Astyanax bransfordii

When first described by ichthyologist Theodore Gill in 1877, A. bransfordii was called Bramocharax bransfordii, classified in the new genus Bramocharax;^[3] by way of monotypy, it became the type species therein.^[4] A. bransfordii remained in the genus Bramocharax for the majority of its existence as a species. Species Bramocharax elongatus, described by Seth Eugene Meek in 1907, was synonymized with then-Bramocharax bransfordii by ichthyologist Donn Eric Rosen in 1970.^[5] Other species that once belonged to Bramocharax include Astyanax baileyi, Astyanax caballeroi, and Astyanax dorioni.^[6]

Based on genetic factors, the genus Bramocharax is largely considered obsolete in favor of Astyanax.^[7] A comprehensive 2008 study pointed to several Bramocharax species that formed clades with species of Astyanax, as opposed to with each other; A. bransfordii was found to form a clade with Astyanax nicaraguensis.^[8] A large-scale examination of Astyanax in Central and North America by Mexican ichthyologist Juan J. Schmitter-Soto corrected the classification of A. bransfordii, giving it its first official designation as a member of Astyanax. There are three subgenera in Astyanax - Astyanax, Poecilurichthys, and Zygogaster - and Schmitter-Soto placed A. bransfordii in the first.^[9]

The morphological differences previously used to separate genera are now thought to be an adaptation that has arisen independently in a handful of species that face similar environmental pressures;^[8] as such, there is still use in the labels, but as a matter of morphology, not one of phylogeny. When describing a specific set of features including jaw length, body shape, and dentition, species with the Bramocharax morph are long-snouted, slender, and have unicuspid teeth, while species with the Astyanax morph are short-snouted, deep-bodied, and multicuspid.^[10]

The specific epithet "bransfordii" was chosen by Theodore Gill to honor John F. Bransford, an assistant surgeon in the U.S. Navy responsible for collecting the type specimens. Bransford also co-authored the paper that introduced the original description of A. bransfordii.^[11] The genus name "Astyanax" is a reference to the Iliad, where Astyanax was a Trojan warrior and son of prince Hector.^[12]

Astyanax bransfordii, given the shape of its body and mouth, is sometimes referred to as the longjaw tetra.^[1][13]

Astyanax bransfordii reaches a maximum of 15.0 cm (5.9 in) standard length (SL).^[12] The body is long and of moderate depth, with a snout equal to or larger than the eye diameter. The jaws are also elongated, and are of equal length, equipped with large unicuspid teeth in the premaxilla.^[14] There are 9–11 dorsal-fin rays, 25 anal-fin rays (though occasionally up to 28), and 16 pectoral-fin rays. There are 10 or more procurrent rays (rays inserted farther apart as they move away from the fin's origin) in the caudal fin. There are 37–38 scales in the lateral line, and 12 scales in the predorsal series.^[9] The lateral line is complete.^[3] Sexual dimorphism is unknown.

The base scale color is a silver-white, with a darker back in an olive-gray and a gray lateral stripe. The fins are largely transparent, but some specimens demonstrate a light red blotch on the anal fin and/or an orange or red caudal fin.^[14] The only part of the fins to not be transparent is the median handful of caudal-fin rays, which are always dark due to a rhomboidal blotch of dark pigment, which continues onto the rays. There is an oval-shaped or indistinct humeral spot.^[15] The scales themselves are middling in size, with distinct margins and radiating lines.^[3] These radii, while distinct, are weakly-developed, especially in comparison to the dense circuli (rings). Biologist Theodore Cockerell made a comparison to the scales of fish in the family Clupeidae.^[16]

The type specimens of A. bransfordii were collected from Lake Nicaragua.^[17] Further specimens have been collected from Lake Managua and Cano Palma.^[12][18] It has also appeared in various areas of the Sarapiquí River.^[14] It appears to prefer waterways of little current, though it can be found in some areas with moderate velocity. It has been collected from elevations of 5 to 530 meters.^[12]

Due to its presence in blackwater habitats, A. bransfordii demonstrates that it does not have high-oxygen needs. Cano Palma, for example, is a blackwater canal with muddy substrate and low visibility.^[18] Blackwater conditions are caused when microbial decay acts upon leaf litter and other plant materials, leaching dark-colored tannins into the water; the microbes responsible consume much of the available dissolved oxygen.^[19][20]