Nekton

Nektons generally have powerful tails and appendages (e.g. fins, pleopods, flippers or jet propulsion) that make them strong enough swimmers to counter ocean currents, and have mechanisms for sufficient lift and/or buoyancy to prevent sinking.

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.

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.

[1][2] 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.