Blind Cave Tetra

fish 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. Despite profound eye degeneration, cavefish still respond weakly to light and show an endogenous circadian rhythm. During the start of development, larvae still exhibit a shadow response which is controlled by the pineal eye. 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 interbreed and produce fertile offspring. 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. 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 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. Another cave-adapted population of Astyanax, varying from blind and depigmented to individuals showing intermediate features, is known from the Granadas Cave, part of the Balsas River drainage in Guerrero, southern Mexico, but it is a part of A. aeneus (itself sometimes included in A. mexicanus).

The surface and cave forms of the Mexican tetra have proven powerful subjects for scientists studying evolution. One of the most striking changes to evolve was the loss of eyes. This is referred to as a "regressive trait" because the surface fish that originally colonized caves possessed eyes. or indirect selection through antagonistic pleiotropy, rather than genetic drift and neutral mutation, the traditionally favored hypothesis for regressive evolution. In a cave environment where vision is not helpful, natural selection will have prioritized energy savings and therefore favored the fish without eyes, over time leading to eye degeneration. Pleiotropy is hypothesized to be important in cave fish because there are genes that might be selected for one trait and automatically cause another trait to be selected for it if it is governed by the same gene. As selective pressure on one trait can coordinate change in others, pleiotropy could explain why independent adaptation to the cave environment has been observed in multiple populations of the species. One example is the relationship between taste bud amplification and eye loss controlled by sonic hedgehog expression (Shh) in cave fish. The Shh gene is expressed in higher levels in the cave form than in the surface form, and it has been shown that with an over expression of Shh there is an increased number of taste buds and reduced eye development. Darwin said of sightless fish: Modern genetics has made clear that the lack of use does not, in itself, necessitate a feature's disappearance. In this context, the positive genetic benefits have to be considered, i.e., what advantages are obtained by cave-dwelling tetras by losing their eyes? Possible explanations include: *Not developing eyes allows the individual more energy for growth but not egg production. Inhibition of the HSP90 protein has a dramatic effect in the development of the blind tetra.

The blind cave tetras seen in the aquarium trade are all based on stock collected in the Cueva Chica Cave in the southern part of the Sierra del Abra system in 1936. Experiments have shown that keeping these fish in bright aquarium set-ups has no effect on the development of the skin flap that forms over their eyes as they grow.