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Aggregate of actively swimming aquatic organisms in a body of water

Nekton or necton (from the νηκτόν (nekton)) 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.

The term was first proposed and used by the German biologist Ernst Haeckel in 1891 in his article Plankton Studien where he contrasted it with plankton, the aggregate of passively floating, drifting, or somewhat motile organisms present in a body of water, primarily tiny algae and bacteria, small eggs and larvae of marine organisms, and protozoa and other minute consumers. Today it is sometimes considered an obsolete term because it often does not allow for the meaningful quantifiable distinction between these two groups. The colonization of water column is extremely crucial and important for the evolution of marine animals. During the Devonian Nekton Revolution (DNR) well known as the ‘age of fishes’ accounted more than eighty five percent of nekton were widespread during the Carboniferous period, that took place during the Paleozoic era. Some biologists no longer use it.

As a guideline, nekton are larger and tend to swim largely at biologically high Reynolds numbers (>103 and up beyond 109), where inertial flows are the rule, and eddies (vortices) are easily shed. Plankton, on the other hand, are small and, if they swim at all, do so at biologically low Reynolds numbers (0.001 to 10), where the viscous behavior of water dominates, and reversible flows are the rule. Organisms such as jellyfish and others are considered plankton when they are very small and swim at low Reynolds numbers, and considered nekton as they grow large enough to swim at high Reynolds numbers. Many animals considered classic examples of nekton (e. g., Mola mola, squid, marlin) start out life as tiny members of the plankton and then, it was argued, gradually transition to nekton as they grow.