A Plankton:

Plankton are any drifting organism that inhabits the pelagic zone of oceans, seas, or bodies of fresh water. It is a description of life-style rather than a genetic classification. They are widely considered to be some of the most important organisms on Earth, due to the food supply they provide to most aquatic life.The name plankton is derived from the Greek word πλανκτος ("planktos"), meaning "wanderer" or "drifter".While some forms of plankton are capable of independent movement and can swim up to several hundreds of meters vertically in a single day (a behavior called diel vertical migration), their horizontal position is primarily determined by currents in the body of water they inhabit. By definition, organisms classified as plankton are unable to resist ocean currents. This is in contrast to nekton organisms that can swim against the ambient flow of the water environment and control their position (e.g. squid, fish, and marine mammals).Within the plankton, itself, holoplankton are those organisms that spend their entire life cycle as part of the plankton (e.g. most algae, copepods, salps, and some jellyfish). By contrast, meroplankton are those organisms that are only planktonic for part of their lives (usually the larval stage), and then graduate to either the nekton or a benthic (sea floor) existence. Examples of meroplankton include the larvae of sea urchins, starfish, crustaceans, marine worms, and most fish.Plankton abundance and distribution are strongly dependent on factors such as ambient nutrients concentrations, the physical state of the water column, and the abundance of other plankton.The study of plankton is termed planktology. Individual plankton are referred to as plankters.