The Mexican tetra (Astyanax mexicanus), also known as the blind cave fish, blind cave characin, and blind cave tetra, is a freshwater fish of the family Characidae of the order Characiformes. The type species of its genus, it is native to the Nearctic realm, originating in the lower Rio Grande and the Neueces and Pecos Rivers in Texas, as well as the central and eastern parts of Mexico. Growing to a maximum total length of , the Mexican tetra is of typical characin shape, with silvery, unremarkable scalation. Its blind cave form, however, is notable for having no eyes or pigment; it has a pinkish-white color to its body (resembling an albino). This fish, especially the blind variant, is reasonably popular among aquarists. A. mexicanus is a peaceful species that spends most of its time in midlevel water above the rocky and sandy bottoms of pools and backwaters of creeks and rivers of its native environment. Coming from a subtropical climate, it prefers water with 6.5–8 pH, a hardness of up to 30 dGH, and a temperature range of . In the winter, some populations migrate to warmer waters. Its natural diet consists of crustaceans, insects, and annelids, although in captivity it is omnivorous. The Mexican tetra has been treated as a subspecies of A. fasciatus, but this is not widely accepted. Additionally, the hypogean blind cave form is sometimes recognized as a separate species, A. jordani, but this directly contradicts phylogenetic evidence. Blind cave form A. mexicanus is famous for its blind cave form, which is known by such names as blind cave tetra, blind tetra (leading to easy confusion with the Brazilian Stygichthys typhlops), blind cave characin and blind cavefish. Depending on the exact population, cave forms can have degenerated sight or have total loss of sight and even their eyes, due to down-regulation of the protein αA-crystallin and consequent lens cell death. The fish in the Pachón caves have lost their eyes completely whilst the fish from the Micos cave only have limited sight. Cave fish and surface fish are able to produce fertile offspring. These fish can still, however, find their way around by means of their lateral lines, which are highly sensitive to fluctuating water pressure. Blindness in A. mexicanus induces a disruption of early neuromast patterning, which further causes asymmetries in cranial bone structure. One such asymmetry is a bend in the dorsal region