Defense in the Plankton
Predation is generally thought to be an extremely important
source of mortality for most organisms in the plankton. Jellies (which
belong to the invertebrate groups Cnidaria
and Ctenophora), arrow worms (Chaetognatha)
and fishes are among the most voracious predators of other zooplankton
(follow this link for more information about Feeding
in the Plankton). In response to high levels of predation, planktonic
organisms have evolved a variety of adaptations that presumably function
as defenses against predators. One of the striking observations to be made
about the plankton is the similarity of defenses that can be found among
very different groups of animals.
Small body size and transparency are general characteristics
of many planktonic organisms that presumably make it difficult for visual
predators, such as fish, to see their prey. The spines, setae (bristles),
and shells of crab (Arthropoda),
polychaete worm (Annelida),
and bivalve (Mollusca) larvae
act as morphological defenses that may increase the difficulty of ingestion.
Some sea star and urchin (Echinodermata)
larvae store toxic chemicals that make them unpalatable to predators. Planktonic
organisms can also exhibit behaviors that act to reduce predation.
When attacked, polychaete and crab larvae will stop swimming and escape
by passively sinking.
Copepods, which spend their entire lives in the plankton,
often perform an astonishing feat of vertical migration each day (see Locomotion
in the Plankton). Many copepods (and other zooplankton) spend the night
feeding on phytoplankton near the surface of the ocean, only to move down
to depths as deep as 500 m or more during the day. The most common explanation
for this behavior is that, by feeding at night and spending the day below
the depths to which light penetrates, copepods avoid being seen and eaten
by their predators. It has also been suggested that benthic adult invertebrates
may release larvae or gametes into the plankton at times when predation
is lower (see Reproduction
in the Plankton).
The diverse array of antipredatory defenses of many zooplankton
suggest that predation has been an important evolutionary force in shaping
behavior and morphology. Even so, there has been little experimental work
on planktonic predator-prey relationships, in part because of the difficulties
inherent in acquiring even the most basic information about either group
in the field. Our limited understanding of predation and defense
in the plankton provides exciting opportunities for research into the ecology
and evolution of many marine organisms.
Cnidaria & Ctenophora