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.
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