Gymnotus occur in virtually any freshwater habitat in their range, even those with little oxygen (survives by breathing air directly from the water surface), areas affected by pollution, and for a period on land if their aquatic habitat dries out. Large species tend to live near floating vegetation along the edges of large rivers or floodplains, while smaller tend to live among leaf-litter or near banks of small streams. The genus includes both widespread and common species that occur in many different habitat types, and more restricted and rare species that occur in fewer habitats. There are species that remain in the same habitat throughout their lives, while others breed in specific habitats and spend the rest of their time elsewhere. At least as many as five species of Gymnotus may occur together in the same region and habitat.
Gymnotus species are nocturnal and mainly feed on aquatic insects, crustaceans, small fish and other small animals, but may also take plant material. Being electric fish, they generate weak electric fields used for navigation, finding prey and communicating with other individuals of their species. At least some species are highly territorial and will react aggressively if detecting the electric field of another individidual of their species, especially between conspecific males. The electric signal is species specific, and tends to differ between males and females. However, Gymnotus are not able to generate a strong electric field that can be used for incapacitating prey or enemies, like the related electric eel.
Nothing is known about the breeding behavior of most members of this genus, but in two species, G. carapo and G. mamiraua, males make a "nest" (a depression in the bottom in the former species and within vegetation in floating meadows in the latter) and guard the young. Additionally, males of at least G. carapo will mouthbrood.
Gymnotus are generally brownish with a banded pattern, but this can also be more mottled or spotted in some species. Small scales are always present on these fish. The mouth is superior, meaning it is turned upwards. The anal fin terminates at a point near the tip of the tail. Like other Neotropical knifefish, they often lose their tail due to attacks by predators or aggressive encounters with conspecifics, but they are able to regenerate it. The largest Gymnotus are up to 100 cm (3.3 ft) in total length. Most species reach less than one-third that size and the smallest only around 10 cm (4 in) long.
There are currently 43 recognized species in this genus:
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^ abcdCraig, J.M.; W.G.R. Crampton; J.S. Albert (2017). "Revision of the polytypic electric fish Gymnotus carapo (Gymnotiformes, Teleostei), with descriptions of seven subspecies". Zootaxa. 4318 (3): 401–438. doi:10.11646/zootaxa.4318.3.1.
^ abcNelson, J.S. (2006). Fishes of the World. John Wiley & Sons, Inc. ISBN 0-471-25031-7.
^ abcdefvan der Sleen, P.; J.S. Albert, eds. (2017). Field Guide to the Fishes of the Amazon, Orinoco, and Guianas. Princeton University Press. pp. 332–334. ISBN 978-0691170749.
^ abGiora, J.; L.R. Malabarba (2016). "Gymnotus refugio, a new and endangered species of electric fish of the Gymnotus pantherinus species-group from southern Brazil (Gymnotiformes: Gymnotidae)". Zootaxa. 4066 (5): 581–590. doi:10.11646/zootaxa.4066.5.6.
^ abAlbert, J.S.; Crampton, W.G.R. (2001). "Five new species of Gymnotus (Teleostei: Gymnotiformes) from an Upper Amazonian floodplain, with descriptions of electric organ discharges and ecology". Ichthyological Exploration of Freshwaters. 12 (3): 241–266.
^ abRangel-Pereira, G.S. (2014). "Gymnotus capitimaculatus, a new species of electric fish from rio Jucuruçu basin, northeastern Brazil (Ostariophysi: Gymnotiformes: Gymnotidae)". Vertebrate Zoology. 64 (2): 169–175.
^ abDavis, E.A.; C.D. Hopkins (1988). "Behavioural analysis of electric signal localization in the electric fish, Gymnotus carapo (Gymnotiformes)". Animal Behaviour. 36 (6): 1658–1671. doi:10.1016/S0003-3472(88)80106-4.
^ abcCrampton, W.G.R.; C.D. Hopkins (2005). "Nesting and Paternal Care in the Weakly Electric Fish Gymnotus (Gymnotiformes: Gymnotidae) with Descriptions of Larval and Adult Electric Organ Discharges of Two Species". Copeia. 2005 (1): 48–60.
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^Milhomem S.S.R.; Crampton W.G.R.; Pieczarka J.C.; Shetka G.H.; Silva D.S.; Nagamachi C.Y. (2012). "Gymnotus capanema, a new species of electric knife fish (Gymnotiformes, Gymnotidae) from eastern Amazonia, with comments on an unusual karyotype". Journal of Fish Biology. 80 (4): 802–815. doi:10.1111/j.1095-8649.2012.03219.x.
^Cognato, D.P., Richer-de-Forges, M.M., Albert, J.S. & Crampton, W.G.R. (2008). "Gymnotus chimarrao, a new species of electric fish (Gymnotiformes: Gymnotidae) from Southern Brazil". Ichthyologial Exploration of Freshwaters. 18 (4): 375–382.
^Craig, J.M.; L.R. Malabarba; W.G.R. Crampton; J.S. Albert (2018). "Revision of banded knifefishes of the Gymnotus carapo and G. tigre clades (Gymnotidae Gymnotiformes) from the Southern Neotropics". Zootaxa. 4379 (1): 47–73. doi:10.11646/zootaxa.4379.1.3.
^Maxime E.L.; Albert J.S. (2014). "Redescription of the tuvirão, Gymnotus inaequilabiatus Valenciennes, 1839, using high-resolution X-ray computed tomography". Copeia. 2014 (3): 462–472. doi:10.1643/ci-13-054.
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