The "Blind Cave Tetra", "Astyanax fasciatus mexicanus", is a very unusual fish. It is completely devoid of skin pigments and has a pink skin due to the haemoglobin in its blood. The most distinctive part is that it has no eyes as an adult. For the first two weeks of life it does have eyes and certainly appears to be able to see.
There are several other common names for the Blind Cave Tetra, these include: the "Blind Cave Fish", the "Mexican Tetra", and the "Silvery Tetra".
It grows to about three and a half inches long (9cm). The life span is about 5 years.
The Blind Cave Tetras can find their way around an aquarium without much trouble. It is not completely clear how they do this. They have a goodsense of smell which helps them locate food, but this does not fully explain how they navigate around an aquarium. There are several theories and they are being studied.
A Recent Example of Evolution?
For some people, "Evolution" is a dirty word so I hesitated about even using it in a descriptive article about fish. If you prefer other explanations for the formation of this sub species, I am quite happy with that.
What appears to have happened is that some fish of the species Astyanax fasciatus found their way into an underground cave system in Mexico. These fish had eyes and could see as most fish can. In the darkness their eyes were of little use, and eyes use up energy as well as a substantial amount of brain power to interpret images. The fish that did not use so much energy and brain power for their eyes had an advantage and bred more. Over many generations the fish without eyes replaced the fish with eyes and the new sub species was formed.
I should explain that the phase "Sub species" does not suggest any form of inferiority, but is simply a taxonomic group below the level of species, but above that of variety.
The Blind Cave tetra is not considered a separate species from the fish that stayed on the surface and kept their eyes. (In light, vision is an advantage.) The blind cave tetra will still breed freely with their sighted cousins, so this is not an example of the formation of a new species.