Mexican tetra  (Astyanax mexicanus)

Subspecies: Astyanax mexicanus jordani (Sardina ciega) is classified as Vulnerable (VU) on the IUCN Red List (1).

The Mexican tetra is a small fish with a rather unremarkable appearance, but behind its uninteresting first impression is a fascinating tale of evolution, in which different populations have evolved very different features and habitats, resulting in much debate over this species’ taxonomy (3) (4) (5). One form is a surface-dwelling fish, silvery in colour with a black band that extends along each side to the tail (4) (5). The tail and lower fins of some individuals may be tinted yellow or red (4). The other form has evolved to inhabit caves, which has resulted in a loss of colouration and, most significantly, the loss of functioning eyes (3) (6). Both forms of the Mexican tetra have a forked tail, a small dorsal fin (4), and an anal fin that, on breeding males, possesses tiny hooks (5). It has comparatively large and strong teeth (4) (5). Much confusion exists over the correct taxonomy of the Mexican tetra, with some scientists believing that the surface-dwelling and cave forms are actually different species (7) (8).

The Mexican tetra occurs in eastern and central Mexico (2) and in Texas and New Mexico in the United States (4). It has apparently also been introduced into several other southern states where it did not previously occur (4).

The surface-dwelling form of the Mexican tetra inhabits a wide range of freshwater habitats. Adults show a preference for rocky and sandy bottomed pools in creeks, streams and rivers, while young Mexican tetras are found in shallower waters, often near vegetation that overhangs the bank of a river or stream (5). The other form inhabits freshwater in caves, where a lack of light has resulted in the changes seen in these populations (3).

The Mexican tetra is a schooling fish (6), which can form schools of up to several hundred or even thousands of individuals (5). It is primarily a carnivorous fish, feeding on aquatic insects, crustaceans, worms, snails and smaller fish (2) (5), but it is also reported to feed on plant matter and algae (5).

In Texas, breeding activity has been observed in the surface-dwelling form from late April until September, although elsewhere, such as the lower reaches of the Rio Grande River, reproduction is said to occur year-round. Spawning taking place in late spring and early summer, when a mass of sticky eggs is released into the surrounding waters (5). Mexican tetras develop quickly, which relates to their short lifespan. Those that are born in the spring reproduce for the first time in autumn, and few live for longer than two years (5).

The Mexican tetra is not believed to be in danger of extinction (5). However, a lack of clarification on the taxonomy of Astyanax species (8) makes it hard to determine the status of this fish and what threats it may face. The cave form has been captured for aquariums, although it is easily bred in captivity and so it is thought that most Mexican tetras sold today are captive bred, thus this trade will not impact wild populations (9).

There are no known conservation measures specifically in place for the Mexican tetra. As the confusion surrounding the taxonomy of this species currently prohibits the status of this species being determined, clarifying this issue should be a priority.

View information on this species at the UNEP World Conservation Monitoring Centre.