Nekton or necton (from the ) is any aquatic organism that can actively and persistently propel itself through a water column (i.e. swim) without touching the seabed. Nekton generally have powerful and (e.g. fish fin, , flippers or jets) that make them strong enough swimmers to counter , and have mechanisms for sufficient lift and/or buoyancy to prevent sinking. Examples of extant taxon nekton include most fish (especially pelagic fish like tuna and ), (, sirenia and ) and marine reptile (specifically ), , coleoid ( and cuttlefish) and several species of decapod crustaceans (specifically , shrimp and krill).
The term was proposed by German biologist Ernst Haeckel to differentiate between the active swimmers in a body of water, and the plankton that are 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 Ichthyoplankton, and transition to nektonic juveniles and 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.
History
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 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 the
Age of Fishes, accounted for more than eighty-five percent of nekton, which were widespread during the Carboniferous period that took place during the
Paleozoic era. Some biologists no longer use the term.
[Haeckel, E. 1891. Plankton-Studien. Jenaische Zeitschrift für Naturwissenschaft 25 / (Neue Folge) 18: 232–336. BHL.]
Definition
As a guideline, nekton are larger and tend to swim largely at biologically high
(>1000 to 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 then, it is argued, gradually transition to nekton as they grow.
Oceanic nekton
Oceanic nekton comprises
largely from three
:
-
(phylum Chordata), particularly pelagic fish, and , form the largest contribution; these animals have made of and cartilage and propel themselves via a powerful tail and paddle-shaped such as fish fin, flippers or webbed feet.
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(phylum Mollusca), specifically Decapodiformes such as squids and cuttlefish, are pelagic nekton that swim using a combination of jet propulsion and cephalopod fin. Octopodiformes coleoids such as can also swim quite robustly, but they are mostly benthic using cephalopod limb to crawl around.
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(phylum Arthropoda), especially decapod Eucarida such as , shrimp and krill that can swim actively using specialized arthropod leg known as (a.k.a. ) and a tail fan formed by the telson and . Benthic decapods such as and crayfish, though normally moving by walking, can also temporarily swim quickly backwards as an escape response. Some crab species can also swim in open waters using their last pair of legs () for paddling.
There are organisms whose initial life stage is identified as planktonic, but when they grow and increase in body size they become gradually more nektonic. A typical example is the medusa of the jellyfish, which can actively propel itself (though generally insufficient to overcome strong currents).
See also
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Neuston (organisms, including microscopic, living at the surface of the water)
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Benthos (organisms, including microscopic, living at the bottom of a body of water)
External links
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Stefan Nehring and Ute Albrecht (1997): "Hell und das redundante Benthon: Neologismen in der deutschsprachigen Limnologie". In: Lauterbornia H. 31: 17–30, Dinkelscherben, December 1997 E-Text (PDF-Datei)
