Just about any sample of seawater taken will be full of plankton. Plankton is made up of microscopic plants (phytoplankton) and animals (zooplankton). Zooplankton are mostly too small to be seen with the naked eye. They are made up of the larva of some sea creatures, and other adult animals that never grow above microscopic sizes. Zooplankton are the second step in most food chains. Zooplankton eat phytoplankton, and often other zooplankton as well. They are in turn eaten by very small animals.
Some zooplankton swim while others simply drift. Some examples of the kinds of animals that can be found in zooplankton are krill (the tiny shrimp eaten blue whales), tiny jellyfish, and crab larvae. Many of the ocean's animals spend at least part of their lives drifting through the soup of plankton.