The Mexican tetra or blind cave fish (Astyanax mexicanus) is a fresh-water fish of the characin family (family Characidae) of order Characiformes.  The type species of its genus, it is native to the Nearctic ecozone, 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 overall length of 12 cm (4.7 in), the Mexican tetra is of typical characin shape, with unremarkable, drab coloration. Its blind cave form, however, is notable for having no eyes and being albino, that is, completely devoid of pigmentation; it has a pinkish-white color to its body.
This fish, especially the blind variant, is reasonably popular among aquarists.
A. mexicanus is a peaceful species that spends most of its time in the mid-level of the 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 6.0–7.8 pH, a hardness of up to 30 dGH, and a temperature range of 20 to 25 °C (68 to 77 °F). In the winter it migrates to warmer waters. Its natural diet consists of crustaceans, insects, and annelids, although in captivity it is omnivorous.
Blind cave form
A. mexicanus is famous for its blind cave form, which is known by such names as blind cave tetra, blind tetra, and blind cavefish. These forms have lost the power of sight and even their eyes. These fish can still, however, find their way around by means of their lateral lines, which are highly sensitive to fluctuating water pressure. Currently, 29 cave populations are known, dispersed over three geographically distinct areas in a karst region of northeastern Mexico. Recent studies suggest that there are at least two distinct genetic lineages among the blind populations, and that the current distribution of populations arose by at least five independent invasions.
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.
Astyanax jordani is another blind cave fish which is sometimes confused with the cave form of A. mexicanus.
The surface and cave forms of the Mexican tetra have proven powerful subjects for scientists studying evolution. When the surface-dwelling ancestors of current cave populations entered the subterranean environment, the change in ecological conditions rendered their phenotype—which included many biological functions dependent on the presence of light—subject to natural selection and genetic drift. 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. In addition to regressive traits, cave forms evolved "constructive traits." In contrast to regressive traits, the purpose or benefit of constructive traits is generally accepted. Active research focuses on the mechanisms driving the evolution of regressive traits, such as the loss of eyes, in A. mexicanus. Recent studies have produced evidence that the mechanism may be direct selection, or indirect selection through antagonistic pleiotropy, rather than genetic drift and neutral mutation, the traditionally favored hypothesis for regressive evolution.
In the aquarium
|This section possibly contains original research. (September 2007)|
A. mexicanus only grows to a length of 12 cm (4.7 in) in the recommended minimum of 30 US gallons. The blind cave tetra is a fairly hardy species. They are not picky on food, as they will eat anything from standard flakes to sinking carnivore pellets. Their lack of sight does not hinder their ability to get any and all food before it hits the bottom. They prefer subdued lighting with a rocky substrate, like gravel. It is recommended that the tank mimic their natural environment, so artificial or natural rock is highly appreciated. They become semi-aggressive as they age, but are by nature schooling fish, and a group of at least three is a good idea. However, it isn't imperative, as they can be kept as a single specimen. As stated earlier, they can get to their food rather swiftly, even more so than fish with the ability to see, so they aren't really good for a community. They are very quick and agile, so catching them with a net is a chore in itself while conducting maintenance.
Blind cave tetras and creationism
The blind form of the Mexican tetra is different from the surface-dwelling form in a number of ways, including having unpigmented skin, having a better olfactory sense by having taste buds all over its head, and by being able to store four times more energy as fat allowing it to deal with irregular food supplies more effectively. However, it is the lack of eyes that has been at the centre of discussion of the Mexican cave tetras among creationists.
Darwin said of sightless fish:
By the time that an animal had reached, after numberless generations, the deepest recesses, disuse will on this view have more or less perfectly obliterated its eyes, and natural selection will often have affected other changes, such as an increase in the length of antennae or palpi, as compensation for blindness.—Charles Darwin, Origin of Species (1859)
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 and reproduction 
- there remains less chance of accidental damage and infection, since the previously useless and exposed organ is sealed with a flap of protective skin
Another likely explanation for the loss of its eyes is that of selective neutrality and genetic drift: in the dark environment of the cave, the eyes are neither advantageous nor disadvantageous and thus any genetic factors that might impair the eyes (or their development) can take hold with no consequence on the individual or species. Because there is no selection pressure for sight in this environment, any number of genetic abnormalities that give rise to the damage or loss of eyes could proliferate among the population with no effect on the fitness of the population.
Among some creationists the cave tetra is seen as evidence against evolution. One argument claims that this is an instance of "devolution"—showing an evolutionary trend of decreasing complexity. But evolution is a non-directional process, and while increased complexity is a common effect, there is no reason why evolution cannot tend towards simplicity if that makes an organism better suited to its environment.
- Froese, Rainer and Pauly, Daniel, eds. (2006). "Astyanax mexicanus" in FishBase. March 2006 version.
- "Astyanax mexicanus". Integrated Taxonomic Information System. Retrieved 1 July 2006.
- Yoshizawa, M.; Yamamoto, Y. O'Quin, K. E. Jeffery, W. R. (Dec 2012). "Evolution of an adaptive behavior and its sensory receptors promotes eye regression in blind cavefish". BMC Biology 10: 108. doi:10.1186/1741-7007-10-108.
- Gross, J.B. (Jun 2012). "The complex origin of Astyanax cavefish". BMC Evolutionary Biology 12: 105. doi:10.1186/1471-2148-12-105.
- Retaux, S.; Casane, D. (Sep 2013). "Evolution of eye development in the darkness of caves: adaptation, drift, or both?". Evodevo 4: 26. doi:10.1186/2041-9139-4-26.
- Soares, D.; Niemiller, M. L. (Apr 2013). "Sensory Adaptations of Fishes to Subterranean Environments". Bioscience 63 (4): 274–283. doi:10.1525/bio.2013.63.4.7.
- Wilkens, H (Nov 2012). "Genes, modules and the evolution of cave fish". Heredity 105 (5): 413–422. doi:10.1038/hdy.2009.184.
- Protas, M; Tabansky, I. Conrad, M. Gross, J. B. Vidal, O. Tabin, C. J. Borowsky, R. (Mar-Apr 2008). "Multi-trait evolution in a cave fish, Astyanax mexicanus". Evolution & Development 10 (2): 196–209.
- Jeffery, WR (2009). "Regressive Evolution in Astyanax Cavefish". Annual Review of Genetics 43: 25–47. doi:10.1146/annurev-genet-102108-134216.
- Helfman G., Collette B., & Facey D.: The Diversity of Fishes, Blackwell Publishing, p 315, 1997, ISBN 0-86542-256-7
- Dawkins, R.: Climbing Mount Improbable, W. W. Norton & Co, 1997, ISBN 0-393-31682-3