1 Description 2 Taxonomy 3 Distribution and habitat 4 Evolution 5 See also 6 References 7 External links
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.
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
These fish possess electric organs that allow them to produce
electricity. 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.
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. They may also be used to send signals between fish of the same species. In addition to this low-level field, the electric eel also has the capability to produce much more powerful discharges to stun prey. Taxonomy There are currently about 250 valid gymnotiform species in 34 genera and five families, with many additional species known but yet to be formally described. The actual number of species in the wild is unknown. Gymnotiformes
Gymnotiformes is thought to be the sister group to the Siluriformes from which they diverged in the Cretaceous
Cretaceous period (about 120 million years ago). The families are classified over suborders and superfamilies as below. Order Gymnotiformes
Distribution and habitat
Gymnotiform fishes inhabit freshwater rivers and streams throughout
the humid Neotropics, ranging from
Guatemala to northern Argentina. They are nocturnal fishes. The families Gymnotidae
Gymnotidae and Hypopomidae
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
Apteronotidae and Sternopygidae
Sternopygidae are most diverse and abundant in large rivers. Species of Rhamphichthyidae
Rhamphichthyidae are moderately diverse in all these habitat types. Evolution Gymnotiformes
Gymnotiformes are among the more derived members of Ostariophysi, a lineage of primary freshwater fishes. The only known fossils are from the Miocene
Miocene about 7 million years ago (Mya) of Bolivia. Gymnotiformes
Gymnotiformes has no extant species in Africa. This may be because they did not spread into Africa
Africa before South America and Africa
Africa split, or it may be that they were out-competed by Mormyridae, which are similar in that they also use electrolocation. Gymnotiformes
Gymnotiformes and Mormyridae
Mormyridae have developed their electric organs and electrosensory systems (ESSs) through convergent evolution. As Arnegard et al. (2005) and Albert and Crampton (2005) show, 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 completely unique waveform that allows the individual fish to identify between species, genders, individuals and even between mates with better fitness levels. 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. The most common predators of Gymnotiformes
Gymnotiformes include the closely related Siluriformes
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. Sexual selection is another driving force with an unusual influence, in that females exhibit preference for males with low-frequency signals (which are energetically expensive and easily detected by predators), but most males exhibit this frequency only intermittently. They also prefer males with longer pulses, also energetically expensive, and large tail lengths. These signs indicate some ability to exploit resources, thus indicating better lifetime reproductive success. See also
Electric fish Bioelectromagnetism
^ Froese, Rainer, and Daniel Pauly, eds. (2007). "Gymnotiformes" in
FishBase. Apr 2007 version.
^ a b Ferraris, Carl J. (1998). Paxton, J.R.; Eschmeyer, W.N., eds.
Encyclopedia of Fishes. San Diego: Academic Press. pp. 111–112.
^ Albert, J.S. 2001. Species diversity and phylogenetic systematics of
American knifefishes (Gymnotiformes, Teleostei). Misc. Publ. Mus.
Zool. University of Michigan, 190:1-127.
^ 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.
^ Vincent Fugère, Hernán Ortega and Rüdiger Krahe. 2010. Electrical
signalling of dominance in a wild population of electric fish. Biol.
^ a b 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.
^ Eschmeyer, W. N., & Fong, J. D. (2016). Catalog of fishes:
Species by family/subfamily.
^ 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. ^ "Fink and Fink, 1996">FINK, S. V., & FINK, W. L. (1981). Interrelationships of the ostariophysan fishes (Teleostei). Zoological Journal of the Linnean Society, 72(4), 297-353. ^ "Arcila et al., 2017">Arcila, D., Ortí, G., Vari, R., Armbruster, J. W., Stiassny, M. L., Ko, K. D., ... & Betancur-R, R. (2017). Genome-wide interrogation advances resolution of recalcitrant groups in the tree of life. Nature Ecology & Evolution, 1, 0020. ^ Nelson ^ Albert, J.S. and W.L. Fink. 2007. Phylogenetic relationships of fossil Neotropical
Neotropical electric fishes (Osteichthyes: Gymnotiformes) from the Upper Miocene
Miocene of Bolivia. Journal Vertebrate Paleontology 27(1):17-25. ^ Hopkins, C. D. 1995. Convergent designs for electrogenesis and electroreception. Current Opinion in Neurobiology 5:769-777. ^ 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. ^ Arnegard, M. E., S. M. Bogdanowicz, and C. D. Hopkins. 2005. Multiple cases of striking genetic similarity between alternate electric fish signal morphs in sympatry. Evolution 59:324-343. ^ a b Arnegard, M. E., P. B. McIntyre, L. J. Harmon, M. L. Zelditch, W. G. R. Crampton, J. K. Davis, J. P. Sullivan, S. Lavoue, and C. D. Hopkins. 2010. Sexual signal evolution outpaces ecological divergence during electric fish species radiation. American Naturalist 176:335-356. ^ a b c d Hopkins, C. D. 1999a. Design features for electric communication. Journal of Experimental Biology 202:1217-1228. ^ Stoddard, P. K. 1999. Predation enhances complexity in the evolution of electric fish signals. Nature 400:254-256. ^ Stoddard, P. K. 2002b. The evolutionary origins of electric signal complexity. Journal of Physiology-Paris 96:485-491.
Photos of various gymnotiforms
v t e
Actinopterygii orders by subclass
Kingdom Animalia Phylum Chordata Subphylum Vertebrata Infraphylum Gnathostomata Superclass Osteichthyes
Elopiformes Albuliformes Notacanthiformes Anguilliformes
Clupeiformes Alepocephaliformes Gonorynchiformes Cypriniformes Characiformes Gymnotiformes Siluriformes
Argentiniformes Galaxiiformes Salmoniformes Esociformes
Ateleopodiformes Aulopiformes Myctophiformes Lampriformes Polymixiiformes
Percopsiformes Zeiformes Stylephoriformes Gadiformes
Beryciformes Trachichthyiformes Holocentriformes Ophidiiformes Batrachoidiformes Kurtiformes Gobiiformes Syngnathiformes Scombriformes Synbranchiformes Anabantiformes Istiophoriformes Carangiformes Pleuronectiformes Cichliformes Atheriniformes Cyprinodontiformes Beloniformes Mugiliformes Gobiesociformes Blenniiformes Gerreiformes Uranoscopiformes Labriformes Moroniformes Ephippiformes Chaetodontiformes Acanthuriformes Lutjaniformes Lobotiformes Spariformes Scatophagiformes Priacanthiformes Caproiformes Lophiiformes Tetraodontiformes Pempheriformes Centrarchiformes Perciformes
Wd: Q752264 ADW: Gymnotiformes EoL: 5477 Fossilworks: 92061 GBIF: 1165 ITIS: 553135 NCBI: 8002 W