Nekton refers to the aggregate of actively swimming aquatic organisms in a body of water (usually oceans or lakes) able to move independently of water currents.
Nekton are contrasted with plankton which refers to the aggregate of passively floating, drifting, or somewhat motile organisms occurring in a body of water, primarily comprising tiny algae and bacteria, small eggs and larvae of marine organisms, and protozoa and other minute predators.
As a guideline, nekton are larger and tend to swim largely at biologically high Reynolds numbers (>10^3 and up beyond 10^9), 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 gradually transition to nekton as they grow.
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Oceanic nekton comprises animals largely from three clades
The term nekton was coined in 1890 by Ernst Haeckel; it is rooted in the Greek adjective νηκτός nēktós ("the swimming") derived from the verb νήχειν nḗkhein ("to swim"). The study of swimming organisms (biofluidynamics, biomechanics, functional morphology of fluid locomotion, locomotor physiology) is called nektology. One who studies swimming in all its forms is called a nektologist.