Zooplankton are tiny aquatic animals that cannot move against currents in the water. Zooplankton are either herbivorous, feeding on phytoplankton, or carnivorous, feeding on other zooplankton. They themselves are fed upon by other zooplankton, fish, and even whales. Zooplankton is the vital transition between marine primary production (phytoplankton) and large animals (fish).
Zooplankton, like all plankton, exist in the epipelagic zone of the ocean. It is possible for these creatures to move up and down in the water, (diel vertical migration). However, if the organism sinks too low, it will not be able to reach a suitable height, and will be washed out of the system.
Some types of marine zooplankton include:
Meroplankton (organisms that spend only their larval stages as plankton)
Holoplankton (organisms that spend their entire lives as plankton)