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. Some thirty distinct populations of Mexican tetras live in deep caves and 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.
The eyed and eyeless forms of A. mexicanus, being members of the same species, are closely related and can interbreed. Astyanax jordani, however, is another blind cave fish, independently and recently evolved from the sighted surface form, which is sometimes confused with the cave form of A. mexicanus. However, when born, the cave dwelling form of A. mexicanus has eyes. As they grow older, skin just grows over them and the eyes degenerate completely, because there is no need for sight in the pitch-black world of a cave.
In the aquarium
|This section may contain 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.
The surface and cave forms of the Mexican tetra have proven popular subjects for scientists studying evolution.
One hypothesis for the cave fish's evolution says that because of its dark habitat, the fish embryo saves energy it would normally use to develop eyes to develop other body parts, and this developmental choice would eventually dominate the population. This is called economical adaptation. However, studies have shown that blind cave fish embryos begin to grow eyes during development but then something actively stops this process and flesh grows over the partially grown eyes. Another hypothesis is that some Mexican tetra randomly don't develop eyes (which is represented by broken genes in the fish's genome), and this lack of eyes spreads to the rest of the population despite having no advantage or disadvantage. This is called the neutral hypothesis, or genetic drift.
In one experiment studying eye development, University of Maryland scientists transplanted lenses from the eyes of sighted surface-form embryos into blind cave-form embryos, and vice versa. In the cave form, lens development begins within the first 24 hours of embryonic development, but quickly aborts, the lens cells dying; most of the rest of the eye structures never develop. Researchers found that the lens seemed to control the development of the rest of the eye, as the surface-form tetras which received cave-form lenses failed to develop eyes, while cave-form tetras which received surface-form lenses grew eyes with pupils, corneas, and irises. (It is not clear whether they possessed sight, however.) 
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 Daniel Pauly, eds. (2006). "Astyanax mexicanus" in FishBase. March 2006 version.
- "Astyanax mexicanus". Integrated Taxonomic Information System. Retrieved 1 July 2006.
- Froese, Rainer, and Daniel Pauly, eds. (2006). "Astyanax jordani" in FishBase. March 2006 version.
- Dowling, T. E., D. P. Martasian, and W. R. Jeffrey (April 1, 2002). "Evidence for Multiple Genetic Forms with Similar Eyeless Phenotypes in the Blind Cavefish, Astyanax mexicanus". Molecular Biology and Evolution 19 (4): 446–55. PMID 11919286.
- Yamamoto, Yoshiyuki, and William R. Jeffrey (2000). "Central Role for the Lens in Cave Fish Eye Degeneration". Science 289 (5479): 631–3. Bibcode:2000Sci...289..631Y. doi:10.1126/science.289.5479.631. PMID 10915628.
- Pennisi, Elizabeth (2000). "Embryonic Lens Prompts Eye Development". Science 289 (5479): 522–3. doi:10.1126/science.289.5479.522b. PMID 10939956.
- 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