Nekton

Nekton or necton (from the Greek: νηκτόν, translit. nekton, lit. "to swim") is an aquatic organism that can actively and persistently propel itself by swimming through a water column. Neckton, such as whales, squid and pelagic fish like tuna, can generally swim strongly enough to counter ocean currents.

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 current. As a guideline, nektonic organisms have a high Reynolds number (greater than 1000) and planktonic organisms a low one (less than 10). Some organisms begin their life cycle as planktonic eggs and 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 avoid using this term.