Aquatic animals and protozoans whose movements are largely dependent upon currents. This diverse assemblage includes organisms that feed on bacteria, phytoplankton, and other zooplankton, as well as organisms that may not feed at all. Zooplankton may be divided into holoplankton, organisms which spend their entire lives as plankton, such as krill, and meroplankton, organisms that exist in the plankton for only part of their lives, such as crab larvae. Fish may live their entire lives in the water column, but are only classified as zooplankton while in their embryonic and larval stages.