Kirsten Mattern
Abstract
The feeding behavior and prey selection of the deep-water calanoid
copepod Gaussia princeps was observed under three different light wavelengths
and four different food conditions. Both the free-swimming and tethered Gaussia
showed no significant preference for any food type, light wavelength, or
combination of food and light. Two possible foraging strategies and prey types of
Gaussia are suggested. Comparison of the observed foraging strategies of the
Gaussia to the known foraging tactics of the shallow-water calanoid copepods of
similar structure show that Gaussia may be a suspension feeding herbivore or a
cruise and sink feeding omnivore. This conclusion was made on te basis that
the time copepods spend beating their mouthparts is a measure of foraging
strategy.
Kirsten Mattern
Introduction
Gaussia princeps is a calanoid copepod found in the mesopelagic zone.
They have been found between 300 and 1000 meters and are thought to be
vertical migrators (Bob North, pers. comm.). Both the adult male and female
copepods are 9.0-12.0 mm in length. The bioluminescent capabilities of these
crustaceans have been extensively studied, but much of their biology is unknown
(Barnes and Case, 1972; Latz, Bowlby and Case, 1990; Bowlby and Case,
1991).
The diet and foraging behavior of Gaussia has not been described, but
the shallow-water copepod feeding strategies and food preferences have been
widely characterized. The shallow-water calanoids are much smaller than
Gaussia, but their similar features may help elucidate much about the deep-water
copepods. These shallow-water copepods employ several different feeding
strategies and select different prey types. The feeding strategies used by the
shallow-water copepods are influenced by what type of prey they are seeking
and the physical characteristics of their environment, such as light and water
movement. By characterizing and comparing the feeding behavior of the
Gaussia to the shallow-water calanoids, the foraging tactics and possible prey
type of the Gaussia may be determined.
The purpose of this research was to determine what type of prey Gaussia
preferentially feeds on and what methods these deep-water copepods use to
catch their prey.
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Materials and Methods
To examine copepod activity with different food and light types, I observed
sixteen Gaussia each in two tanks maintained at 5°0 with a current created by
continuous water flowing in a vertical circular pattern. As these crustaceans are
accustomed to low light intensities, even a total absence of light, black sheets of
plastic surrounded the tank, the front panel removable by velcro strips. The
copepods were gently swept around by this current, and the current was
necessary for them to remain suspended.
lused three light types and four food conditions. White light came from
the normal room light, UV light from an overhead light set on top of the
observation tank, and red light from a normal light covered with two layers of red
celophane, also set on top of the tank. I used the white light for a contrast from
the light intensity the Gaussia encounter in their natural habitat and to provide a
comparison to the shallow-water copepod environment. The UV light is absorbed
at depths above that at which Gaussia have been found, and has been observed
to cause the luminous cells in the copepods to fluoresce as well (Bowlby, 1991).
Red light is also absorbed at a depth above that at which the Gaussia live and
so allows the copepods to be observed in an environment closest to their own as
they may be insensitive to this wavelength of light.
The three food types consisted of phytoplankton, brine shrimp nauplii and
natural zooplankton. The phytoplankton (Dunaliella and Nannochloropsis) and
nauplii were obtained from the Monterey Bay Aquarium and the zooplankton was
obtained from 10 minute tows done at Hopkins Marine Station. The zooplankton
consisted primarily of shallow-water copepods, radiolaria, diatoms, cladocera,
and chaetognatha. I did not feed theGaussia in one tank more than one food
type per day, and the tank was cleaned regularly.
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To specifically observe which food type Gaussia prefers and to determine
how they capture their microscopic prey, I superglued individual Gaussia to a
fine glass rod and suspended it in a sealed glass flask through which Ilet water
flow with the different food types.  observed the Gaussia under a microscope
with white light. The cold temperature was maintained by packing ice around the
flask. The appendages of the Gaussia and the food types were clearly visible
under these circumstances.
To observe individuals under each food condition, 1 placed 12 Gaussia in
12 separate glass tubes 4 inches tall and 1 inch in diameter. Three tubes had no
food and served as the control, three had phytoplankton, three had brine shrimp
nauplii, and the last three had zooplankton. I maintained the tubes in a 500
water bath. The tubes were also kept in the dark when not being observed.
When I studied these tubes, I took them out of their water bath and set them
under the different light types for 10 minute intervals to see whether the Gaussia
displayed different behavior patterns under the different food conditions. Äfter
the first week, I cleaned the tubes and rotated the food types, excluding the
control tubes. I repeated this process after the second week of observations so
each set of copepods was exposed to each food type.
defined two types of behavior most often seen during my initial
observations. One, the dart, is an abrupt voluntary swimming movement that
was not a result of touching or colliding with another individual. Second, the
side is a behavior where the individual hangs upsidedown facing the current,
either with its dorsal or ventral side facing the tank wall. My observations of this
copepod activity included watching them for 30 minute intervals and recording
how often each type of behavior occurred per minute. I broke these time
intervals into three 10 minute periods, each time using a different light source.
observed the Gaussia for these 30 minutes under four different food conditions:
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no food, phytoplankton, brine shrimp nauplii and zooplankton. For each 10
minute interval I calculated the averages of darts and sides. I conducted
between two and five replicates of each food and light combination. Data were
analyzed by two-way analysis of variance with light source and food type as fixed
factors.
Results
Under the microscope, the Gaussia displayed several different types of
movement patterns. When I kept the water flowing at a steady rate or at a zero
velocity, the tethered Gaussia rapidly moved its antennae to create a current flow
past its maxillipeds in the posterior direction. This pattern could be seen when
added a dense suspension of food to the water. IfI suddenly changed the rate of
water flow, the copepod would either completely stop all movement for a moment
then resume activity with a rapid appendage contraction which included pumping
of the legs and contraction of the maxillipeds and antennules, or immediately
perform the rapid appendage contraction before resuming the antennae
moyement. This movement of the legs and contraction of the maxillipeds and
antennules are how the Gaussia propel themselves through the water, producing
the dart behavior. I also observed this behavior when large pieces of debris
associated with the plankton food suspension became entangled with the
copepod's maxillipeds or antennules. The copepod could also move just the
antennule or maxilliped on which the piece was stuck.
When the copepod contracted its maxillipeds, they moved toward its
mouth. This movement is extremely fast and although the microscopic plankton
and individual nauplii were visible, I could not tell whether the Gaussia had
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captured a prey or what prey type it was. The tethered copepods showed no
specific preference for either plankton, algae or nauplii. Any contraction of the
maxillipeds was sporadic and did not occur to a greater extent in any one food
suspension.
found no significant differences of copepod activity between each food
type or between each light type. There was no significant interaction between
food and light (Tables 1 & 2). Gaussia showed the most activity under white light
with no food (Fig. 1). The least amount of dart activity occurred with the brine
shrimp nauplii, regardless of light condition. The greatest average of individuals
on the side per minute occurred under red light and with phytoplankton (Fig. 2).
The side activity under UV light peaked with zooplankton. Under white light, the
average activity remained fairly constant, regardless of food condition. Again,
none of these differences were significant.
When I placed the Gaussia in the glass tubes, they immediately
sank to the bottom and made no attempt to swim unless the tube was disturbed.
After I added each food type, no dart or side activity occurred. The copepods lay
on either their dorsal or ventral surface and continued with the normal rapid
antenna movement. This sinking behavior also occurred when I turned off the
current in the large tanks. With no current, there was no activity under any
combination of food or light.
Discussion
The foraging behavior of the shallow-water calanoid copepods has been
extensively characterized. Koehl and Strickler (1981) investigated the sieve
feeding tactic of the copepods. They scan the water by beating their feeding
appendages, producing a postero-lateral current. The assymetric swinging of the
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maxillipeds orients the copepod to capture the section of water containing the
algae. Only the water containing the algae is pushed through the bristles on the
second maxillae. The food particles are transferred to the mouth via the first
maxillae. The preferred prey is phytoplankton, and these copepods can scan the
water they filter for certain food. Captured prey can be ingested or rejected.
Greene (1988) characterized the same filtering tactic used by the suspension
feeding calanoids. He also described the omnivorous and carnivorous calanoids
that recognize prey and seize the prey raptorially with the second maxillae and
maxillipeds then use their first maxillae to guide the prey to their mouths. Greene
(1988) stressed the strong interaction between feeding and swimming because
these behaviors utilize the same appendages. He characterized three foraging
tactics: the suspension feeders who remain stationary, the cruising, raptorial
predators, and the omnivorous calanoids who employ a 'cruise and sink
behavior. Tiselius and Jonsson (1990) described three hydrodynamically based
foraging strategies of shallow-water calanoids. Stationary feeding is a balance
between gravity and upward thrust, cruising copepods encounter more prey and
can approach these prey in a hydrodynamically quieter manner than the
stationary feeders, and the ambush predators sink slowly and passively and are
hydrodynamically quieter than both the stationary and cruising calanoids. The
time the calanoids spend moving their mouthparts is a measure of their foraging
strategy. The upward thrust created by this movement counteracts the sinking
brought about by gravity so the stationary feeders exhibit continuous movement
while the ambush predators should only use their mouthparts to capture the prey.
Gaussia exhibit the continuous movement of the mouthparts, or
antennae, both in the presence and absence of food. These calanoids also sank
to the tank floor and to the bottom of the glass tubes in the absence of a current,
vet continued the movement. When in the tank, the upside down position on the
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sides of the tank was also accompanied by the antennae movement. This may
indicate that the Gaussia are suspension feeders, a strategy employed by the
shallow-water algal feeders. In their natural habitat, Gaussia may be sinking
while moving their antennae then moving upwards by the darting behavior, such
as the cruise and sink method Greene (1988) attributed to omnivourous
copepods. Based on data from shallow-water copepods, (Tiselius and Jonsson,
1990) the ambush method may be eliminated as a strategy of the Gaussia
because they continuously move their mouthparts. The Gaussia may then be
either suspension feeders or cruising feeders, depending on the current velocity,
and their prey preference could be either herbivorous or omnivorous.
Buskey, et al. (1989) found that certain oceanic copepods found ca 80m
showed negative phototaxis over a wide range of light intensities, that is, the
copepods were found in sections farthest from the light. Such copepods would
then migrate downwards during the day and towards the water surface at night.
This differs from the coastal and estuarine vertically migrating copepods which
stay towards the bottom but within the photosensitive zone during the day
According to Buskey et al. (1989) the maximum photosensitivity displayed by the
deep-water copepods corresponded to the light wavelengths most prevalent in
their environment and to the wavelengths of most marine bioluminescence. The
exact effect of light on vertical migration is not clear. It is clear, however, that
light can be used to predict the position of copepods if their photosensitivity
thresholds are known. Light affects where the copepod is during a 24 hour
period, and prey types at certain depths affect what strategy the copepod uses in
its particular environment to capture the prey.
Gaussia showed no differences in activity under any wavelength, white
red or UV. Because they are deep-water copepods, they may be insensitive to
the red and UV light because these light wavelengths are absorbed at a depth
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above that which Gaussia inhabits. These light wavelengths could imitate their
natural environment. The white light may have had some effect as the least
amount of side behavior consistently occured under this wavelength. If light is
not the factor that regulates preferred depth, Buskey et al. (1989) suggested that
pressure may control the vertical migration of the deep-water copepods.
To understand the foraging tactics and prey selection of the Gaussia to a
greater extent, these calanoid copepods should be viewed in an environment
more like their natural surroundings. The layer they live in not only has different
water movements compared to the shallow-water environment and artificial tank
currents, but is also under greater pressure. In addition, identity and abundance
of the possible prey organisms in Gaussia’s natural habitat is necessary to fully
determine what they eat. Two possible methods of discovering possible prey of
the Gaussia are analyzing the gut contents of freshly caught Gaussia, and
performing a fecal analysis.
Defining the feeding biology of the mesopelagic calanoid copepod
Gaussia princeps will not only show what types of species inhabit this deep
zone, but will also help to determine the physical characteristics of this zone and
how these species interact with each other and with their environment.
Acknowledgements
Thank you to Bob North at the Monterey Bay Aquarium for supplying the
Gaussia and equipment for my research.  would also like to thank Professor
Mark Denny for his help with my set-up and with background material, and
Professor Stuart Thompson for the use of his microscope.
would also
especially like to thank Professor Jim Watanabe for his help with research ideas,
background material, statistical analysis, and reviewing my paper.
Kirsten Mattern
Literature Cited
Barnes, A.T. 1972. Bioluminescence in the mesopelagic copepodGaussia
princeps. J. Exp. Mar. Biol. Ecol. amp; 53-71.
Bowiby, M.R., and J.F. Case. 1991. Ultrastructure and Neuronal Control of
Luminous Cells in the Copepod Gaussia princeps. Biological Bulletin. 180: 440.
446.
Buskey, E.J., K.S. Baker, R.C. Smith and E. Swift. 1989. Photosensitivity of the
oceanic copepods Pleuromamma gracilis and Pleuromamma xiphias and its
relationship to light penetration and daytime depth distribution. Marine Ecology
Progress Series. 55: 207-216.
Davis, C.C. The Pelagic Copepoda of the northwestern Pacific Ocean. Seattle,
University of Washington Press, 1949. pp. 50-51
Greene, C.H. 1988. Foraging tactics and prey-selection patterns of omnivorous
and carnivorous calanoid copepods. Hydrobiologia. 167/168: 295-302.
1981. Copepod feeding currents: Food
Koehl, M.A.R., and J.R. Strickler.
capture at low Reynolds number. Limnol. Oceanogr. 2666): 1062-1073.
Latz, M.I., M.R. Bowiby, and J.F. Case. 1990. Recovery and stimulation of
copepod bioluminescence. J. Exp. Mar. Biol. Ecol. 136: 1-22.
Tiselius, P., and P.R. Jonsson. 1990. Foraging behavior of six calanoid
copepods: observations and hydrodynamic analysis. Marine Ecology Progress
Series. 66: 23-33.
Kirsten Mattern
Table 1: ANOVA of logdart: Average number of darts per minute during a 10
minute period. Data were log-transformed prior to analysis (In (x+1)).
E-BATIO
SOURCE
MEAN¬
SQUARE
2.138
0.114
Food
0.450
0.778
0.05
0.253
Light
Food X Light
0.12
0.574
0.748
0.2
ERROR 34
Kirsten Mattern
Table 2: ANOVA of side: Average number resting per minute during a 10 minute
period.

SOURCE

MEAN¬
E-BATIO
of
SQUARE
0.987
0.411
Food
1.12
0.057
3.568
3.128
Light

0.489
Food X Light
926
1.056
—
34
ERROR
1.141
Kirsten Mattern
Figure Legend
Figure 1: The average number of darts min
during a ten minute period per
light wavelength and food type.
Figure 2: The average number of sides min
during a ten minute period per
light wavelength and food type.
Kirsten Mattern
FIGURE 1: AVERAGE NUMBER OF DARTS PER MINUTE.
3-
— RED LIGHT
—---- UV LIGHT
------: WHITE LIGHT

0 +
NONE NAUPLII PLANKTON ALGAE 0
FOOD
Kirsten Mattern
FIGURE 2: AVERAGE NUMBER OF SIDES PER MINUTE

—----- UV LIGHT

WHITE LGIT

ooo
NONE NAUPLII PLANKTON ALGAE
0
FOOD