Gymnotiformes
225170
224754636
2008-07-10T07:55:00Z
ماني
7206760
{{Taxobox
| name = Gymnotiformes
| fossil_range = Late [[Jurassic]] - Recent<ref name=fishbase>{{FishBase order|order=Gymnotiformes|year=2007|month=Apr}}</ref>
| image = Black Ghost Knifefish 400.jpg
| image_caption = [[Black ghost knifefish]], ''[[Apteronotus albifrons]]''
| regnum = [[Animal]]ia
| phylum = [[Chordate|Chordata]]
| classis = [[Actinopterygii]]
| ordo = '''Gymnotiformes'''
| subdivision_ranks = Families
| subdivision =
*[[Apteronotidae]]
*[[Gymnotidae]]
*[[Hypopomidae]]
*[[Rhamphichthyidae]]
*[[Sternopygidae]]
}}
The '''Gymnotiformes''' is a lineage of [[ostariophysan]] [[teleost]] [[electric fish]]es. Common names found in the literature include the '''Neotropical electric fishes''', South American electric fishes, or '''American knifefishes'''. They are primarily freshwater inhabitants and have [[organ (anatomy)|organ]]s adapted to the generation of [[electric fields]].
Perhaps the best-known species is the [[electric eel]] (''Electrophorus electricus'') which uses powerful electric shocks (up to 600 V) for hunting and self-defense. Other species familiar in the aquarium trade are: the [[black ghost knifefish]] (''Apteronotus albifrons''), the [[glass knifefish]] (''[[Eigenmannia virescens]]''), and the banded knifefish ''[[Gymnotus carapo]]''.
==Taxonomy==
There are currently about 150 known species in 32 genera contained in 5 families, and at least 50 or so additional species are known and are yet to be formally described<ref name="Albert&Crampton, 2005a">Albert, J.S., and W.G.R. Crampton. 2005. Electroreception and electrogenesis. Pp. 431-472 in The Physiology of Fishes, 3rd Edition. D.H. Evans and J.B. Claiborne (eds.). CRC Press.</ref>. The actual number of species in the wild is unknown<ref name="Albert&Crampton, 2005b">Albert, J.S. and W.G.R. Crampton. 2005. Diversity and phylogeny of Neotropical electric fishes (Gymnotiformes). Pp. 360-409 in Electroreception. T.H. Bullock, C.D. Hopkins, A.N. Popper, and R.R. Fay (eds.). Springer Handbook of Auditory Research, Volume 21 (R.R. Fay and A. N. Popper, eds). Springer-Verlag, Berlin.</ref>. This group is thought to be the sister group to the [[Siluriformes]]<ref>Fink and Fink, 1996</ref> from which they diverged in the Cretacous Period (about 120 million yearrs ago).
The families are classified over suborders and superfamilies as below.<ref>Nelson</ref>
Order '''Gymnotiformes'''
: Suborder '''Gymnotoidei'''
::: Family [[Gymnotidae]] (banded knifefishes and electric eel)
: Suborder '''Sternopygoidei'''
:: Superfamily '''Rhamphichthyoidea'''
::: Family [[Rhamphichthyidae]] (sand knifefishes)
::: Family [[Hypopomidae]] (bluntnose knifefishes)
:: Superfamily '''Apteronotoidea'''
::: Family [[Sternopygidae]] (glass knifefishes)
::: Family [[Apteronotidae]] (ghost knifefishes)
==Distribution and habitat==
Gymnotiform fishes inhabit freshwater rivers and streams throughout the humid [[Neotropic]]s, ranging from [[Guatemala]] to Northern [[Argentina]]. They are [[nocturnal]] fishes.
==Evolution==
Gymnotiformes are regarded as the among more derived members of [[Ostariophysi]], a lineage of primary freshwater fishes. They arose in the western portion of Gondwana in what is now South America before the geological sepatation with Africa about 120 million years ago. The only known [[fossils]] are from the [[Miocene]] about 10 million years ago of [[Bolivia]]<ref name="Albert, J.S. and W.L. Fink. 2007">Albert, J.S. and W.L. Fink. 2007. Phylogenetic relationships of fossil Neotropical electric fishes (Osteichthyes: Gymnotiformes) from the Upper Miocene of Bolivia. Journal Vertebrate Paleontology 27(1):17-25.</ref>.
Gymnotiformes has no extant species in [[Africa]]. This may be because they did not spread into Africa before South America and Africa split, or it may be that they were outcompeted by [[Mormyridae|mormyrids]], which are similar in that they also use [[electrolocation]].<ref name="Albert&Crampton, 2005a"/>.
==Physical characteristics==
Gymnotiformes have a knife-shaped (culteriform), eel-like body that is either compressed or cylindrical. They lack pelvic fins or a dorsal fin. The anal fin is extremely long; this fin is undulated to allow the fish to move both forwards and backwards. The caudal fin is absent or, in the apteronotids, greatly reduced. The gill opening is restricted. The anal opening is under the head or the pectoral fins<ref name=Albert>Albert, J.S. 2001. Species diversity and phylogenetic systematics of American knifefishes (Gymnotiformes, Teleostei). Misc. Publ. Mus. Zool. University of Michigan, 190:1-127.</ref>.
These fish possess [[electric organ]]s that allow them to produce electricity. In most Gymnotiforms, the electric organs are derived from muscle cells. However, in adult apteronotids they are derived from nerve cells (spinal electromotor neurons). All gymnotiform species generate weak (<1 V) electric fields, using them for navigation and communication. The electric discharge is continuous, being generated day and night throughout the entire life of the individual. Certain aspects of the electric signal are unique to each species, especially a combination of the pulse waveform, duration and repetition rate {Crampton and Albert, 2006}.
==See also==
*[[Electric fish]]
*[[Bioelectromagnetism]]
==References==
*Crampton, W.G.R. and J.S. Albert. 2006. Evolution of electric signal diversity in gymnotiform fishes. Pp. 641-725 in Communication in Fishes. F. Ladich, S.P. Collin, P. Moller & B.G Kapoor (eds.). Science Publishers Inc., Enfield, NH.
<references />
==External links==
* [http://www.flmnh.ufl.edu/fish/tropical/JSA/gymno.htm Gymnotiform overview]
* [http://www.amazonian-fish.co.uk/indexster.html Photos of various gymnotiforms]
{{Actinopterygii}}
[[Category:Gymnotiformes| ]]
[[de:Neuwelt-Messerfische]]
[[es:Gymnotiformes]]
[[fa:برقماهیشکلان]]
[[fr:Gymnotiformes]]
[[lt:Elektražuvės]]
[[hu:Elektromoskéshal-alakúak]]
[[nl:Mesaalachtigen]]
[[no:Elektroåler]]
[[oc:Gymnotiformes]]
[[pl:Strętwokształtne]]
[[pt:Gimnoto]]
[[simple:Gymnotiformes]]
[[fi:Veitsikalat]]
[[zh:电鳗亚目]]