Nekton

Nekton or necton (from the Greek: νηκτόν, translit. nekton, lit. "to swim") refers to aquatic organisms that can actively and persistently propel themselves (i.e. swim) through a water column. The term was proposed by German biologist Ernst Haeckel to differentiate between the active swimmers in a body of water, and the planktons that were passively carried along by the water current. As a guideline, nektonic organisms have a high Reynolds number (greater than 1000) and planktonic organisms a low one (less than 10). However, some organisms can begin life as planktonic larvae and transition to nektonic juveniles and adults later on in life, sometimes making distinction difficult when attempting to classify certain plankton-to-nekton species as one or the other. For this reason, some biologists choose not to use this term.