[4][5] The type species of its genus, it is native to the Nearctic realm, originating in the lower Rio Grande, and the Neueces and Pecos Rivers in Texas, into the Central Plateau and eastern states of Mexico.

[4][6][7] 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.

Likewise, the blind cave tetra has fully "devolved" (lost) the use of its eyes by living in an environment completely devoid of natural light, with only empty sockets in their place.

The blind tetra instead has 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.

[9] 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.

[4] Additionally, the hypogean blind cave form is sometimes recognized as a separate species, A. jordani, but this directly contradicts the phylogenetic evidence.

[21] 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.

[28] 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.

[36] As selective pressure on one trait can coordinate change in others, pleiotropy could explain why independent adaptation to the cave environment has been observed in multiple populations of the species.

[37] One example is the relationship between taste bud amplification and eye loss controlled by sonic hedgehog expression (Shh) in cave fish.

[38] It is hypothesized that since caves are food and nutrient limited, having an increased amount of taste buds is important and may be under strong selection while at the same time causing evolution of eye loss.

[39] Darwin said of sightless fish:[40]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 effected other changes, such as an increase in the length of antennae or palpi, as compensation for blindness.Modern genetics has made clear that the lack of use does not, in itself, necessitate a feature's disappearance.

Possible explanations include: It is important to note that even if natural selection is positively acting to reduce eye growth drift is still present.

[45] Experiments have shown that keeping these fish in bright aquarium set-ups has no effect on the development of the skin flap that forms over their eyes as they grow.