Heterotidinae

Arapaiminae is a subfamily of freshwater osteoglossiform (bony-tongued) fishes belonging to the family Osteoglossidae. It includes the South American arapaimas of the Amazon and Essequibo basins and the African arowana (''Heterotis niloticus'') from the watersheds of the Sahelo-Sudanese region, Senegal, Gambia, and parts of Eastern Africa. This subfamily is sometimes raised to the rank of family, as Arapaimidae. A commonly used synonym is Heterotidinae, but according to the ICZN, Arapaiminae has priority.

Arapaimines, along with other osteoglossomorphs, are of phylogenetic and evolutionary interest due to their trans-oceanic distribution, excellent fossil record, and position as one of the oldest living teleost lineages. The type-species of the group, '' Arapaima gigas'', is an important South American food source and charismatic representative of the region. Both ''Arapaima'' and ''Heterotis'' are cultured for food in their respective countries due to their heartiness and meat, and the arapaima is a prized sport-fish, being the largest truly freshwater fish.

Phylogeny and systematics

The internal placement of Osteoglossomorpha within crown-group teleosts is contested, with competing morphological and molecular analyses placing them either as sister to all other extant teleosts, or internal to Elopimorpha and sister to the clade consisting of Otocephala and Euteleostei.

The placement and name of the clade containing ''Arapaima'' and ''Heterotis'' is also uncertain. Some include this clade in the family Osteoglossidae with the South American and Asian arowana. Others place ''Arapaima'' and ''Heterotis'' together in their own family, Arapaimidae.

Taxonomy

''Arapaima'' taxonomy was recently revised to revalidate old names and describe a new species, proposing 6 existing species (see below) and invalidating current museum specimens. However, these four proposed or reestablished species are known only from singular holotype specimens, and only that of ''A. mapae'' and ''A. leptostoma'' still currently exist. Typically, all species of ''Arapaima'' described by Valenciennes, Spix, and Agassiz are referred to as ''A. gigas'', though current taxonomy could be revised with more thorough evidence. Currently, population genetic evidence supports a singular ''Arapaima'' species with two distinct genetic populations: an Amazonas population (exhibiting a pattern of isolation by distance), and an Araguaia-Tocantins basin population. There is little debate that ''Heterotis'' is a monotypic genus represented by only ''H. niloticus''.

* Arapaiminae Bonaparte, 1846
** '' Arapaimidarum'' tolith** '' Heterotidinarum'' Nolf, Rana & Prasad 2008 tolith** '' Thrissopterus'' Heckel 1856
** '' Joffrichthys''? Li & Wilson 1996
** '' Sinoglossus''? Su 1986
** '' Heterotis'' Rüppell 1829 ex Ehrenberg 1836 (African arowana)
*** '' H. niloticus'' (Cuvier, 1829)
** '' Arapaima'' Müller 1843 (arapaima)
*** '' A. agassizii'' (Valenciennes, 1847)
*** '' A. gigas'' (Schinz, 1822)
*** '' A. leptosoma'' Stewart, 2013
*** '' A. mapae'' (Valenciennes, 1847)
*** ''A. sp.'' '' incertae sedis''

Description and biology

Description

Arapaimines are characterized by elongate, slender bodies with large scales and long dorsal and anal fins positioned close to a short caudal peduncle. The pelvic fins are small and abdominal if present. They lack chin barbels, have a glossolaryngeal (tongue) bone with teeth present, and the premaxillae are fixed to the skull. Branched caudal fin rays are less than sixteen, branchiostegal rays between three and seven, and hypurals less than 6. ''Heterotis'' possesses a specialized suprabranchial organ for concentrating and filtering small food particles.

Biology

Both genera make use of similar freshwater habitats in the respective region, with ''Arapaima'' found in the floodplains of the Amazon and Esequibo river basins of South America and ''Heterotis'' found in littoral zones of large, open rivers in all Sahelo-Sudanese basins of Africa. ''Arapaima'' is typically a top-water fish predator, while ''Heterotis'' is a benthic mud-filterer primarily feeding on phytoplankton and small crustaceans with their suprabranchial organ. Both groups are obligate air-breathers and nest-builders, with males guarding eggs and young.

Evolution

A genetic study shows that Arapaiminae diverged from Osteoglossinae about 220 million years ago, during the Late Triassic. Within Osteoglossinae, the lineage leading to the South American '' Osteoglossum'' arowanas diverged about 170 million years ago, during the Middle Jurassic. The Asian and Australian arowanas in the genus '' Scleropages'' separated about 140 million years ago, during the Early Cretaceous.

Originally, it was thought that the breakup of Gondwana 150 – 30 million years ago was the evolutionary cause of the trans-continental distribution of the osteoglossomorphs. However, minimum ages of intercontinental clades and presence of marine forms in the fossil records imply that ancestral trans-oceanic dispersal is possible. Tests of these hypotheses are currently inconclusive as they are dependent on an '' a priori'' calibrated age of crown-group Teleostei, about which fossil and molecular evidence disagree. I.e., hypotheses do not fail only if Teleostei are of Permian origin, but molecular inferences push crown ages further back.

Use by humans

Both ''Arapaima'' and ''Heterotis'' are farmed in their respective regions as relatively large and hardy food-fish.

Etymology

The subfamily is named after the monotypic genus ''Arapaima'', whose name derives from the Tupi-Guyarana indigenous name for ''Arapaima gigas''.

File:Arapaima gigas in aqua park.jpg, ''A. gigas'' in aquarium
File:Arapaima gigas at Beijing aquarium.JPG, ''A. gigas'' in aquarium
File:Heterotis niloticus.png, ''H. niloticus'' in aquarium
File:Heterotis niloticus MHNT ICHT 1995 69.jpg, Museum specimen of ''H. niloticus''

References

{{Taxonbar, from1=Q107552925, from2=Q525579, from3=Q18910351

Osteoglossidae
Ray-finned fish subfamilies