Astyanax brachypterygium

Astyanax brachypterygium was first described in 2001 by Brazilian ichthyologists Vinicius A. Bertaco and Luiz R. Malabarba.[3] Its only known synonym is Astyanax brachypterygyrium, with an extra "yr", which was provided in error as part of the original treatment.[2][4] It is known to be affiliated with species in the Astyanax scabripinnis species complex,[5] which makes it a part of the subgenus Astyanax.[6] There are three total subgenera in the genus Astyanax; the other two, aside from Astyanax itself, are Poecilurichthys and Zygogaster.[7]

Some preliminary research suggests that A. brachypterygium be considered a junior synonym of congener Astyanax cremnobates,[6] which was named in the same 2001 paper.[8]

Astyanax brachypterygium reaches a maximum length of 6.4 cm (2.5 in) standard length, SL.[4] There are 13–15 anal-fin rays, which is a much lower number than almost every other member of the genus; counts therein range from 20 to 45. There are 11 or 12 pectoral-fin rays, occasionally 10 or 13. There are 6 or 7 pelvic-fin rays, and 8–10 rays in both lobes of the caudal fin. There are 35–37 scales in the lateral line.[5] The eye takes up roughly one-quarter of the head's length.[11]

Astyanax brachypterygium has a brown-green back and a silver belly. The fins are mostly tinted reddish, with the exception of the pectorals, which are clear, and the adipose, which is slightly yellowish. There is also a patch of yellow on the middle caudal-fin rays.[12] There is a main humeral spot that demonstrates vertical elongation, and a secondary humeral spot that connects to the dark lateral stripe. This secondary spot may be diffuse, and is not always obvious upon examination.[5] On occasion, the area just behind the primary humeral spot may be pale.[13]

Astyanax brachypterygium bears enough of a resemblance to congener Astyanax cremnobates that the two have been considered for synonymy by some researchers.[6] Several morphometric differences exist, however. A. cremnobates has a larger eye diameter, more anal-fin rays, and a thinner caudal peduncle in comparison to A. brachypterygium; further, the native ranges are different.[14] A. cremnobates is also ~3 cm (~1 in) longer than A. brachypterygium.[15]

Another species, Astyanax pampa, bears further similarities to A. brachypterygium, but can be told apart by its much longer anal-fin base in comparison.[16]

Astyanax brachypterygium is endemic to headwater streams of the upper Uruguay and Jacuí River basins in Brazil.[4][12] This distribution is considered relatively restricted, which is not uncommon for species in the A. scabripinnis complex.[17][18] Amongst sites with specific occurrence records, it is relatively more common in the Lajeadinho stream, and slightly less so in the Marco river.[19]

Astyanax brachypterygium was originally collected at an elevation of 1050 m, but it can be collected as far as 1100 to 1200 m above sea level.[20] In these locales, biodiversity is low, which makes for fewer sympatric species; however, those collected alongside it include Pareiorhaphis hystrix, Rhamdia quelen, and Jenynsia eirmostigma.[19] It generally prefers shallow streams with a healthy flow.[1] Dietary needs are unknown.

The Lajeadinho stream floods on a regular basis, and is surrounded by floodplains as a result.[21] Unsurprisingly, the locale is subject to significant rainfall, though not on a constant basis.[22] The Marco river is a waterway with a strong current and rocky substrate, as well as a sparse riparian zone.[23]

Astyanax brachypterygium has been evaluated by the IUCN as a near threatened species. Recently, there has been a notable decline in the quality of its habitat, attributable to several anthropogenic factors; these include destruction of the riparian zones, introduction of invasive species, and the construction of hydroelectric dams.[1]