus
MAKOTO
OMORI
finmarchicus
on Artemia
nauplii
_---___-
Copcpcdid
stage
mite
---___
Tempcra hire
-
14 November
21
,I
28
5 Jam&y
1961
1961
1961
1962
(“C) -.-~
V
IV
V
V
8
8
8
8
Volume
of bottle
Number
Artemia
given
(ml)
150
150
150
150
50
15
15
10
-_ .__--
that C. finmarchicus ate the same amount of
T. fluviatilis whether 01’ not Artemia were
present. When only Artemia were available,
a few were eaten, but the feeding decreased
greatly in a mixed culture. Thus, it would
appear that Calanus is predominantly
an
herbivore.
Acartia tonsa
Despite its small body size, relatively
high feeding rates were found (Fig. 2) for
A. tonsa. In pure cultures this copepod ate
cithcr T. fluviatilis or Artemia. As might
have been expected, this species also ate
both animal and plant food equally in the
mixed cultures. The consumption of both
reduced
when
items
was markedly
compared with the amounts caten in pure
cultures, whether plant or animal alone.
There was thus no apparent preference in
the feeding habits of Acartia, at least insofar as these experiments were concerned.
of
Mean
No. of
Artemia consumed/
copepod/G
hr
-~
1.8
0
1.3
2.4
- - -~
Mean
Artemia
_
mg
Mean
thy wt/
copcpocl/6
hr
Artemia
mg
dry wt/
mg copepod/6
0.0021
0
0.0015
0.0027
hr
0.026
0
0.025
0.027
Centropages ham&us
The grazing rate for C. hamatus was low
with a mixed diet when compared with that
in a diatom suspension alone ( Fig. 3).
This species did, however, prey upon Artemia to a considerable extent even in the
mixed suspension. As the grazing of C.
h.umatus was not active in summer (Table
2)) the values obtained in the experiments
just described may be considerably lower
than those at other times of year. In any
event, this species ate both phytoplankton
and zooplankton, but the latter seemed to
be preferred if there were a choice as in
mixed cultures.
Centropages typicus
This copepod fed rather well on either
T. fluviatilis
or Artemia in pure culture
( Fig, 4)) but it ate relatively few diatoms
when a mixed diet was available. Thus, C.
.7-T.
y
.
--
ACARTIA
TONSA
y
2. Feeding rates of Acurtk tonsa on diatoms ( Thnlnssiosira
fhhtilis
) only, diatoms and
Artemh,
and Artemia
alone.
The cxpcrimcnts
wcrc carried out on 3 October (Series a and b)
an d on 22 October 1961 ( Scrics c). The Artemia
concentration
was 90 per 65-ml bottle.
FIG.
FIG.
3. Feeding rates of Centropages
ham&us
on diatoms ( Thnlassiosirn fluviutilis)
only, diatoms
and Artemia, and Artemia alone. Feeding at 15°C
only was tested on 3 July 1961 for a period of 11
hr. Thirty Artemia per 65-ml bottle were used. A
full series was run on 10 August 19Gl over 24-hr.
The Artemia concentration
then was 50 per 65 ml.
FEEDING
- .. , 1, .
AND
MOUTII-PARTS
OF MARINE
119
COPEPODS
Labidocera
I
I._ ---+----A
20
,-.j
FIG. 4. Feeding rates of Centropages typicus on
diatoms
( Thalassiosira
fluvintilis)
only, diatoms
and Artemia, and Artemia alone. The cxpcriments
w(:rc made on 7 Novcmbcr
( Scrics a and b ) and
on 10 Novcmbcr
1961 (Scrics c). Forty Artemia
per 65ml bottle were used.
typicus feeds on either phytoplankton
or
zoloplankton, but it appears to bc more
markedly predatory than C. hamatus.
aestiva
During a preliminary
experiment
( 18
July 1961) diatoms were fed to L. aestiva
at each of 4 temperatures (2”, 8”, 15”,
2Z.S°C). Two copepods were put into a
65ml bottle with the usual density of
T. fluviatilis.
Six hours later, nothing had
apparently been catcn. Next 80 Artemia
nauplii per adult female of L. aestiva were
provided over a period of 24 hr ( Table 3).
Feeding was active at the higher tempcratures and indeed in August, the animals dcvoured more Artemia than their own weight.
Although there may have been some error
in weighing the nauplii, this is neverthclcss
an extremely high feeding rate. Since this
species did not cat T. fluvintiZis, a mixed
diet was not tried.
Tortanus
cliscaudatus
Preliminary
experiments indicated that
neither the males nor the females ate any
\
-
FIG. 5. Second antenna. A. Calanus
D. Centropages typicus, El. Centropages
finmarchicus,
B. Labidocera
humatus, I?. Acartia tonsa.
aestiva,
C. Tortanus
discuudatus,
120
t+G.
MASATERU
6.
Centropages
Mandible.
typicus,
ANRAKU
by Ccntro2. Amount of water filtered
pages ham&us at different seasons (in ml)
--- - -~
---=
--.-._--21
10
21
8
rrA131,E
---.__
----
Mcan/copcpocl/c-lay
Mean/mg
MAKOTO
A. Calanzrs finmarchicus,
B. Labidocern
E. Centropnges hnmatus, I?. Acartia tonsa.
T. fluviatilis
so that a mixed diet was
Artemia
omitted from our cxpcriments.
nauplii alone were then used at 15°C
( Table 4a). This species was predatory at
all times. Therefore, on 13 July, Pseudocalanus minutus was used instead of Artemia
nauplii. Nine living PseudocaZanus females
were put in a 65ml bottle with 10 adult
Tortanus females. This culture was then
immersed in the water bath at 15°C for a
day. The Tortanus ate Pseudocalanus as
well as each other (Table 4b). On another
occasion, 10 dead Pseudocalanus were providcd as food for 10 Tortanus and only 2
-
AND
dry weight/day
--
May
---
-
JIIIW
Aug
act
39
17
8
25
1,63S I., 140 670 1,497
OMORI
ncstivu,
C. Tortcuaus
discaudatus,
D.
were eaten. So it appears that T. cliscnudatus preys more successfully on living individuals than on dead ones. In the laboratory, Torlanus usually eats other species
such czs Temora longicornis, Centropages
hamatus, and Pseudocalanus minutus.
COMPARISON
OF THE
Second antenna
MOUTII-PARTS
(Fig. 5)
In C. finmarchicus (A) and two species
of Centropnges ( D, E ) the 2nd antennae are
very much alike. The exopods are longer
than the endopods and both are furnished
with well-developed setae. The cxopods of
L. aestiva ( 8)) T. discaudatus ( C), and A.
tonsa ( F ) arc, on the contrary, shorter than
the endopods. In the latter ( F ) this appendage is rather dchcate. It consists of
the slender endopod and a very short 3scgmen ted cxopod. The setac on both rami
arc dcvclopcd. The number of segments as
FEEDING
MOUTH-PARTS
Predation
3.
TABLE
AND
OF MARINE
rates of Labidocera
121
COPEPODS
nauplii
aestiva on Artemia
=
Predation
Date
-
Mean
Mean
Mean
Mean
Mean
Mean
Mean
Mean
Mean
9 Aug 1961
22 Aug 1961
7’ Nov 1961
No.
mg
mg
No.
mg
mg
No.
mg
mg
rate
of Artemia/copepod/day
Artemia/copepod/day
Artemia/mg
dry wt/day
of Artemia/copepod/day
Artemia/copepod/day
Artemia/mg
dry wt/day
of Artemh/copepod/day
Artemin/copepod/day
Artemi.a/mg dry wt/day
well as of setae are reduced in L. aestiva
(18 ) and I’. discaudatus (C ); the long setae
are only found at the distal end of each
branch.
Mandible ( Fig. 6)
The mandible is composed of a denticulate cutting edge and a well-developed
biramous palp. The mandibular palp of
these species is generally alike in structure, but it is much more slender in T.
discaudatus ( C ) and has fewer setae.
The toothed cutting edge ( Fig. 7) varies
co,nspicuously from one species to another.
In C. finmarchicus ( A ) , the grinding surface has 9 teeth and one marginal denticulate spine, Each tooth has a few small
processes. In C. typicus (D) and C. hamatus ( E ), this edge is provided with 8 teeth
TABLE
4.
Predation
a: Feeding
-
June
July
Nov
Nov
Jan
Jan
of
Tortanus
1961
1961
1961
1961
1962
1962
b: Feeding
T. discnzrclott~s
P. ntinu tars
15°C
discaudatus
minutus
5.6
0.007
0.116
5.0
0.006
0.147
0.7
0.001
0.011
16.6
0.020
0.283
22.1
0.026
0.571
18.5
0.022
0.320
41.7
0.049
0.860
34.1
0.044
0.749
31.1
0.037
0.523
22.5”C
63.3
0.075
1.400
63.5
0.075
1.159
31.6
0.037
0.601
and one marginal spine on its innermost
border. Six teeth on the inner side diverge
at the tip. Since there are no small processes
on the teeth, the chewing parts arc on the
whole sharper than in CaZunus. In L. aestiva
(B ) the cutting edge also has 8 teeth with
fine hairs in the “valley” between the 3rd
and 4th and the 4th and 5th teeth. The
three on the inner side, however, are reduced to tiny points with fine hairs protruding toward the spine. This marginal
spine with denticulations is well developed.
In T. discaudatus ( C ), the edge is composed of one small and four large teeth and
has one small marginal spine and many fine
hairs. A. tonsa ( F ) has teeth of a somewhat
different
shape; some are sharp, others
rounded.
on Artemia
(h )
nauplii
(a) and
on Pseudocalanus
on Artemia
Nunhcr of
Artemia/bottle
Date
15
31
14
14
20
20
rates
8°C
2°C
Etl?’ -:
(n21>
50
50
44
100
7
15
65
150
65
150
65
150
Mcnn number
of Artemia cat&
copepocl/clay
36.0
25.0
33.8
30.3
8.8
13.2
Mcnn mg
Artemia/
copepod/day
Mean mg
Artemia/mg
dry wt/day
0.042
0.030
0.052
0.036
0.010
0.016
1.681
0.932
0.221
0.380
on copepods
Survived
Dead
7
3
Survived
Dead
2
7
(1 complctc
for urosome.
(6 without
cxccpt for urosomc and first antenna. 2 complete
Anal segment and caudal furca remained. )
abdomen,
1 half body. )
except
122
MASATERU
ANRAKU
AND
MAKOTO
OMORI
First maxilla
(Fig. 8)
Interpretation of the segmentation of this
appendage differs among various investigators (Giesbrecht 1892; Sars 1903; Hansen
1925; Borradaile 1926; Gurney 1931). According to Gurney ( 1931) the 1st maxilla of
Calanus has 4 segments. The 1st segment,
the precoxa, carries the gnathobasc supplied with stout bristles. The 2nd, the coxa,
has an epipod and a 1st internal lobe. The
3rd, the basis, has a second internal lobe
and a small external lobe with one seta.
The 4th has an exopod and endopod.
The 1st maxilla in C. finmarchicus
( A)
and both species of Centropages (D, E ) are
normally developed. The 1st and 2nd internal lobes are equal in length. On the
other hand, the 1st lobe is longer than the
2nd in L. aestivn ( B ) but in A. tonsa ( F ) ,
the 2nd internal lobe has disappeared and
the endopod is replaced by a seta. In T.
discaudatus ( C ) , only the gnathobase and
the 1st internal lobe remain. The latter is
enlarged and furnished with three heavy
claw-like setae.
Second maxilla
~-00.1
MM-i
FIG. 7. Cutting
edge of mandible.
A. Calanus
finmarchicus,
B. Labidoceru
aestiua, C. Tortanus
discaudatus,
D. Centropnges
typicus, E. Centropages hnmatus, F. Acartia tonsa.
(Fig. 9)
This appendage consists of two basal segments and an endopod of not more than 5
segments. The first segment of the endopod
is generally larger than the others. C. finmarchicus ( A) has many long setac on each
In both Centropages species
segment.
( D, E ), this maxilla is rather stout. The
setae on the endopod are almost claw-like
while those on the 1st and 2nd basal segments are thin and short. L. aestiva ( B )
has many long stout setae on the endopod
and on the distal portion of the second
basal segment, The others are reduced and
rather delicate. In A. tonsa ( F ), the long
heavy setae also occur on the distal portion.
They are fewer, however, and are about
equal in length in contrast to those in Centropages or L. aestiva. T. discaudatus (C)
has 7 very strong falciform setae, but long
setae are not present on its proximal portion.
Maxilliped (Fig. 10)
The maxilliped has a &segmented basal
joint, and a 5 or B-segmented endopod.
FEEDING
FIG.
8.
Centropages
AND
MOUTH-PARTS
OF MARINE
First maxilla. A. Calanus finmarchicus,
B. Labidocera
typicus, E. Centropages ham&us, F. Acartia tonsa.
The structure is very much alike in C. finmarchicus
(A)
and both Centropages
species ( D, E ) . It is considerably longer,
cspccially in Calanus, than the 2nd maxilla.
In. the other three species, on the other hand,
these are smaller than the 2nd maxilla. In
L. aestiva ( B ) and A. tonsa ( F), the 1st
basal segment is broad with several thick
setae, but the other part is rather slender
and has only short setae. In T. discaudatus
(C ) the first basal segment is more developed, while the rest of the appendage is
reduced.
In the filter-feeding
copepods, the 2nd
antennae, mandibular palps, distal portions
of 1st maxillae, and the maxillipeds seem to
be important for maintaining
a pair of
“f ecding swirls” (Esterly
1916; Cannon
1928; Lowndes 1935). The resistance to
water would become more effective were
the surface area of the appendages
in-
123
COl?EPODS
aestiva,
C. Tortanus
discaudntus,
D.
creased. Their structure, then, in C. finmarchicus and both species of Centropages
indicates that these species are efficient for
filtering.
L. aestiva and T. discaudatus,
especially the latter, are not well adapted
to grazing because their appendages are
simpler and have fewer sctae. In carnivorous fresh-water Cyclopoida, the 1st maxillae serve as the clasping organ to hold
prey. The 2nd maxillae and maxillipeds
also assist in grasping their food (Fryer
195713). The 1st maxilla of T. discaudatus
appears to be a typical predatory type
which can seize its prey. Some modification
also exists in A. tonsa and L. aestiva, but
the mouth-parts still seem to function somewhat in producing “feeding swirls.” The
maxillipeds in T. discaudatus, L. aestiva,
and A. tonsa are reduced and modified with
strong setae on the basal segment apparently developed for grasping. In another
genus such as in Pareuchaeta norvegica, the
maxillipeds are much longer than the 2nd
124
MASATERU
ANIIAKU
AND
MAKOTO
OhiORI
FIG. 9. Second maxilla. A. Calanus finmarchicus
(without
the external scta ), B. Lahidocera aestiva,
C. Tortanus discaudatus, II. Centropages typicus, E. Centropages hamatus, I?. Acartia tonsa.
maxillae with stout claw-like setae on the
endopod, with which individuals
of this
species capture C. finmarchicus.
Indeed, P.
norvegica females were almost entirely filled
with the remains of Calanus (Lowndes
1935 ) .
The 2nd maxillae of Calanus remain
practically motionless and serve as a filtering net (Cannon 1928; Lowndes 1935).
However, in Centropages, they are prehensile (Lowndes 1935). In the latter the sctae
on the basal segment serve as a filter but
those on the endopod have become grasping organs.
The results of our feeding experiments
were also indicative of these similarities in
the structure of the mouth-parts.
Calanus
did not often attack Artemia nauplii but
C. typicus and C. hamatus could definitely
feed on prey of this sort. Indeed, in the
laboratory, C. typicus was frequently seen
to grasp other species by the long prehensile setae of the endopod of the 2nd maxilla.
The stout bristles on the gnathobasc of the
1st maxillae aid in this, The 2nd maxillae
of A. tonsa are like those of C. typicus and
C. hamatus, although they carry only about
two-thirds as many setae. These serve as a
scoop in collecting diatoms ( Conover 1956).
By a gathering motion of this appendage,
Acartia clausi collects phytoplankton
and
captures motile organisms ( Petipa 1959a ) .
In L. aestiva, the powerful prehensile setac
also occur on the distal half of the 2nd
maxilla. On the other hand, the short setae
on the proximal half may be useful for filtering. In contrast, these same setae are
rcduccd or absent in 7’. discaudatus, and
accordingly this appendage appears to be
entirely useless for filtering.
The movement of the cutting edges of
the mandibles cannot be easily observed in
living specimens since they are covered by
the labrum. If their function is chewing,
however, the shape of the teeth should vary
depending on the feeding habits. These
need not be strong in herbivores, but they
must be large and sharp in predators. In
FEEDING
FIG. 10.
Centropages
AND
Maxilliped.
A. Calanus
typicus, E. Centropages
MOUTH-PARTS
OF MARINE
finmarchicus,
B. Labidocera
hamatus, F. Acartia tonsa.
C, finmarchicus, for example, the teeth are
rather smooth and not particularly
heavy,
but they are sharp and robust in T. discaudatus and L. aestiva. Those of C. typicus
and C. ham&us are intermediate in form.
The comparison of feeding on Thalassiosira fluviatilis,
Artemia nauplii, and a
mixture of both revealed that the preferen,ce for a particular food was correlated
with the morphological
structure of the
mouth-parts. C. finmarchicus ate predominantly diatoms, and is apparently a definite
herbivore depending almost entirely upon
phytoplankton
and on the suspended matter, in its normal environment.
This has
already been observed by many workers.
On the other hand, it is noteworthy that
Calanus, which has been classified as a
strict herbivore ( Ussing 1938; Gauld 1953),
can definitely capture small motile animals
when there is no plant food present. C.
typicus and C. hamatus ate both T. fluviatilk; and Artemia nauplii and are thus om-
125
COPEPODS
nestiua,
C. Tortanus
dixaudutw,
D.
nivores. The preference for an animal diet
is, however, stronger in the former. A.
tonsa also eats either plants or animals.
This species in all likelihood can live on
phytoplankton
when it is abundant in the
sea. But in other environments individuals
can subsist entirely on animals as already
observed ( Petipa 194913). Since L. aestiva
did not eat T. fluviatilis
at all, it would
then be classified as a predator. However,
the structure of the 1st and 2nd maxillae
suggest that it might have some capability
for filtering and it can perhaps eat much
larger diatoms than T. fluviatilis.
The structure of the mouth-parts of T. discaudatus is
more like those of carnivores than any
others examined.
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