Copepods

Free-living copepods form a crucial link in the food chain and are often assigned the role of "primary consumers." Although some large forms of copepods are predators, free-living copepods are generally herbivores, feeding only on plant plankton which they filter from the water. Specifically, they eat small plant plankton and are, in turn, eaten by larger animals, like herring or mackerel. More fish and other aquatic animals feed on copepods than any other kind of animal in existence. One of the dominant forms of animal plankton, some scientists estimate that there are more copepods on the planet than all other multicellular animals combined.