Astyanax mexicanus, or “blind cave fish”, as it is commonly known, is an evolutionary wonder. The tetra lives freshwater rivers in Mexico, particulary in dark caves in which eyesight is redundant.
The most fascinating aspect of the blind cave fish, as the name implies, is its characteristic lack of eyes.
The Mexican Tetra is in fact a moniker for two varieties of the same species, only one of them blind, that are genetically related, and are, in fact, only isolated reproductively by geographic location.
Stating the obvious, this means that the blind tetra and the surface tetra can mate and reproduce, while the blind tetra grows to full maturity without ever developing eyes.
The evolutionary tale doesn’t end here, though. There is another related species confounded with the Mexican Tetra called Astyanax jordani. This species probably evolved from the surface version of A. mexicanus, and, amazingly, develops eyes which are consequently covered by skin until they degenerate.
That is a stunning demonstration of neutral evolution in action: a surface fish that changes its habitat to cave-dwelling loses eyesight without adaptive costs, but without arresting completely the development of eyes.
There’s also material evidence that the surface and eyeless versions of A. mexicanus are, in fact, the same species or , at the very least, extremely related: lens cells were cross-planted in both varieties of A. mexicanus, causing the surface fish embryos to become eyeless and the blind fish embryos to develop fully formed eyes, lenses, irises, lock, stock and barrel.