Metazoan Zooplankton

Numerous metazoan, or animal, groups live in the plankton during some part of their life cycle. Many metazoan zooplankton feed or graze on phytoplankton, and their life cycles and population densities are synchronized with, or tied to, the availability of food. A large number of marine organisms have planktonic larvae that feed in the plankton before undergoing metamorphosis and settling out in a suitable habitat. Examples of organisms that produce meroplanktonic larvae include: corals, mollusks, gastropods, crustaceans, sea urchins, sea stars, annelids, and fish.

Holoplanktonic metazoan taxa found in marine waters include: snails (pteropods and heteropods), cnidarians (corals and jellyfish), ctenophores (comb jellies), crustaceans (cope-pods, ostracods, amphipods, and krill), and salps. Copepods are the most abundant and diverse group of metazoan zooplankton, dominating oceanic and coastal waters. Krill are shrimplike crustaceans that are abundant in regions of high productivity. In the Antarctic, krill are the primary food of baleen whales. The gelatinous zooplankton are made up of a poly-phyletic array of metazoans that includes scyphozoans (true jellyfish), ctenophores (comb jellies), salps, and larvaceans. Other metazoan groups with holoplanktonic members include chaetognaths (arrow worms), pteropods (snails), and polychaete worms.

The holoplanktonic metazoan taxa that are important in freshwater ecosystems are rotifers and cladoceran and copepod crustaceans. Rotifers are small (100 to 250 ^m) metazoans with a distinctive rotating mode of locomotion. They often account for more than 50 percent of the zooplankton in freshwater ecosystems, occurring in densities as high as 20,000 individuals per liter of seawa-ter. Cladocerans, also known as "water fleas," are small, transparent crustaceans, and are often the most abundant metazoans in the zooplankton. Both calanoid and cyclopoid crustaceans are also ubiquitous components of the zooplankton in freshwater systems. Freshwater jellyfish (limnomedusae) are also found in various lakes in different localities all over the world.

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