''Hypopygus'' is a

genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...

of South American gymnotiform knifefishes native to the

Amazon Amazon most often refers to: * Amazon River, in South America * Amazon rainforest, a rainforest covering most of the Amazon basin * Amazon (company), an American multinational technology company * Amazons, a tribe of female warriors in Greek myth ...

Orinoco The Orinoco () is one of the longest rivers in South America at . Its drainage basin, sometimes known as the Orinoquia, covers approximately 1 million km2, with 65% of it in Venezuela and 35% in Colombia. It is the List of rivers by discharge, f ...

and upper

Paraguay Paraguay, officially the Republic of Paraguay, is a landlocked country in South America. It is bordered by Argentina to the Argentina–Paraguay border, south and southwest, Brazil to the Brazil–Paraguay border, east and northeast, and Boli ...

basins, as well as rivers in the

Guianas The Guianas, also spelled Guyanas or Guayanas, are a geographical region in north-eastern South America. Strictly, the term refers to the three Guianas: Guyana, Suriname, and French Guiana, formerly British Guiana, British, Surinam (Dutch colo ...

. They are often common, and found near submerged roots, aquatic vegetation and leaf-litter in streams, edges of rivers and floodplains. They are regularly found among vegetation in floating meadows, a habitat that often contains little oxygen, but they are well-adapted to this. They are well-

camouflaged Camouflage is the use of any combination of materials, coloration, or illumination for concealment, either by making animals or objects hard to see, or by disguising them as something else. Examples include the leopard's spotted coat, the b ...

and brown in color with a banded/mottled pattern. They generally resemble the related ''

Steatogenys ''Steatogenys'' is a genus of freshwater ray-finned fishes belonging to the family Rhamphichthyidae, the sand kinifefishes. The fishes in this genus are found in the Amazon, Orinoco and Essequibo river basins in tropical South America. The wides ...

'', but are smaller, reaching up to in total length depending on the exact species of ''Hypopygus''. The smallest is ''H. hoedemani'', which is the second-smallest knifefish, after ''

Microsternarchus ''Microsternarchus'' is a genus of bluntnose knifefish that is found in creeks and streams, often in areas with submerged vegetation, roots and leaf litter, in the Amazon, Orinoco, Essequibo–Rupununi and Río de la Plata basins in South Ameri ...

brevis''. They are

nocturnal Nocturnality is a ethology, behavior in some non-human animals characterized by being active during the night and sleeping during the day. The common adjective is "nocturnal", versus diurnality, diurnal meaning the opposite. Nocturnal creatur ...

and feed on small invertebrates. During the day they remain hidden, often in groups that may number several dozen individuals.

Taxonomy and species

''Hypopygus'' has traditionally been included in the family

Hypopomidae The Hypopomidae are a family of fishes in the order Gymnotiformes known as the bluntnose knifefish. They may also be called grass or leaf knifefishes. These electric fish are not often eaten, of little commercial importance, rarely kept as aquari ...

, but a comprehensive molecular study from 2015 showed it belongs in

Rhamphichthyidae Sand knifefish are freshwater electric fish of the family Rhamphichthyidae, from freshwater habitats in South America. Just like most part of the members of the Gymnotiformes group, they also have elongated and compressed bodies and electric org ...

, and this has been followed by recent authorities. ''Hypopygus'' contains the following species: * '' Hypopygus benoneae'' L. A. W. Peixoto, Dutra, de Santana & Wosiacki, 2013 * ''

Hypopygus cryptogenes ''Hypopygus cryptogenes'' is a species of bluntnose knifefish endemic to Brazil where it is found in the Rio Negro basin. This species can reach a total length of up to . It is the only member of the genus ''Stegostenopos'' according to FishBa ...

'' ( Triques, 1997) * '' Hypopygus hoedemani'' de Santana & Crampton, 2011 (Hoedeman's Hypopygus) * ''

Hypopygus isbruckeri ''Hypopygus'' is a genus of South American gymnotiform knifefishes native to the Amazon, Orinoco and upper Paraguay basins, as well as rivers in the Guianas. They are often common, and found near submerged roots, aquatic vegetation and leaf-litt ...

'' de Santana & Crampton, 2011 (Isbrücker's Hypopygus) * ''

Hypopygus lepturus ''Hypopygus'' is a genus of South American gymnotiform knifefishes native to the Amazon, Orinoco and upper Paraguay basins, as well as rivers in the Guianas. They are often common, and found near submerged roots, aquatic vegetation and leaf-litt ...

'' Hoedeman, 1962 * ''

Hypopygus minissimus ''Hypopygus'' is a genus of South American gymnotiform knifefishes native to the Amazon, Orinoco and upper Paraguay basins, as well as rivers in the Guianas. They are often common, and found near submerged roots, aquatic vegetation and leaf-litt ...

'' de Santana & Crampton, 2011 * ''

Hypopygus neblinae ''Hypopygus neblinae'' is a species of freshwater ray-finned fish belonging to the family Rhamphichthyidae, the sand knifefishes. This species is found in Brazil, Colombia, Guyana and Venezuela, living among the leaves and roots of submerged marg ...

'' Mago-Leccia, 1994 * ''

Hypopygus nijsseni ''Hypopygus'' is a genus of South American gymnotiform knifefishes native to the Amazon, Orinoco and upper Paraguay basins, as well as rivers in the Guianas. They are often common, and found near submerged roots, aquatic vegetation and leaf-litt ...

'' de Santana & Crampton, 2011 (Nijssen's Hypopygus) * ''

Hypopygus ortegai ''Hypopygus'' is a genus of South American gymnotiform knifefishes native to the Amazon, Orinoco and upper Paraguay basins, as well as rivers in the Guianas. They are often common, and found near submerged roots, aquatic vegetation and leaf-litt ...

'' de Santana & Crampton, 2011 (Ortega's Hypopygus) * ''

Hypopygus varii ''Hypopygus'' is a genus of South American Gymnotiformes, gymnotiform knifefishes native to the Amazon basin, Amazon, Orinoco and upper Paraguay River, Paraguay basins, as well as rivers in the Guianas. They are often common, and found near subme ...

'' Campos-da-Paz 2018

References

Hypopomidae Freshwater fish of South America Freshwater fish genera Ray-finned fish genera {{gymnotiformes-stub