1. Floating animal organisms collectively: Tiny creatures, or zooplankton, consist of rotifers, copepods, and krill, or microorganisms once classified as animals, such as dinoflagellates and other protozoans.
2. A small animal organism present in natural waters: The majority of zooplankton are little crustaceans, for example, copepods and krill, arrowworms, and gelatinous creatures that feed primarily on phytoplankton.
3. Microscopic animals that move passively in aquatic ecosystems, such as, protozoans: To the zooplankton belong the protozoa, the sea anemones, the corals, and the incredibly shaped jellyfishes.
4. Microscopic drifting lifeforms much of which live on or near the surface of the water, but some are at greater depths: The zoologist was studying zooplankton and the nutritional requirements and efficiencies of food conversion of marine animals, or zooplankters, feeding on marine phytoplankters or on other small animals.