The name Blind Cave Fish could refer to any species of fish that have no eyes and are found in caves but the name is normally associated with Astyanax mexicanus, also known as the Mexican Tetra.
This fish is the most studied of subterranean species because it’s easy to transport and keep in the laboratory. In fact, it’s so easy to care for that it can often be found in home aquariums. Aside from A. mexicanus, only 6 species have been successfully bred in the lab.
These fish were discovered in the 1930’s in northeastern Mexico but can also be found in parts of Texas. They are found in cave streams and pools but probably started out in above-ground streams.
Surface streams often become cave streams through a swallet (the point where they start to flow underground). Anything living in the stream gets swept underground at this point and this is probably how these fish became cave fish.
These fish are not actually born blind but the eyes do not fully develop and they quickly degenerate. Within 3 months, tissue covers the degenerated eye.
Why these fish lose their eyesight is open to debate. Some say it is to conserve energy, since sight is not needed in a dark cave. However, these fish have been found in areas where there is plenty of food and no need to conserve energy.
Another adaptation to cave life by these fish is its lack of pigmentation, making it albino. These adaptations are estimated to have taken from 1 to 2 million years to develop.
The Mexican Tetra is one of over 125 different species of cave fish. Some surface dwelling fish are washed into caves and lose their pigment. These can be mistaken for true cave fish but they are not adapted to the cave environment and eventually die.This video is an excellent example of a Mexican Tetra being raised in a home aquarium. As you can see, being blind is not a problem for these fish. They can still locate food quite well.