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 of their skull, which is propounded to increase water flow to the opposite side of the face, functionally enhancing sensory input and spatial mapping in the dark waters of caves. Scientists suggest that gene cystathionine beta synthase-a mutation restricts blood flow to cavefish eyes during a critical stage of growth so the eyes are covered by skin.
Currently, about 30 cave populations are known, dispersed over three geographically distinct areas in a karst region of San Luis Potosí and far southern Tamaulipas, northeastern Mexico. Among the various cave population are at least three with only full cave forms (blind and without pigment), at least eleven with cave, "normal" and intermediate forms, and at least one with both cave and "normal" forms but no intermediates. Studies suggest at least two distinct genetic lineages occur among the blind populations, and the current distribution of populations arose by at least five independent invasions. Furthermore, cave populations have a very recent origin (< 20,000 years) in which blindness or reduced vision evolved convergently after surface ancestors populated several caves independently and at different times. This recent origin suggests that the phenotypic changes in cavefish populations, namely eye degeneration, arose as a result of the high fixation of genetic variants present in surface fish populations in a short period of time.
The eyed and eyeless forms of A. mexicanus, being members of the same species, are closely related and can interbreed making this species an excellent model organism for examining convergent and parallel evolution, regressive evolution in cave animals, and the genetic basis of regressive traits. This, combined with the ease of maintaining the species in captivity, has made it the most studied cavefish and likely also the most studied cave organism overall.