Zooplankton

Zooplankton are the consumers of the plankton, the community of organisms that drift with currents because they cannot swim strongly against them. They include single-celled protists such as foraminifera and radiolarians, small crustaceans such as copepods and krill, arrow worms, tiny swimming snails, comb jellies and jellyfish. Some, called holoplankton, drift for their whole lives. Others, called meroplankton, are plankton only as eggs and larvae, including young fish, crabs, sea stars and many other animals that settle on the bottom or swim freely as adults.

Most zooplankton graze on phytoplankton, and they in turn are eaten by fish, seabirds, squid and filter-feeding whales, so they form the vital link that passes energy from the ocean's producers to larger animals. Copepods are among the most numerous animals on Earth, and Antarctic krill support penguins, seals and baleen whales. Many zooplankton rise to the surface at night to feed and sink to darker depths by day to avoid predators. This daily vertical migration is thought to be the largest regular movement of animals on the planet, and it helps carry carbon into the deep ocean.

Because they respond quickly to temperature, pH and food supply, zooplankton are watched as early signs of change in ocean ecosystems.