The Mexican tetra (Astyanax mexicanus) is a freshwater 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 12.0 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 is reasonably popular among aquarists. This is especially true of the blind cave form.

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 30.0 dGH, and a temperature range of 20–58°C (68–77°F). In the winter it migrates to warmer waters. Its natural diet consists of crustaceans, insects, and worms, although in captivity it is omnivorous.

The Mexican tetra is sometimes considered a subspecies of A. fasciatus, the banded tetra. The blind forms were once considered to constitute a separate genus, Anoptichthys. Anoptichthys jordani and Anoptichthys hubbsi are thus obsolete synonyms for Astyanax mexicanus.

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. A recent study (Dowling, Martasian, and Jeffery) suggests that there are at least two distinct genetic lineages among the blind populations, arguing that these represent a case of convergent evolution.

The blind Mexican tetra is often held up as one of the proofs of the theory of evolution. Consequently, opponents of evolutionary theory argue that the blind form cannot be explained by natural selection.

References

• "Astyanax mexicanus (http://www.fishbase.org/Summary/SpeciesSummary.cfm?genusname=Astyanax&speciesname=mexicanus)". FishBase. Ed. Ranier Froese and Daniel Pauly. September 2004 version. N.p.: FishBase, 2004.
• "Astyanax mexicanus" (TSN 162850) (http://www.itis.usda.gov/servlet/SingleRpt/SingleRpt?search_topic=TSN&search_value=162850). Integrated Taxonomic Information System. N.p.: Integrated Taxonomic Information System, 2004. Accessed on November 3, 2004.
• Dowling, T. E., D. P. Martasian, and W. R. Jeffrey. "Evidence for Multiple Genetic Forms with Similar Eyeless Phenotypes in the Blind Cavefish, Astyanax mexicanus." Molecular Biology and Evolution 19 (April 2002): 446–55. Abstract on line (http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?holding=npg&cmd=Retrieve&db=PubMed&list_uids=11919286&dopt=Abstract).
• Sharpe, Shirlie. "Blind Cave Fish (http://freshaquarium.about.com/cs/characins2/a/blindcavefish.htm)". Your Guide to Freshwater Aquariums. Accessed on 3 November 2004.
• Wieland, Carl. "New Eyes for Blind Cave Fish? (http://www.answersingenesis.org/docs2/4361news8_9_2000.asp)". Answers in Genesis. Accessed on 17 December 2004.