The Mexican tetra (Astyanax mexicanus), also known as the blind cave fish, blind cave characin or the blind cave tetra, is a species of freshwater ray-finned fish belonging to the family Acestrorhamphidae, the American characins. This fish is found in the lower Rio Grande, and the Nueces and Pecos Rivers in Texas, into the Central Plateau and eastern states of Mexico. Maturing at a total length of about 12 cm (4.7 in), the Mexican tetra is of typical characin form, albeit with silvery, unremarkable scalation, likely an evolutionary adaptation to its natural environment. By comparison, the species' blind "cave" form has scales which evolved a depigmented, pinkish-white color, somewhat resembling an albino, as it inhabits pitch-black caverns and subterranean streams and has no need for a colorful appearance (i.e. for attracting mates, camouflage, UV protection). Likewise, the blind cave tetra has fully lost functionality of its eyes as a result of inhabiting an environment completely devoid of natural light, with only empty sockets covered with skin in their place. The blind tetra instead has highly developed sensory organs along its body, as well as a heightened nervous system (and senses of smell and touch), and can immediately detect where objects or other animals are located by slight changes in the surrounding water pressure, a process vaguely similar to echolocation—another adaptation known from cave-dwelling, as well as aquatic, species, such as the bats and cetaceans. The Mexican tetra's blind variant has experienced a steady surge in popularity among modern aquarists. A. mexicanus is a peaceful, sociable schooling species, like most tetras, that spends most of its time in midlevel waters above the rocky and sandy bottoms of pools, and backwaters of creeks and streams. Coming from an environment somewhere between subtropical climate, it prefers water with 6.5–8 pH, a hardness of up to 30 dGH, and a temperature range of 20 to 25 °C (68 to 77 °F). In the winter, some populations migrate to warmer waters. The species' natural diet consists largely of crustaceans, annelids and arthropods and their larvae, including both aquatic insects, such as water beetles, and those that land on or fall in the water, like flies or arachnids. It will also supplement its diet with algae or aquatic vegetation; in captivity, it is largely omnivorous, often doing well on a variety of foods such as frozen/thawed or live cultured blackworms, bloodworms, brine shrimp, daphnia, and mysis shrimp, among other commercially available fish foods. The Mexican tetra has been treated as a subspecies of A. fasciatus, though this is not widely accepted. Additionally, the hypogean blind cave form is sometimes recognized as a separate species, A. jordani, but this directly contradicts phylogenetic evidence.
Blind cave 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. These fish can still, however, find their way around by means of their lateral lines, which are highly sensitive to fluctuating water pressure. 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. Studies suggest at least two distinct genetic lineages occur among the blind populations, and the current distribution of populations arose by at least five independent invasions. 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.
… excerpt ends here. Continue reading the full article.
