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{{Short description|Order of bony fishes}}
{{Short description|Order of bony fishes}}
{{Automatic taxobox
{{Automatic_taxobox
| name = South American knifefish
| name = South American knifefish
| fossil_range = {{Fossil range |Late Jurassic |Recent}}<ref name=fishbase>{{FishBase order |order=Gymnotiformes |year=2007 |month=Apr}}</ref>
| fossil_range = {{Fossil range |Late Jurassic |Recent}}<ref name=fishbase>{{FishBase order |order=Gymnotiformes |year=2007 |month=Apr}}</ref>
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| taxon = Gymnotiformes
| taxon = Gymnotiformes
| type_species = ''[[Gymnotus carapo]]''
| type_species = ''[[Gymnotus carapo]]''
| type_species_authority = [[Carl Linnaeus |Linnaeus]], [[10th edition of Systema Naturae |1758]]
| type_species_authority = [[Carl Linnaeus|Linnaeus]], [[10th edition of Systema Naturae|1758]]
}}
}}
[[File:Electric Eel- despite the name, these are note eels at all (27900224295).jpg |thumb |Despite the name, the Electric Eel is a type of knifefish]]
[[File:Electric Eel- despite the name, these are note eels at all (27900224295).jpg |thumb|Despite the name, the electric eel is a type of knifefish.]]
The '''Gymnotiformes''' {{IPAc-en|dʒ|ɪ|m|ˈ|n|ɒ|t|ᵻ|f|ɔːr|m|iː|z}} are an order of [[teleost]] [[bony fish]]es commonly known as '''Neotropical knifefish''' or '''South American knifefish'''. They have long bodies and swim using undulations of their elongated [[anal fin]]. Found almost exclusively in [[fresh water]] (the only exceptions are species that occasionally may visit [[brackish water]] to feed), these mostly [[nocturnal]] fish are capable of [[Electroreception and electrogenesis |producing electric fields to detect prey]], for navigation, communication, and, in the case of the [[electric eel]] (''Electrophorus electricus''), attack and defense.<ref name=Sleen2017>{{cite book |editor1=van der Sleen, P. |editor2=Albert, J. S. |year=2017 |title=Field Guide to the Fishes of the Amazon, Orinoco, and Guianas |publisher=Princeton University Press |pages=322–345 |isbn=978-0691170749 }}</ref> A few species are familiar to the [[fishkeeping |aquarium trade]], such as the [[black ghost knifefish]] (''Apteronotus albifrons''), the [[glass knifefish]] (''Eigenmannia virescens''), and the [[banded knifefish]] (''Gymnotus carapo'').
The '''Gymnotiformes''' {{IPAc-en|dʒ|ɪ|m|ˈ|n|ɒ|t|ᵻ|f|ɔːr|m|iː|z}} are an order of [[teleost]] [[bony fish]]es commonly known as '''Neotropical knifefish''' or '''South American knifefish'''. They have long bodies and swim using undulations of their elongated [[anal fin]]. Found almost exclusively in [[fresh water]] (the only exceptions are species that occasionally may visit [[brackish water]] to feed), these mostly [[nocturnal]] fish are capable of [[Electroreception and electrogenesis|producing electric fields to detect prey]], for navigation, communication, and, in the case of the [[electric eel]] (''Electrophorus electricus''), attack and defense.<ref name=Sleen2017>{{cite book |editor1=van der Sleen, P. |editor2=Albert, J. S. |year=2017 |title=Field Guide to the Fishes of the Amazon, Orinoco, and Guianas |publisher=Princeton University Press |pages=322–345 |isbn=978-0691170749 }}</ref> A few species are familiar to the [[fishkeeping|aquarium trade]], such as the [[black ghost knifefish]] (''Apteronotus albifrons''), the [[glass knifefish]] (''Eigenmannia virescens''), and the [[banded knifefish]] (''Gymnotus carapo'').


== Description ==
== Description ==
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Aside from the electric eel (''Electrophorus electricus''), Gymnotiformes are slender fish with narrow bodies and tapering tails, hence the common name of "knifefishes". They have neither [[pelvic fin]]s nor [[dorsal fin]]s, but do possess greatly elongated [[anal fin]]s that stretch along almost the entire underside of their bodies. The fish swim by rippling this fin, keeping their bodies rigid. This means of propulsion allows them to move backwards as easily as they move forward.<ref name=EoF>{{cite book |editor=Paxton, J.R. |editor2=Eschmeyer, W.N. |last=Ferraris |first=Carl J. |year=1998 |title=Encyclopedia of Fishes |publisher=Academic Press |location=San Diego |pages=111–112 |isbn=0-12-547665-5}}</ref>
Aside from the electric eel (''Electrophorus electricus''), Gymnotiformes are slender fish with narrow bodies and tapering tails, hence the common name of "knifefishes". They have neither [[pelvic fin]]s nor [[dorsal fin]]s, but do possess greatly elongated [[anal fin]]s that stretch along almost the entire underside of their bodies. The fish swim by rippling this fin, keeping their bodies rigid. This means of propulsion allows them to move backwards as easily as they move forward.<ref name=EoF>{{cite book |editor=Paxton, J.R. |editor2=Eschmeyer, W.N. |last=Ferraris |first=Carl J. |year=1998 |title=Encyclopedia of Fishes |publisher=Academic Press |location=San Diego |pages=111–112 |isbn=0-12-547665-5}}</ref>


The knifefish has approximately one hundred and fifty fin rays along its ribbon-fin. These individual fin rays can be curved nearly twice the maximum recorded curvature for [[Actinopterygii |ray-finned fish]] fin rays during [[Fish locomotion |locomotion]]. These fin rays are curved into the direction of motion, indicating that the knifefish has active control of the fin ray curvature, and that this curvature is not the result of passive bending due to fluid loading.<ref>{{cite journal |last1=Youngerman |first1=Eric D. |last2=Flammang |first2=Brooke E. |last3=Lauder |first3=George V. |title=Locomotion of free-swimming ghost knifefish: anal fin kinematics during four behaviors |journal=Zoology |date=October 2014 |volume=117 |issue=5 |pages=337–348 |doi=10.1016/j.zool.2014.04.004 |pmid=25043841 }}</ref>
The knifefish has approximately one hundred and fifty fin rays along its ribbon-fin. These individual fin rays can be curved nearly twice the maximum recorded curvature for [[Actinopterygii|ray-finned fish]] fin rays during [[Fish locomotion|locomotion]]. These fin rays are curved into the direction of motion, indicating that the knifefish has active control of the fin ray curvature, and that this curvature is not the result of passive bending due to fluid loading.<ref>{{cite journal |last1=Youngerman |first1=Eric D. |last2=Flammang |first2=Brooke E. |last3=Lauder |first3=George V. |title=Locomotion of free-swimming ghost knifefish: anal fin kinematics during four behaviors |journal=Zoology |date=October 2014 |volume=117 |issue=5 |pages=337–348 |doi=10.1016/j.zool.2014.04.004 |pmid=25043841 |bibcode=2014Zool..117..337Y }}</ref>


Different wave patterns produced along the length of the elongated anal fin allow for various forms of thrust. The wave motion of the fin resembles traveling [[Sine wave|sinusoidal waves]]. A forward traveling wave can be associated with forward motion, while a wave in the reverse direction produces thrust in the opposite direction.<ref name=":2">{{cite journal |last1=Shirgaonkar |first1=Anup A. |last2=Curet |first2=Oscar M. |last3=Patankar |first3=Neelesh A. |last4=MacIver |first4=Malcolm A. |title=The hydrodynamics of ribbon-fin propulsion during impulsive motion |journal=Journal of Experimental Biology |date=1 November 2008 |volume=211 |issue=21 |pages=3490–3503 |doi=10.1242/jeb.019224 |pmid=18931321 |s2cid=10911068 |doi-access=free }}</ref> This undulating motion of the fin produced a system of linked vortex tubes that were produced along the bottom edge of the fin. A jet was produced at an angle to the fin that was directly related to the vortex tubes, and this jet provides propulsion that moves the fish forward.<ref>{{cite journal |last1=Neveln |first1=I. D. |last2=Bale |first2=R. |last3=Bhalla |first3=A. P. S. |last4=Curet |first4=O. M. |last5=Patankar |first5=N. A. |last6=MacIver |first6=M. A. |title=Undulating fins produce off-axis thrust and flow structures |journal=Journal of Experimental Biology |date=15 January 2014 |volume=217 |issue=2 |pages=201–213 |doi=10.1242/jeb.091520 |pmid=24072799 |s2cid=2656865 |doi-access=free }}</ref> The wave motion of the fin is similar to that of other marine creatures, such as the undulation of the body of an [[eel]], however the [[Wake turbulence |wake vortex]] produced by the knifefish was found to be a reverse [[Kármán vortex street |Kármán vortex]]. This type of vortex is also produced by some fish, such as [[trout]], through the oscillations of their [[Fish fin |caudal fins]].<ref name=":3">{{cite journal |last1=Neveln |first1=I. D. |last2=Bai |first2=Y. |last3=Snyder |first3=J. B. |last4=Solberg |first4=J. R. |last5=Curet |first5=O. M. |last6=Lynch |first6=K. M. |last7=MacIver |first7=M. A. |title=Biomimetic and bio-inspired robotics in electric fish research |journal=Journal of Experimental Biology |date=1 July 2013 |volume=216 |issue=13 |pages=2501–2514 |doi=10.1242/jeb.082743 |pmid=23761475 |s2cid=14992273 |doi-access=free }}</ref> The speed at which the fish moved through the water had no correlation to the amplitude of its undulations, however it was directly related to the frequency of the waves generated.<ref name=":4">{{cite journal |last1=Xiong |first1=Grace |last2=Lauder |first2=George V. |title=Center of mass motion in swimming fish: effects of speed and locomotor mode during undulatory propulsion |journal=Zoology |date=August 2014 |volume=117 |issue=4 |pages=269–281 |doi=10.1016/j.zool.2014.03.002 |pmid=24925455 }}</ref>
Different wave patterns produced along the length of the elongated anal fin allow for various forms of thrust. The wave motion of the fin resembles traveling [[Sine wave|sinusoidal waves]]. A forward traveling wave can be associated with forward motion, while a wave in the reverse direction produces thrust in the opposite direction.<ref name=":2">{{cite journal |last1=Shirgaonkar |first1=Anup A. |last2=Curet |first2=Oscar M. |last3=Patankar |first3=Neelesh A. |last4=MacIver |first4=Malcolm A. |title=The hydrodynamics of ribbon-fin propulsion during impulsive motion |journal=Journal of Experimental Biology |date=1 November 2008 |volume=211 |issue=21 |pages=3490–3503 |doi=10.1242/jeb.019224 |pmid=18931321 |s2cid=10911068 |doi-access= }}</ref> This undulating motion of the fin produced a system of linked vortex tubes that were produced along the bottom edge of the fin. A jet was produced at an angle to the fin that was directly related to the vortex tubes, and this jet provides propulsion that moves the fish forward.<ref>{{cite journal |last1=Neveln |first1=I. D. |last2=Bale |first2=R. |last3=Bhalla |first3=A. P. S. |last4=Curet |first4=O. M. |last5=Patankar |first5=N. A. |last6=MacIver |first6=M. A. |title=Undulating fins produce off-axis thrust and flow structures |journal=Journal of Experimental Biology |date=15 January 2014 |volume=217 |issue=2 |pages=201–213 |doi=10.1242/jeb.091520 |pmid=24072799 |s2cid=2656865 |doi-access=free }}</ref> The wave motion of the fin is similar to that of other marine creatures, such as the undulation of the body of an [[eel]], however the [[Wake turbulence|wake vortex]] produced by the knifefish was found to be a reverse [[Kármán vortex street|Kármán vortex]]. This type of vortex is also produced by some fish, such as [[trout]], through the oscillations of their [[Fish fin|caudal fins]].<ref name=":3">{{cite journal |last1=Neveln |first1=I. D. |last2=Bai |first2=Y. |last3=Snyder |first3=J. B. |last4=Solberg |first4=J. R. |last5=Curet |first5=O. M. |last6=Lynch |first6=K. M. |last7=MacIver |first7=M. A. |title=Biomimetic and bio-inspired robotics in electric fish research |journal=Journal of Experimental Biology |date=1 July 2013 |volume=216 |issue=13 |pages=2501–2514 |doi=10.1242/jeb.082743 |pmid=23761475 |s2cid=14992273 |doi-access=free }}</ref> The speed at which the fish moved through the water had no correlation to the amplitude of its undulations, however it was directly related to the frequency of the waves generated.<ref name=":4">{{cite journal |last1=Xiong |first1=Grace |last2=Lauder |first2=George V. |title=Center of mass motion in swimming fish: effects of speed and locomotor mode during undulatory propulsion |journal=Zoology |date=August 2014 |volume=117 |issue=4 |pages=269–281 |doi=10.1016/j.zool.2014.03.002 |pmid=24925455 |bibcode=2014Zool..117..269X }}</ref>


Studies have shown that the natural angle between the body of the knifefish and its fin is essential for efficient forward motion, for if the anal fin was located directly underneath, then an upwards force would be generated with forward thrust, which would require an additional downwards force in order to maintain [[neutral buoyancy]].<ref name=":3" /> A combination of forward and reverse wave patterns, which meet towards the center of the anal fin, produce a [[Ship motions |heave]] force allowing for hovering, or upwards movement.<ref name=":2" />
Studies have shown that the natural angle between the body of the knifefish and its fin is essential for efficient forward motion, for if the anal fin was located directly underneath, then an upwards force would be generated with forward thrust, which would require an additional downwards force in order to maintain [[neutral buoyancy]].<ref name=":3" /> A combination of forward and reverse wave patterns, which meet towards the center of the anal fin, produce a [[Ship motions|heave]] force allowing for hovering, or upwards movement.<ref name=":2" />


The ghost knifefish can vary the undulation of the waves, as well as the [[angle of attack]] of the fin to achieve various directional changes. The pectoral fins of these fishes can help to control [[Ship motions |roll]] and [[Ship motions |pitch]] control.<ref>{{cite journal |last1=Salazar |first1=R. |last2=Fuentes |first2=V. |last3=Abdelkefi |first3=A. |title=Classification of biological and bioinspired aquatic systems: A review |journal=Ocean Engineering |date=January 2018 |volume=148 |pages=75–114 |doi=10.1016/j.oceaneng.2017.11.012 }}</ref> By rolling they can generate a vertical thrust to quickly, and efficiently, ambush their prey.<ref name=":3" /> The forward movement is determined exclusively by the ribbon fins and the contribution of the [[Fish fin |pectoral fins]] for forward movement was negligible.<ref>{{cite journal |last1=Jagnandan |first1=Kevin |last2=Sanford |first2=Christopher P. |title=Kinematics of ribbon-fin locomotion in the bowfin, Amia calva |journal=Journal of Experimental Zoology Part A: Ecological Genetics and Physiology |date=December 2013 |volume=319 |issue=10 |pages=569–583 |doi=10.1002/jez.1819 |pmid=24039242 }}</ref> The body is kept relatively rigid and there is very little motion of the [[center of mass]] motion during locomotion compared to the body size of the fish.<ref name=":4" />
The ghost knifefish can vary the undulation of the waves, as well as the [[angle of attack]] of the fin to achieve various directional changes. The pectoral fins of these fishes can help to control [[Ship motions|roll]] and [[Ship motions|pitch]] control.<ref>{{cite journal |last1=Salazar |first1=R. |last2=Fuentes |first2=V. |last3=Abdelkefi |first3=A. |title=Classification of biological and bioinspired aquatic systems: A review |journal=Ocean Engineering |date=January 2018 |volume=148 |pages=75–114 |doi=10.1016/j.oceaneng.2017.11.012 |bibcode=2018OcEng.148...75S }}</ref> By rolling they can generate a vertical thrust to quickly, and efficiently, ambush their prey.<ref name=":3" /> The forward movement is determined exclusively by the ribbon fins and the contribution of the [[Fish fin|pectoral fins]] for forward movement was negligible.<ref>{{cite journal |last1=Jagnandan |first1=Kevin |last2=Sanford |first2=Christopher P. |title=Kinematics of ribbon-fin locomotion in the bowfin, Amia calva |journal=Journal of Experimental Zoology Part A: Ecological Genetics and Physiology |date=December 2013 |volume=319 |issue=10 |pages=569–583 |doi=10.1002/jez.1819 |pmid=24039242 |bibcode=2013JEZA..319..569J }}</ref> The body is kept relatively rigid and there is very little motion of the [[center of mass]] motion during locomotion compared to the body size of the fish.<ref name=":4" />


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>{{cite book |last1=Albert |first1=James S |title=Species diversity and phylogenetic systematics of American knifefishes (Gymnotiformes, Teleostei) |date=2001 |publisher=Museum of Zoology |oclc=248781367 |hdl=2027.42/56433 |hdl-access=free }}</ref>
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>{{cite book |last1=Albert |first1=James S |title=Species diversity and phylogenetic systematics of American knifefishes (Gymnotiformes, Teleostei) |date=2001 |publisher=Museum of Zoology |oclc=248781367 |hdl=2027.42/56433 |hdl-access=free }}</ref>


=== Electroreception and electrogenesis ===
=== Electroreception and electrogenesis ===
{{further |Electroreception and electrogenesis}}
{{Further |Electroreception and electrogenesis}}


These fish possess [[Electric organ (biology) |electric organ]]s that allow them to produce electric fields, which are usually weak. In most gymnotiforms, the electric organs are derived from muscle cells. However, adult apteronotids are one exception, as theirs are derived from nerve cells (spinal electromotor neurons). In gymnotiforms, the electric organ discharge may be continuous or pulsed. If continuous, it is 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, amplitude, phase and frequency.<ref name="Crampton and Albert 2006">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.</ref>
These fish possess [[Electric organ (biology)|electric organ]]s that allow them to produce electric fields, which are usually weak. In most gymnotiforms, the electric organs are derived from muscle cells. However, adult apteronotids are one exception, as theirs are derived from nerve cells (spinal electromotor neurons). In gymnotiforms, the electric organ discharge may be continuous or pulsed. If continuous, it is 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, amplitude, phase and frequency.<ref name="Crampton and Albert 2006">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.</ref>


The electric organs of most Gymnotiformes produce tiny discharges of just a few [[millivolt]]s, far too weak to cause any harm to other fish. Instead, they are used to help navigate the environment, including locating the bottom-dwelling invertebrates that compose their diets.<ref name="Bullock Bodznick Northcutt 1983">{{cite journal |last1=Bullock |first1=Theodore H. |author1-link=Theodore Holmes Bullock |last2=Bodznick |first2=D. A. |last3=Northcutt |first3=R. G. |date=1983 |title=The phylogenetic distribution of electroreception: Evidence for convergent evolution of a primitive vertebrate sense modality |journal=Brain Research Reviews |volume=6 |issue=1 |pages=25–46 |doi=10.1016/0165-0173(83)90003-6 |pmid=6616267 |hdl=2027.42/25137 |s2cid=15603518 |url=https://deepblue.lib.umich.edu/bitstream/2027.42/25137/1/0000573.pdf |hdl-access=free }}</ref><!--Bullock page 37--> They may also be used to send signals between fish of the same species.<ref name=Fugereetal>{{cite journal |last1=Fugère |first1=Vincent |last2=Ortega |first2=Hernán |last3=Krahe |first3=Rüdiger |title=Electrical signalling of dominance in a wild population of electric fish |journal=Biology Letters |date=23 April 2011 |volume=7 |issue=2 |pages=197–200 |doi=10.1098/rsbl.2010.0804 |pmid=20980295 |pmc=3061176 }}</ref> In addition to this low-level field, the electric eel also has the capability to [[electric fish |produce much more powerful discharges]] to stun prey.<ref name=EoF/>
The electric organs of most Gymnotiformes produce tiny discharges of just a few [[millivolt]]s, far too weak to cause any harm to other fish. Instead, they are used to help navigate the environment, including locating the bottom-dwelling invertebrates that compose their diets.<ref name="Bullock Bodznick Northcutt 1983">{{cite journal |last1=Bullock |first1=Theodore H. |author1-link=Theodore Holmes Bullock |last2=Bodznick |first2=D. A. |last3=Northcutt |first3=R. G. |date=1983 |title=The phylogenetic distribution of electroreception: Evidence for convergent evolution of a primitive vertebrate sense modality |journal=Brain Research Reviews |volume=6 |issue=1 |pages=25–46 |doi=10.1016/0165-0173(83)90003-6 |pmid=6616267 |hdl=2027.42/25137 |s2cid=15603518 |url=https://deepblue.lib.umich.edu/bitstream/2027.42/25137/1/0000573.pdf |hdl-access=free }}</ref><!--Bullock page 37--> They may also be used to send signals between fish of the same species.<ref name=Fugereetal>{{cite journal |last1=Fugère |first1=Vincent |last2=Ortega |first2=Hernán |last3=Krahe |first3=Rüdiger |title=Electrical signalling of dominance in a wild population of electric fish |journal=Biology Letters |date=23 April 2011 |volume=7 |issue=2 |pages=197–200 |doi=10.1098/rsbl.2010.0804 |pmid=20980295 |pmc=3061176 }}</ref> In addition to this low-level field, the electric eel also has the capability to [[electric fish|produce much more powerful discharges]] to stun prey.<ref name=EoF/>


==Taxonomy==
==Taxonomy==


There are currently about 250 valid gymnotiform species in 34 genera and five families, with many additional species [[Undescribed taxon|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><ref name="Eschmeyer&Fong 2016">Eschmeyer, W. N., & Fong, J. D. (2016). Catalog of fishes: Species by family/subfamily.{{pn |date=April 2021}}</ref><ref name="ReferenceA">{{cite journal |last1=Ferraris Jr |first1=Carl J. |last2=de Santana |first2=Carlos David |last3=Vari |first3=Richard P. |title=Checklist of Gymnotiformes (Osteichthyes: Ostariophysi) and catalogue of primary types |journal=Neotropical Ichthyology |date=2017 |volume=15 |issue=1 |doi=10.1590/1982-0224-20160067 |doi-access=free }}</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> Gymnotiformes is thought to be the sister group to the [[Siluriformes]]<ref>"Fink and Fink, 1996">{{cite journal |last1=Fink |first1=Sara V. |last2=Fink |first2=William L. |title=Interrelationships of the ostariophysan fishes (Teleostei) |journal=Zoological Journal of the Linnean Society |date=August 1981 |volume=72 |issue=4 |pages=297–353 |doi=10.1111/j.1096-3642.1981.tb01575.x }}</ref><ref>"Arcila et al., 2017">{{cite journal |last1=Arcila |first1=Dahiana |last2=Ortí |first2=Guillermo |last3=Vari |first3=Richard |last4=Armbruster |first4=Jonathan W. |last5=Stiassny |first5=Melanie L. J. |last6=Ko |first6=Kyung D. |last7=Sabaj |first7=Mark H. |last8=Lundberg |first8=John |last9=Revell |first9=Liam J. |last10=Betancur-R |first10=Ricardo |title=Genome-wide interrogation advances resolution of recalcitrant groups in the tree of life |journal=Nature Ecology & Evolution |date=13 January 2017 |volume=1 |issue=2 |page=20 |doi=10.1038/s41559-016-0020 |pmid=28812610 |s2cid=16535732 }}</ref> from which they diverged in the [[Cretaceous]] period (about 120 million years ago). The families have traditionally been classified over suborders and superfamilies as below.<ref name=Nelson>{{cite book |title=Fishes of the World |last1=Nelson |first1=Joseph S. | last2=Grande |first2=Terry C. |last3=Wilson |first3=Mark V. H. |publisher=John Wiley & Sons |year=2016 |edition=5 |isbn=978-1118342336 }}{{pn |date=April 2021}}</ref><ref name="ReferenceA"/>
There are currently about 250 valid gymnotiform species in 34 genera and five families, with many additional species [[Undescribed taxon|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><ref name="Eschmeyer&Fong 2016">Eschmeyer, W. N., & Fong, J. D. (2016). Catalog of fishes: Species by family/subfamily.{{page needed |date=April 2021}}</ref><ref name="ReferenceA">{{cite journal |last1=Ferraris Jr |first1=Carl J. |last2=de Santana |first2=Carlos David |last3=Vari |first3=Richard P. |title=Checklist of Gymnotiformes (Osteichthyes: Ostariophysi) and catalogue of primary types |journal=Neotropical Ichthyology |date=2017 |volume=15 |issue=1 |doi=10.1590/1982-0224-20160067 |doi-access=free }}</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> Gymnotiformes is thought to be the sister group to the [[Siluriformes]]<ref>"Fink and Fink, 1996">{{cite journal |last1=Fink |first1=Sara V. |last2=Fink |first2=William L. |title=Interrelationships of the ostariophysan fishes (Teleostei) |journal=Zoological Journal of the Linnean Society |date=August 1981 |volume=72 |issue=4 |pages=297–353 |doi=10.1111/j.1096-3642.1981.tb01575.x }}</ref><ref>"Arcila et al., 2017">{{cite journal |last1=Arcila |first1=Dahiana |last2=Ortí |first2=Guillermo |last3=Vari |first3=Richard |last4=Armbruster |first4=Jonathan W. |last5=Stiassny |first5=Melanie L. J. |last6=Ko |first6=Kyung D. |last7=Sabaj |first7=Mark H. |last8=Lundberg |first8=John |last9=Revell |first9=Liam J. |last10=Betancur-R |first10=Ricardo |title=Genome-wide interrogation advances resolution of recalcitrant groups in the tree of life |journal=Nature Ecology & Evolution |date=13 January 2017 |volume=1 |issue=2 |page=20 |doi=10.1038/s41559-016-0020 |pmid=28812610 |bibcode=2017NatEE...1...20A |s2cid=16535732 }}</ref> from which they diverged in the [[Cretaceous]] period (about 120 million years ago). The families have traditionally been classified over suborders and superfamilies as below.<ref name=Nelson>{{cite book |title=Fishes of the World |last1=Nelson |first1=Joseph S. | last2=Grande |first2=Terry C. |last3=Wilson |first3=Mark V. H. |publisher=John Wiley & Sons |year=2016 |edition=5 |isbn=978-1118342336 }}{{page needed |date=April 2021}}</ref><ref name="ReferenceA"/>


Order '''Gymnotiformes'''
Order '''Gymnotiformes'''
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|2={{clade
|2={{clade
|1=''[[Gymnotus]]'' (banded knifefishes) [[File:Farm-Fresh lightning.png|15px]] [[File:Gymnotus sp.jpg|120px]]
|1=''[[Gymnotus]]'' (banded knifefishes) [[File:Farm-Fresh lightning.png|15px]] [[File:Gymnotus sp.jpg|120px]]
|2=''[[Electric_eel|Electrophorus]]'' (electric eels) [[File:Farm-Fresh lightning.png|15px]] [[File:Lightning Symbol.svg|12px]] [[File:Lateral view of Electrophorus electricus.png|160px]]
|2=''[[Electric eel|Electrophorus]]'' (electric eels) [[File:Farm-Fresh lightning.png|15px]] [[File:Lightning Symbol.svg|12px]] [[File:Lateral view of Electrophorus electricus.png|160px]]
}}
}}
}}
}}
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==Distribution and habitat==
==Distribution and habitat==
Gymnotiform fishes inhabit freshwater rivers and streams throughout the humid [[Neotropic]]s, ranging from southern [[Mexico]] to northern [[Argentina]]. They are [[nocturnal]] fishes. The families Gymnotidae and Hypopomidae are most diverse (numbers of species) and abundant (numbers of individuals) in small nonfloodplain streams and rivers, and in floodplain "floating meadows" of aquatic macrophytes (e.g., ''Eichornium'', the Amazonian water hyacinth). Apteronotidae and Sternopygidae are most diverse and abundant in large rivers. Species of Rhamphichthyidae are moderately diverse in all these habitat types.
Gymnotiform fishes inhabit freshwater [[river]]s and [[stream]]s throughout the humid [[Neotropic]]s, ranging from southern [[Mexico]] to northern [[Argentina]]. They are [[nocturnal]] fishes. The families Gymnotidae and Hypopomidae are most [[Biodiversity|diverse]] (numbers of species) and abundant ([[Biomass (ecology)|numbers of individuals]]) in small non-floodplain streams and rivers, and in [[floodplain]] "floating meadows" of [[Aquatic_plant#Free-floating|aquatic macrophytes]] (e.g., ''[[Eichhornia|Eichornium]]'', the [[Pontederia crassipes|Amazonian water hyacinth]]). On the other hand, families Apteronotidae and Sternopygidae are most diverse and abundant in large rivers. Species of Rhamphichthyidae are moderately diverse in all these habitat types.


==Evolution==
==Evolution==
{{further |Electric fish}}
{{Further |Electric fish}}


Gymnotiformes are among the more derived members of [[Ostariophysi]], a lineage of primary freshwater fishes. The only known [[fossils]] are from the [[Miocene]] about 7 million years ago ([[Mya (unit) |Mya]]) of [[Bolivia]].<ref name="Albert, J.S. and W.L. Fink. 2007">{{cite journal |last1=Albert |first1=James S. |last2=Fink |first2=William L. |title=Phylogenetic relationships of fossil neotropical electric fishes (Osteichthyes: Gymnotiformes) from the upper Miocene of Bolivia |journal=Journal of Vertebrate Paleontology |date=12 March 2007 |volume=27 |issue=1 |pages=17–25 |doi=10.1671/0272-4634(2007)27[17:PROFNE]2.0.CO;2 |s2cid=35007130 }}</ref>
Gymnotiformes are among the more derived members of [[Ostariophysi]], a lineage of primary freshwater fishes. The only known [[fossils]] are from the [[Miocene]] about 7 million years ago ([[Mya (unit)|Mya]]) of [[Bolivia]].<ref name="Albert, J.S. and W.L. Fink. 2007">{{cite journal |last1=Albert |first1=James S. |last2=Fink |first2=William L. |title=Phylogenetic relationships of fossil neotropical electric fishes (Osteichthyes: Gymnotiformes) from the upper Miocene of Bolivia |journal=Journal of Vertebrate Paleontology |date=12 March 2007 |volume=27 |issue=1 |pages=17–25 |doi=10.1671/0272-4634(2007)27[17:PROFNE]2.0.CO;2 |s2cid=35007130 }}</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 out-competed by [[Mormyridae]], which are similar in that they also use [[electrolocation]].<ref name="Albert&Crampton, 2005a"/>
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 out-competed by [[Mormyridae]], which are similar in that they also use [[electrolocation]].<ref name="Albert&Crampton, 2005a"/>


Approximately 150 Mya, the ancestor to modern-day Gymnotiformes and Siluriformes were estimated to have convergently evolved ampullary receptors, allowing for passive electroreceptive capabilities.<ref>{{Cite journal |last=Crampton |first=William G. R. |date=2019 |title=Electroreception, electrogenesis and electric signal evolution |url=https://onlinelibrary.wiley.com/doi/abs/10.1111/jfb.13922 |journal=Journal of Fish Biology |volume=95 |issue=1 |pages=92–134 |doi=10.1111/jfb.13922 |pmid=30729523 |s2cid=73442571 }}</ref> As this characteristic occurred after the prior loss of electroreception among the subclass Neopterygii<ref>{{Cite journal |last1=Baker |first1=Clare V. H. |last2=Modrell |first2=Melinda S. |last3=Gillis |first3=J. Andrew |date=2013-07-01 |editor-last=Krahe |editor-first=Rüdiger |editor2-last=Fortune |editor2-first=Eric |title=The evolution and development of vertebrate lateral line electroreceptors |url=https://doi.org/10.1242/jeb.082362 |journal=Journal of Experimental Biology |volume=216 |issue=13 |pages=2515–2522 |doi=10.1242/jeb.082362 |pmc=4988487 |pmid=23761476}}</ref> after having been present in the common ancestor of vertebrates, the ampullary receptors of Gymnotiformes are not homologous with those of other jawed non-teleost species, such as chondricthyans.<ref>{{Cite journal |last=Crampton |first=William G. R. |date=2019 |title=Electroreception, electrogenesis and electric signal evolution |url=https://onlinelibrary.wiley.com/doi/abs/10.1111/jfb.13922 |journal=Journal of Fish Biology |volume=95 |issue=1 |pages=92–134 |doi=10.1111/jfb.13922 |pmid=30729523 |s2cid=73442571}}</ref>
Approximately 150 Mya, the ancestor to modern-day Gymnotiformes and Siluriformes were estimated to have convergently evolved ampullary receptors, allowing for passive electroreceptive capabilities.<ref>{{Cite journal |last=Crampton |first=William G. R. |date=2019 |title=Electroreception, electrogenesis and electric signal evolution |journal=Journal of Fish Biology |volume=95 |issue=1 |pages=92–134 |doi=10.1111/jfb.13922 |pmid=30729523 |s2cid=73442571 |doi-access=free |bibcode=2019JFBio..95...92C }}</ref> As this characteristic occurred after the prior loss of electroreception among the subclass Neopterygii<ref>{{Cite journal |last1=Baker |first1=Clare V. H. |last2=Modrell |first2=Melinda S. |last3=Gillis |first3=J. Andrew |date=2013-07-01 |editor-last=Krahe |editor-first=Rüdiger |editor2-last=Fortune |editor2-first=Eric |title=The evolution and development of vertebrate lateral line electroreceptors |url=https://doi.org/10.1242/jeb.082362 |journal=Journal of Experimental Biology |volume=216 |issue=13 |pages=2515–2522 |doi=10.1242/jeb.082362 |pmc=4988487 |pmid=23761476}}</ref> after having been present in the common ancestor of vertebrates, the ampullary receptors of Gymnotiformes are not homologous with those of other jawed non-teleost species, such as chondricthyans.<ref>{{Cite journal |last=Crampton |first=William G. R. |date=2019 |title=Electroreception, electrogenesis and electric signal evolution |journal=Journal of Fish Biology |volume=95 |issue=1 |pages=92–134 |doi=10.1111/jfb.13922 |pmid=30729523 |s2cid=73442571|doi-access=free |bibcode=2019JFBio..95...92C }}</ref>


Gymnotiformes and Mormyridae have developed their electric organs and electrosensory systems (ESSs) through [[convergent evolution]].<ref>{{cite journal |last1=Hopkins |first1=Carl D |title=Convergent designs for electrogenesis and electroreception |journal=Current Opinion in Neurobiology |date=1 December 1995 |volume=5 |issue=6 |pages=769–777 |doi=10.1016/0959-4388(95)80105-7 |pmid=8805421 |s2cid=39794542 }}</ref> As Arnegard et al. (2005) and Albert and Crampton (2005) show,<ref>Albert, J. S., and W. G. R. Crampton. 2006. Electroreception and electrogenesis. Pp. 429-470 in P. L. Lutz, ed. The Physiology of Fishes. CRC Press, Boca Raton, FL.</ref><ref>{{cite journal |last1=Arnegard |first1=Matthew E. |last2=Bogdanowicz |first2=Steven M. |last3=Hopkins |first3=Carl D. |title=Multiple cases of striking genetic similarity between alternate electric fish signal morphs in sympatry |journal=Evolution |date=February 2005 |volume=59 |issue=2 |pages=324–343 |doi=10.1111/j.0014-3820.2005.tb00993.x |pmid=15807419 |s2cid=14178144 |doi-access=free }}</ref> their last common ancestor was roughly 140 to 208 Mya, and at this time they did not possess ESSs. Each species of ''Mormyrus'' (family: Mormyridae) and ''Gymnotus'' (family: Gymnotidae) have evolved a unique waveform that allows the individual fish to identify between species, genders, individuals and even between mates with better fitness levels.<ref name="Arnegard, M. E. 2010">{{cite journal |last1=Arnegard |first1=Matthew E. |last2=McIntyre |first2=Peter B. |last3=Harmon |first3=Luke J. |last4=Zelditch |first4=Miriam L. |last5=Crampton |first5=William G. R. |last6=Davis |first6=Justin K. |last7=Sullivan |first7=John P. |last8=Lavoué |first8=Sébastien |last9=Hopkins |first9=Carl D. |title=Sexual Signal Evolution Outpaces Ecological Divergence during Electric Fish Species Radiation. |journal=The American Naturalist |date=1 September 2010 |volume=176 |issue=3 |pages=335–356 |doi=10.1086/655221 |pmid=20653442 |s2cid=16787431 |url=https://stars.library.ucf.edu/cgi/viewcontent.cgi?article=7957&context=facultybib2010 }}</ref> The differences include the direction of the initial phase of the wave (positive or negative, which correlates to the direction of the current through the electrocytes in the electric organ), the amplitude of the wave, the frequency of the wave, and the number of phases of the wave.
Gymnotiformes and Mormyridae have developed their electric organs and electrosensory systems (ESSs) through [[convergent evolution]].<ref>{{cite journal |last1=Hopkins |first1=Carl D |title=Convergent designs for electrogenesis and electroreception |journal=Current Opinion in Neurobiology |date=1 December 1995 |volume=5 |issue=6 |pages=769–777 |doi=10.1016/0959-4388(95)80105-7 |pmid=8805421 |s2cid=39794542 }}</ref> As Arnegard et al. (2005) and Albert and Crampton (2005) show,<ref>Albert, J. S., and W. G. R. Crampton. 2006. Electroreception and electrogenesis. Pp. 429–470 in P. L. Lutz, ed. The Physiology of Fishes. CRC Press, Boca Raton, FL.</ref><ref>{{cite journal |last1=Arnegard |first1=Matthew E. |last2=Bogdanowicz |first2=Steven M. |last3=Hopkins |first3=Carl D. |title=Multiple cases of striking genetic similarity between alternate electric fish signal morphs in sympatry |journal=Evolution |date=February 2005 |volume=59 |issue=2 |pages=324–343 |doi=10.1111/j.0014-3820.2005.tb00993.x |pmid=15807419 |s2cid=14178144 |doi-access= }}</ref> their last common ancestor was roughly 140 to 208 Mya, and at this time they did not possess ESSs. Each species of ''Mormyrus'' (family: Mormyridae) and ''Gymnotus'' (family: Gymnotidae) have evolved a unique waveform that allows the individual fish to identify between species, genders, individuals and even between mates with better fitness levels.<ref name="Arnegard, M. E. 2010">{{cite journal |last1=Arnegard |first1=Matthew E. |last2=McIntyre |first2=Peter B. |last3=Harmon |first3=Luke J. |last4=Zelditch |first4=Miriam L. |last5=Crampton |first5=William G. R. |last6=Davis |first6=Justin K. |last7=Sullivan |first7=John P. |last8=Lavoué |first8=Sébastien |last9=Hopkins |first9=Carl D. |title=Sexual Signal Evolution Outpaces Ecological Divergence during Electric Fish Species Radiation. |journal=The American Naturalist |date=1 September 2010 |volume=176 |issue=3 |pages=335–356 |doi=10.1086/655221 |pmid=20653442 |s2cid=16787431 |url=https://stars.library.ucf.edu/cgi/viewcontent.cgi?article=7957&context=facultybib2010 }}</ref> The differences include the direction of the initial phase of the wave (positive or negative, which correlates to the direction of the current through the electrocytes in the electric organ), the amplitude of the wave, the frequency of the wave, and the number of phases of the wave.


One significant force driving this evolution is predation.<ref name="Hopkins, C. D 1228">{{cite journal |last1=Hopkins |first1=C. D. |title=Design features for electric communication |journal=Journal of Experimental Biology |date=15 May 1999 |volume=202 |issue=10 |pages=1217–1228 |doi=10.1242/jeb.202.10.1217 |pmid=10210663 |url=https://jeb.biologists.org/content/202/10/1217.short }}</ref> The most common predators of Gymnotiformes include the closely related Siluriformes (catfish), as well as predation within families (''E. electricus'' is one of the largest predators of ''Gymnotus''). These predators sense electric fields, but only at low frequencies, thus certain species of Gymnotiformes, such as those in ''Gymnotus'', have shifted the frequency of their signals so they can be effectively invisible.<ref name="Hopkins, C. D 1228"/><ref>{{cite journal |last1=Stoddard |first1=Philip K. |title=Predation enhances complexity in the evolution of electric fish signals |journal=Nature |date=July 1999 |volume=400 |issue=6741 |pages=254–256 |doi=10.1038/22301 |pmid=10421365 |bibcode=1999Natur.400..254S |s2cid=204994529 }}</ref><ref>{{cite journal |last1=Stoddard |first1=Philip K. |title=The evolutionary origins of electric signal complexity |journal=Journal of Physiology-Paris |date=1 September 2002 |volume=96 |issue=5 |pages=485–491 |doi=10.1016/S0928-4257(03)00004-4 |pmid=14692496 |s2cid=6240530 }}</ref>
One significant force driving this evolution is predation.<ref name="Hopkins, C. D 1228">{{cite journal |last1=Hopkins |first1=C. D. |title=Design features for electric communication |journal=Journal of Experimental Biology |date=15 May 1999 |volume=202 |issue=10 |pages=1217–1228 |doi=10.1242/jeb.202.10.1217 |pmid=10210663 |url=https://jeb.biologists.org/content/202/10/1217.short }}</ref> The most common predators of Gymnotiformes include the closely related Siluriformes (catfish), as well as predation within families (''E. electricus'' is one of the largest predators of ''Gymnotus''). These predators sense electric fields, but only at low frequencies, thus certain species of Gymnotiformes, such as those in ''Gymnotus'', have shifted the frequency of their signals so they can be effectively invisible.<ref name="Hopkins, C. D 1228"/><ref>{{cite journal |last1=Stoddard |first1=Philip K. |title=Predation enhances complexity in the evolution of electric fish signals |journal=Nature |date=July 1999 |volume=400 |issue=6741 |pages=254–256 |doi=10.1038/22301 |pmid=10421365 |bibcode=1999Natur.400..254S |s2cid=204994529 }}</ref><ref>{{cite journal |last1=Stoddard |first1=Philip K. |title=The evolutionary origins of electric signal complexity |journal=Journal of Physiology-Paris |date=1 September 2002 |volume=96 |issue=5 |pages=485–491 |doi=10.1016/S0928-4257(03)00004-4 |pmid=14692496 |s2cid=6240530 }}</ref>
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Sexual selection is another driving force with an unusual influence, in that females exhibit preference for males with low-frequency signals (which are more easily detected by predators),<ref name="Hopkins, C. D 1228"/> but most males exhibit this frequency only intermittently. Females prefer males with low-frequency signals because they indicate a higher fitness of the male.<ref name=":0">{{cite journal |last1=Stoddard |first1=Philip K. |last2=Tran |first2=Alex |last3=Krahe |first3=Rüdiger |title=Predation and Crypsis in the Evolution of Electric Signaling in Weakly Electric Fishes |journal=Frontiers in Ecology and Evolution |date=10 July 2019 |volume=7 |pages=264 |doi=10.3389/fevo.2019.00264 |s2cid=195856052 |doi-access=free }}</ref> Since these low-frequency signals are more conspicuous to predators, the emitting of such signals by males shows that they are capable of evading predation.<ref name=":0" /> Therefore, the production of low-frequency signals is under competing evolutionary forces: it is selected against due to the eavesdropping of electric predators, but is favored by sexual selection due to its attractiveness to females. Females also prefer males with longer pulses,<ref name="Arnegard, M. E. 2010"/> also energetically expensive, and large tail lengths. These signs indicate some ability to exploit resources,<ref name="Hopkins, C. D 1228"/> thus indicating better lifetime reproductive success.
Sexual selection is another driving force with an unusual influence, in that females exhibit preference for males with low-frequency signals (which are more easily detected by predators),<ref name="Hopkins, C. D 1228"/> but most males exhibit this frequency only intermittently. Females prefer males with low-frequency signals because they indicate a higher fitness of the male.<ref name=":0">{{cite journal |last1=Stoddard |first1=Philip K. |last2=Tran |first2=Alex |last3=Krahe |first3=Rüdiger |title=Predation and Crypsis in the Evolution of Electric Signaling in Weakly Electric Fishes |journal=Frontiers in Ecology and Evolution |date=10 July 2019 |volume=7 |pages=264 |doi=10.3389/fevo.2019.00264 |s2cid=195856052 |doi-access=free }}</ref> Since these low-frequency signals are more conspicuous to predators, the emitting of such signals by males shows that they are capable of evading predation.<ref name=":0" /> Therefore, the production of low-frequency signals is under competing evolutionary forces: it is selected against due to the eavesdropping of electric predators, but is favored by sexual selection due to its attractiveness to females. Females also prefer males with longer pulses,<ref name="Arnegard, M. E. 2010"/> also energetically expensive, and large tail lengths. These signs indicate some ability to exploit resources,<ref name="Hopkins, C. D 1228"/> thus indicating better lifetime reproductive success.


Genetic drift is also a factor contributing to the diversity of electric signals observed in Gymnotiformes.<ref name=":1">{{cite journal |last1=Picq |first1=Sophie |last2=Alda |first2=Fernando |last3=Bermingham |first3=Eldredge |last4=Krahe |first4=Rüdiger |title=Drift-driven evolution of electric signals in a Neotropical knifefish |journal=Evolution |date=September 2016 |volume=70 |issue=9 |pages=2134–2144 |doi=10.1111/evo.13010 |pmid=27436179 |s2cid=1064883 |doi-access=free }}</ref> Reduced gene flow due to geographical barriers has led to vast differences signal morphology in different streams and drainages.<ref name=":1" />
Genetic drift is also a factor contributing to the diversity of electric signals observed in Gymnotiformes.<ref name=":1">{{cite journal |last1=Picq |first1=Sophie |last2=Alda |first2=Fernando |last3=Bermingham |first3=Eldredge |last4=Krahe |first4=Rüdiger |title=Drift-driven evolution of electric signals in a Neotropical knifefish |journal=Evolution |date=September 2016 |volume=70 |issue=9 |pages=2134–2144 |doi=10.1111/evo.13010 |pmid=27436179 |s2cid=1064883 |doi-access= }}</ref> Reduced gene flow due to geographical barriers has led to vast differences signal morphology in different streams and drainages.<ref name=":1" />


==See also==
==See also==
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==References==
==References==
{{Reflist}}
{{reflist|25em}}


==External links==
==External links==
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{{Taxonbar |from=Q752264}}
{{Taxonbar |from=Q752264}}
{{Authority control}}


[[Category:Gymnotiformes | ]]
[[Category:Gymnotiformes| ]]
[[Category:Electroreceptive animals]]
[[Category:Electroreceptive animals]]
[[Category:Extant Late Jurassic first appearances]]
[[Category:Extant Late Jurassic first appearances]]

Latest revision as of 01:10, 15 November 2024

South American knifefish
Temporal range: Late Jurassic –Recent [1]
Black ghost knifefish, Apteronotus albifrons
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Class: Actinopterygii
(unranked): Otophysi
Order: Gymnotiformes
Type species
Gymnotus carapo
Despite the name, the electric eel is a type of knifefish.

The Gymnotiformes /dʒɪmˈnɒtɪfɔːrmiːz/ are an order of teleost bony fishes commonly known as Neotropical knifefish or South American knifefish. They have long bodies and swim using undulations of their elongated anal fin. Found almost exclusively in fresh water (the only exceptions are species that occasionally may visit brackish water to feed), these mostly nocturnal fish are capable of producing electric fields to detect prey, for navigation, communication, and, in the case of the electric eel (Electrophorus electricus), attack and defense.[2] A few species are familiar to the aquarium trade, such as the black ghost knifefish (Apteronotus albifrons), the glass knifefish (Eigenmannia virescens), and the banded knifefish (Gymnotus carapo).

Description

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Anatomy and locomotion

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Aside from the electric eel (Electrophorus electricus), Gymnotiformes are slender fish with narrow bodies and tapering tails, hence the common name of "knifefishes". They have neither pelvic fins nor dorsal fins, but do possess greatly elongated anal fins that stretch along almost the entire underside of their bodies. The fish swim by rippling this fin, keeping their bodies rigid. This means of propulsion allows them to move backwards as easily as they move forward.[3]

The knifefish has approximately one hundred and fifty fin rays along its ribbon-fin. These individual fin rays can be curved nearly twice the maximum recorded curvature for ray-finned fish fin rays during locomotion. These fin rays are curved into the direction of motion, indicating that the knifefish has active control of the fin ray curvature, and that this curvature is not the result of passive bending due to fluid loading.[4]

Different wave patterns produced along the length of the elongated anal fin allow for various forms of thrust. The wave motion of the fin resembles traveling sinusoidal waves. A forward traveling wave can be associated with forward motion, while a wave in the reverse direction produces thrust in the opposite direction.[5] This undulating motion of the fin produced a system of linked vortex tubes that were produced along the bottom edge of the fin. A jet was produced at an angle to the fin that was directly related to the vortex tubes, and this jet provides propulsion that moves the fish forward.[6] The wave motion of the fin is similar to that of other marine creatures, such as the undulation of the body of an eel, however the wake vortex produced by the knifefish was found to be a reverse Kármán vortex. This type of vortex is also produced by some fish, such as trout, through the oscillations of their caudal fins.[7] The speed at which the fish moved through the water had no correlation to the amplitude of its undulations, however it was directly related to the frequency of the waves generated.[8]

Studies have shown that the natural angle between the body of the knifefish and its fin is essential for efficient forward motion, for if the anal fin was located directly underneath, then an upwards force would be generated with forward thrust, which would require an additional downwards force in order to maintain neutral buoyancy.[7] A combination of forward and reverse wave patterns, which meet towards the center of the anal fin, produce a heave force allowing for hovering, or upwards movement.[5]

The ghost knifefish can vary the undulation of the waves, as well as the angle of attack of the fin to achieve various directional changes. The pectoral fins of these fishes can help to control roll and pitch control.[9] By rolling they can generate a vertical thrust to quickly, and efficiently, ambush their prey.[7] The forward movement is determined exclusively by the ribbon fins and the contribution of the pectoral fins for forward movement was negligible.[10] The body is kept relatively rigid and there is very little motion of the center of mass motion during locomotion compared to the body size of the fish.[8]

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.[11]

Electroreception and electrogenesis

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Further information: Electroreception and electrogenesis

These fish possess electric organs that allow them to produce electric fields, which are usually weak. In most gymnotiforms, the electric organs are derived from muscle cells. However, adult apteronotids are one exception, as theirs are derived from nerve cells (spinal electromotor neurons). In gymnotiforms, the electric organ discharge may be continuous or pulsed. If continuous, it is 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, amplitude, phase and frequency.[12]

The electric organs of most Gymnotiformes produce tiny discharges of just a few millivolts, far too weak to cause any harm to other fish. Instead, they are used to help navigate the environment, including locating the bottom-dwelling invertebrates that compose their diets.[13] They may also be used to send signals between fish of the same species.[14] In addition to this low-level field, the electric eel also has the capability to produce much more powerful discharges to stun prey.[3]

Taxonomy

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There are currently about 250 valid gymnotiform species in 34 genera and five families, with many additional species yet to be formally described.[15][16][17] The actual number of species in the wild is unknown.[18] Gymnotiformes is thought to be the sister group to the Siluriformes[19][20] from which they diverged in the Cretaceous period (about 120 million years ago). The families have traditionally been classified over suborders and superfamilies as below.[21][17]

Order Gymnotiformes

Suborder Gymnotoidei
Family Gymnotidae (banded knifefishes and electric eels)
Suborder Sternopygoidei
Superfamily Rhamphichthyoidea
Family Rhamphichthyidae (sand knifefishes)
Family Hypopomidae (bluntnose knifefishes)
Superfamily Apteronotoidea
Family Sternopygidae (glass and rat-tail knifefishes)
Family Apteronotidae (ghost knifefishes)

Phylogeny

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Most gymnotiforms are weakly electric, capable of active electrolocation but not of delivering shocks. The electric eels, genus Electrophorus, are strongly electric, and are not closely related to the Anguilliformes, the true eels.[22] Their relationships were analysed by sequencing their mitochondrial genomes in 2019. This shows that contrary to earlier ideas, the Apteronotidae and Sternopygidae are not sister taxa, and that the Gymnotidae are deeply nested among the other families.[23]

Actively electrolocating fish are marked on the phylogenetic tree with a small yellow lightning flash . Fish able to deliver electric shocks are marked with a red lightning flash . There are other electric fishes in other families (not shown).[13][24][25]

Otophysi

Siluriformes (catfish) (some )

Gymnotiformes

Apteronotidae (ghost knifefishes)

Rhamphichthyoidea

Hypopomidae (bluntnose knifefishes)

Rhamphichthyidae (sand knifefishes)

Gymnotidae

Gymnotus (banded knifefishes)

Electrophorus (electric eels)

Sternopygidae (glass knifefishes)

Characoidei (piranhas, tetras, and allies)

Distribution and habitat

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Gymnotiform fishes inhabit freshwater rivers and streams throughout the humid Neotropics, ranging from southern Mexico to northern Argentina. They are nocturnal fishes. The families Gymnotidae and Hypopomidae are most diverse (numbers of species) and abundant (numbers of individuals) in small non-floodplain streams and rivers, and in floodplain "floating meadows" of aquatic macrophytes (e.g., Eichornium, the Amazonian water hyacinth). On the other hand, families Apteronotidae and Sternopygidae are most diverse and abundant in large rivers. Species of Rhamphichthyidae are moderately diverse in all these habitat types.

Evolution

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Further information: Electric fish

Gymnotiformes are among the more derived members of Ostariophysi, a lineage of primary freshwater fishes. The only known fossils are from the Miocene about 7 million years ago (Mya) of Bolivia.[26]

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 out-competed by Mormyridae, which are similar in that they also use electrolocation.[15]

Approximately 150 Mya, the ancestor to modern-day Gymnotiformes and Siluriformes were estimated to have convergently evolved ampullary receptors, allowing for passive electroreceptive capabilities.[27] As this characteristic occurred after the prior loss of electroreception among the subclass Neopterygii[28] after having been present in the common ancestor of vertebrates, the ampullary receptors of Gymnotiformes are not homologous with those of other jawed non-teleost species, such as chondricthyans.[29]

Gymnotiformes and Mormyridae have developed their electric organs and electrosensory systems (ESSs) through convergent evolution.[30] As Arnegard et al. (2005) and Albert and Crampton (2005) show,[31][32] their last common ancestor was roughly 140 to 208 Mya, and at this time they did not possess ESSs. Each species of Mormyrus (family: Mormyridae) and Gymnotus (family: Gymnotidae) have evolved a unique waveform that allows the individual fish to identify between species, genders, individuals and even between mates with better fitness levels.[33] The differences include the direction of the initial phase of the wave (positive or negative, which correlates to the direction of the current through the electrocytes in the electric organ), the amplitude of the wave, the frequency of the wave, and the number of phases of the wave.

One significant force driving this evolution is predation.[34] The most common predators of Gymnotiformes include the closely related Siluriformes (catfish), as well as predation within families (E. electricus is one of the largest predators of Gymnotus). These predators sense electric fields, but only at low frequencies, thus certain species of Gymnotiformes, such as those in Gymnotus, have shifted the frequency of their signals so they can be effectively invisible.[34][35][36]

Sexual selection is another driving force with an unusual influence, in that females exhibit preference for males with low-frequency signals (which are more easily detected by predators),[34] but most males exhibit this frequency only intermittently. Females prefer males with low-frequency signals because they indicate a higher fitness of the male.[37] Since these low-frequency signals are more conspicuous to predators, the emitting of such signals by males shows that they are capable of evading predation.[37] Therefore, the production of low-frequency signals is under competing evolutionary forces: it is selected against due to the eavesdropping of electric predators, but is favored by sexual selection due to its attractiveness to females. Females also prefer males with longer pulses,[33] also energetically expensive, and large tail lengths. These signs indicate some ability to exploit resources,[34] thus indicating better lifetime reproductive success.

Genetic drift is also a factor contributing to the diversity of electric signals observed in Gymnotiformes.[38] Reduced gene flow due to geographical barriers has led to vast differences signal morphology in different streams and drainages.[38]

See also

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References

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  1. ^ Froese, Rainer; Pauly, Daniel (eds.). "Order Gymnotiformes". FishBase. Apr 2007 version.
  2. ^ van der Sleen, P.; Albert, J. S., eds. (2017). Field Guide to the Fishes of the Amazon, Orinoco, and Guianas. Princeton University Press. pp. 322–345. ISBN 978-0691170749.
  3. ^ a b Ferraris, Carl J. (1998). Paxton, J.R.; Eschmeyer, W.N. (eds.). Encyclopedia of Fishes. San Diego: Academic Press. pp. 111–112. ISBN 0-12-547665-5.
  4. ^ Youngerman, Eric D.; Flammang, Brooke E.; Lauder, George V. (October 2014). "Locomotion of free-swimming ghost knifefish: anal fin kinematics during four behaviors". Zoology. 117 (5): 337–348. Bibcode:2014Zool..117..337Y. doi:10.1016/j.zool.2014.04.004. PMID 25043841.
  5. ^ a b Shirgaonkar, Anup A.; Curet, Oscar M.; Patankar, Neelesh A.; MacIver, Malcolm A. (1 November 2008). "The hydrodynamics of ribbon-fin propulsion during impulsive motion". Journal of Experimental Biology. 211 (21): 3490–3503. doi:10.1242/jeb.019224. PMID 18931321. S2CID 10911068.
  6. ^ Neveln, I. D.; Bale, R.; Bhalla, A. P. S.; Curet, O. M.; Patankar, N. A.; MacIver, M. A. (15 January 2014). "Undulating fins produce off-axis thrust and flow structures". Journal of Experimental Biology. 217 (2): 201–213. doi:10.1242/jeb.091520. PMID 24072799. S2CID 2656865.
  7. ^ a b c Neveln, I. D.; Bai, Y.; Snyder, J. B.; Solberg, J. R.; Curet, O. M.; Lynch, K. M.; MacIver, M. A. (1 July 2013). "Biomimetic and bio-inspired robotics in electric fish research". Journal of Experimental Biology. 216 (13): 2501–2514. doi:10.1242/jeb.082743. PMID 23761475. S2CID 14992273.
  8. ^ a b Xiong, Grace; Lauder, George V. (August 2014). "Center of mass motion in swimming fish: effects of speed and locomotor mode during undulatory propulsion". Zoology. 117 (4): 269–281. Bibcode:2014Zool..117..269X. doi:10.1016/j.zool.2014.03.002. PMID 24925455.
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