View source for Nekton

{{Short description|Actively-swimming type of aquatic life}}

'''Nekton''' or '''necton''' (from the {{langx|grc|νηκτόν|nekton|to swim}}) is any [[aquatic organism]] that can actively and persistently [[aquatic locomotion|propel]] itself through a [[water column]] (i.e. swim) without touching the [[seabed|bottom]]. Nekton generally have powerful [[tail]]s and [[appendage]]s (e.g. [[fish fin|fin]]s, [[pleopod]]s, [[flipper (anatomy)|flipper]]s or [[aquatic locomotion#Jet propulsion|jet]]s) that make them strong enough swimmers to counter [[ocean current]]s, and have mechanisms for sufficient [[lift (physics)|lift]] and/or [[buoyancy]] to prevent sinking. Examples of [[extant taxon|extant]] nekton include most [[fish]] (especially [[pelagic fish]] like [[tuna]] and [[shark]]s), [[marine mammal]]s ([[cetacean]]s, [[sirenia]] and [[pinniped]]s) and [[marine reptile|reptile]]s (specifically [[sea turtle]]s), [[penguin]]s, [[coleoid]] [[cephalopod]]s ([[squid]]s and [[cuttlefish]]) and several species of [[decapod crustacean]]s (specifically [[prawn]]s, [[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 [[biological life cycle|life cycle]] as [[Ichthyoplankton|planktonic eggs and larvae]], and transition to nektonic [[juvenile (organism)|juvenile]]s and [[adult]]s 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==
[[File:Neuston, Plankton, Nekton, Benthos.jpg|thumb|upright=1.25| {{center|Nekton (organisms that swim against water currents) can be contrasted with [[plankton]] (organisms that drift with water currents), [[neuston]] (organisms that live at the ocean surface) and [[benthos]] (organisms that live at the ocean floor)}}]]

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{{when|date=September 2025}} 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.<ref name="Aleyev1977">{{cite book|title=Nekton|last1=Aleyev|first1=Yu. G.|year=1977|doi=10.1007/978-94-010-1324-6|isbn=978-94-010-1326-0}}</ref><ref>Haeckel, E. 1891. Plankton-Studien. ''Jenaische Zeitschrift für Naturwissenschaft'' 25 / (Neue Folge) 18: 232–336. [https://www.biodiversitylibrary.org/page/11963729 BHL].</ref>

==Definition==
As a guideline, nekton are larger and tend to swim largely at biologically high [[Reynolds number]]s (>1000 to beyond 10<sup>9</sup>), 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 [[aquatic animal]]s largely from three [[clade]]s:
*[[Vertebrate]]s (phylum [[Chordata]]), particularly [[pelagic fish]], [[cetacean]]s and [[sea turtle]]s, form the largest contribution; these animals have [[endoskeleton]]s made of [[bone]]s and [[cartilage]] and propel themselves via a powerful [[tail]] and [[paddle]]-shaped [[appendage]]s such as [[fish fin|fin]]s, [[flipper (anatomy)|flipper]]s or [[webbed feet]].
*[[Cephalopod]]s (phylum [[Mollusca]]), specifically [[Decapodiformes|decapodiform]] [[coleoid]]s such as squids and [[cuttlefish]], are [[pelagic]] nekton that swim using a combination of [[aquatic locomotion#Jet propulsion|jet propulsion]] and [[cephalopod fin|fin]]s. [[Octopodiformes|Octopodiform]] coleoids such as [[octopus]]es can also swim quite robustly, but they are mostly [[benthic]] [[ambush predator]]s using [[cephalopod limb|arm]]s to crawl around.
*[[Crustacean]]s (phylum [[Arthropoda]]), especially [[decapod]] [[Eucarida|eucarid]]s such as [[prawn]]s, [[shrimp]] and [[krill]] that can swim actively using specialized [[arthropod leg|leg]]s known as [[pleopod]]s (a.k.a. [[swimmeret]]s) and a tail fan formed by the [[telson]] and [[uropod]]s. Benthic decapods such as [[lobster]]s and [[crayfish]], though normally moving by [[walking]], can also temporarily [[caridoid escape reaction|swim quickly backwards]] as an [[escape response]]. Some [[crab]] species can also swim in open waters using their last pair of legs ([[pereiopod]]s) for paddling.

There are organisms whose initial [[biological life cycle|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 (biology)|medusa]] of the jellyfish, which can actively propel itself (though generally insufficient to overcome strong currents).

== See also ==
* [[Neuston]] (organisms, including microscopic, living at the surface of the water)
* [[Benthos]] (organisms, including microscopic, living at the bottom of a body of water)

==References==
{{reflist}}

== External links ==
{{Wiktionary|nekton}}
* 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 [https://web.archive.org/web/20160509040228/http://www.stefannehring.de/downloads/061_Nehring+Albrecht-1997_Lauterbornia-31_benthon.pdf E-Text (PDF-Datei)]

{{aquatic ecosystem topics}}
{{aquatic organisms}}

{{Authority control}}

[[Category:Aquatic ecology]]
[[Category:Aquatic organisms]]
[[Category:Oceanographical terminology]]