Fall 1999 Table
Diet and Feeding Behavior Offer Clues to Fisheries-Related Food
by Tracey Crago, WHOI Sea Grant
its important to study the little things to get the big picture.
ask Rebecca Rapoza, a doctoral student in the WHOI/MIT Joint Program
in biological oceanography. Rapoza studies ctenophores, specifically
their diet and feeding behaviors.
lobate ctenophore, Mnemiopsis leidyi. Found throughout
the world's oceans, these invertebrates average 5-6 cm in size.
Photo: Rebecca Rapoza, WHOI
(pronounced teen-a-fours) are gelatinous marine invertebrates: primitive
animals with simple body forms. Often called comb jellies, they
are completely unrelated to jellyfish, though they are somewhat
similar in appearance. Ctenophores are located throughout the worlds
oceans. They are very effective predators, consuming both mesozooplankton
(200 µ to 2 mm) and microzooplankton (20200 µ).
work, supported by WHOI Sea Grant and The Seaver Institute, focuses
primarily on ctenophores in the Order Lobata, although a portion
of her thesis compares the diet and morphology of Lobata to another
Order of Ctenophora, Cydippida.
both lobates and cydippids prey upon zooplankton and occur in the
same areas, each utilizes a distinct prey-capture strategy. It appears
that the different strategies are related to the corresponding differences
in body structure.
have two elongate tentacles that are used to net prey as the animal
sits motionless. This ensnarement technique allows the cydippids
to capture highly mobile prey.
as their name implies, have two large lobes that surround a body
bearing rows of modest-sized tentacles (see photo). Using four flap-like
ciliated auricles, they draw water into the space between the lobes
(or inter-lobe space). This flow, known as the feeding
current, enables the animal to capture low-speed or weak swimmers
by trapping them on the tentacles. Larger prey typically swim into
the inter-lobe space and are caught on the sticky inner surface
of the lobes. The two methods of prey-capture enable lobates to
effectively prey on both small, passive prey and large, active prey.
to Rapoza and her colleagues, that ability to also feed on microzooplankton
was probably a driving force behind the evolution of lobates from
a cydippid ancestor. Because of their differing feeding strategies,
lobates and cydippids can co-exist and flourish.
a long time," says Rapoza, "it was assumed that lobates
were simple, non-selective feeders: they would consume indiscriminately-whatever
they came upon." Recent observations by Rapoza and others seem
to disprove that long-held notion.
has observed-through videotaped footage of captive animals-that
the behavioral repertoire of these stealth predators is surprisingly
complex. "For example," explains Rapoza, "not only
do the auricles create the feeding current, they enable lobates
to sense approaching prey, reach out, and immobilize them against
the food groove that carries the prey to the mouth."
does the ctenophore create this quiet environment between the lobes,
where prey do not sense approaching doom? To answer this question,
Rapoza used a particle image velocimeter (PIV) to study flow fields
immediately surrounding the animals. The PIV uses a two-dimensional
light sheet to illuminate particles in the water directly surrounding
the animal. By tracking the movement of these particles, characteristics
of flow that are important to prey capture can be mapped and understood.
use of tools such as the PIV, and light and electron microscopy,
have allowed Rapoza and her collaborators, John Costello of Providence
College and Tony Moss of Auburn University, to gain insight into
the functional morphology of lobates, as well as prey-capture techniques.
the feeding and prey-capture dynamics of lobate ctenophores may
not at first seem important to overall food chain dynamics, consider
that their main food source happens to be the food of choice for
the larvae of cod and other commercially important fishes.
learning more about the mechanisms by which lobates select and catch
prey, Rapoza and her colleagues, including her advisor, Larry Madin,
a senior scientist at WHOI, hope to aid scientists in predicting
what kinds of prey will be vulnerable.
the Black Sea, for example, the lobate ctenophore Mnemiopsis leidyi
was introduced through ballast water discharge in 1982. Its predation
on zooplankton and larval fish caused severe damage to commercially
important fish stocks for several years.
studying predator-prey interactions, Rapoza is contributing to the
overall understanding of how ecosystems function. Whether scientists
focus on broad scale distribution of organisms or, as in Rapozas
case, microscale processes of individual organisms, it is the integration
of all relevant information that helps us make better predictions
of how our ocean environment is changing. By adding her research
pieces to the puzzle, Rapoza has gained an appreciation for looking
closely at the little things to gain insight into the big picture.
observations by Rapoza and Costello suggest that different prey-capture
strategies of lobate and cydippid ctenophores may account for
dietary choice. Gut analyses taken from lobates (Bolinopsis
infundibulum), and cydippids, (Pleurobrachia pileus),
present at the same time and location, revealed that the lobates
preferred small, less mobile prey, while the cydippids preferred
larger, stronger prey.