Astyanax acatlanensis, sometimes called the Acatlán tetra, is a small species of fish from the rivers of Mexico. It is a relatively hardy species that is able to tolerate a variety of water conditions, but is only found in freshwater, never brackish or marine. It is known only from the Río Acatlán, which is its type locality, and the Río Jía, which is a small, unknown river in the Oaxaca region. It is a relatively recent addition to the already-speciose Astyanax, named in 2017 as part of an in-depth revision of the genus' presence in Central and North America. Before its species distinction, it was one of various species considered synonymous with congener Astyanax aeneus, partially due to overlapping range and morphological similarity. Description Astyanax acatlanensis is mostly silver in color, with a blue or green tint dorsally. Its sides may bear an indistinct streak in a plainer silver, and it often has a humeral spot in the shape of an inverted triangle. The anal fin has sparse pigmentation, mostly concentrated into the central rays, and it has a spot of pigment on its caudal peduncle that extends onto the fin-rays, where it is slightly less intense. It is a small fish, reaching roughly 7.5 cm (2.9 inches) in standard length (SL), and it has a spine with 32–33 vertebrae. Notable morphometric characteristics include modally 12 predorsal scales, modally 15.5 or fewer scales around caudal peduncle, and a short head, mean 23.8% SL. Mature specimens may develop small fleshy extensions on the anal or pelvic fins, called nuptial spines or tubercles, and these are simple (solid-bodied, as opposed to bifid in some congeners). Nuptial tubercles are only seen on male specimens in the majority of fish species, which suggests relevant sexual dimorphism in A. acatlanensis, but females in other fish species may demonstrate them; details on A. acatlanensis are sparse. Taxonomy Originally, A. acatlanensis was grouped in with Astyanax aeneus, alongside various other species. However, a revision of the genus in 2017 by Mexican ichthyologist Juan J. Schmitter-Soto (specifically members from Central and North America) prompted its recognition as a separate species, albeit an under-studied one. American ichthyologist Robert Rush Miller noted in 1992 literature that the population inhabiting the Río Jía was likely unknown to science. Otherwise, A. acatlanensis has no known synonyms, and has retained its basionym since nomination. Etymology The type locality of A. acatlanensis is the