Nekton ornecton (from theAncient Greek:νηκτόν,romanized: nekton,lit. 'to swim') is anyaquatic organism that can actively and persistentlypropel itself through awater column (i.e. swim) without touching thebottom. Nekton generally have powerfultails andappendages (e.g.fins,pleopods,flippers orjets) that make them strong enough swimmers to counterocean currents, and have mechanisms for sufficientlift and/orbuoyancy to prevent sinking. Examples ofextant nekton include mostfish (especiallypelagic fish liketuna andsharks),marine mammals (cetaceans,sirenia andpinnipeds) andreptiles (specificallysea turtles),penguins,coleoidcephalopods (squids andcuttlefish) and several species ofdecapod crustaceans (specificallyprawns,shrimp andkrill).
The term was proposed by German biologistErnst Haeckel to differentiate between the active swimmers in abody of water, and theplankton that are passively carried along by the current. As a guideline, nektonic organisms have a highReynolds number (greater than 1000) and planktonic organisms a low one (less than 10). Some organisms begin theirlife cycle asplanktonic eggs and larvae, and transition to nektonicjuveniles andadults later in life. This may make 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.
The term was first proposed and used by the German biologistErnst Haeckel in 1891 in his articlePlankton-Studien where he contrasted it withplankton, the aggregate of passively floating, drifting, or somewhat motile organisms present in a body of water, primarily tinyalgae andbacteria, small eggs and larvae of marine organisms, andprotozoa and other minute consumers. Today[when?] it is sometimes considered an obsolete term because it often does not allow for a meaningful quantifiable distinction between these two groups. The colonization of the water column is very important for the evolution of marine animals. The Devonian Nekton Revolution (DNR), well known as theAge of Fishes, accounted for more than eighty-five percent of nekton, which were widespread during theCarboniferous period that took place during thePaleozoic era. Some biologists no longer use the term.[1][2]
As a guideline, nekton are larger and tend to swim largely at biologically highReynolds numbers (>1000 to 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 asjellyfish 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 is argued, gradually transition to nekton as they grow.
Oceanic nekton comprisesaquatic animals largely from threeclades:
There are organisms whose initiallife stage is identified as planktonic, but when they grow and increase in body size they become gradually more nektonic. A typical example is themedusa of the jellyfish, which can actively propel itself (though generally insufficient to overcome strong currents).
