Astyanax brachypterygium

Astyanax brachypterygium is a small species of freshwater fish endemic to a collection of high-elevation streams in Brazil. It was named in 2001 alongside congener Astyanax cremnobates, to which it bears a strong resemblance; physical details (and a separate native range) help to differentiate between the two. Its scales are a greenish-brown on the back and silver on the belly, with reddish fins and a dark humeral spot. One of its defining features is a notably short anal-fin base, which has 13 to 15 rays.

It usually inhabits shallow waters with a good current, but little else of its ecology has been researched, including diet, behavior, and mating practices. Several species are known to live in peaceful sympatry with it. The IUCN has given A. brachypterygium a near threatened classification due to a decline in habitat quality, largely thanks to anthropogenic survival pressures.

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

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

Etymology
The specific name originates in Greek. "Brachys" means short, and "pterygion" means fin; this is in reference to the short anal-fin base. The genus name Astyanax is from the Iliad, wherein Astyanax was the son of Trojan prince Hector.

In Brazil, A. brachypterygium is sometimes referred to as lambari-de-cabeceira; worldwide, it has no accepted common name.

Description
Astyanax brachypterygium reaches a maximum length of 6.4 cm standard length, SL. 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. The eye takes up roughly one-quarter of the head's length.

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. 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. On occasion, the area just behind the primary humeral spot may be pale.

Astyanax brachypterygium bears enough of a resemblance to congener Astyanax cremnobates that the two have been considered for synonymy by some researchers. 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. A. cremnobates is also ~3 cm (~1 in) longer than A. brachypterygium.

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

Sexual dimorphism
Male specimens of A. brachypterygium have bony hooks on the rays of the anal and pelvic fins. Males also have longer pelvic fins than females. The anal fin is smoothly concave in female specimens, while it is straight in males. Females are also generally longer than males, by slightly less than a centimeter.

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

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. 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. It generally prefers shallow streams with a healthy flow. Dietary needs are unknown.

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

Conservation status
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.