Blind cave fish are born with eyes. As they age skin grows over their eyes, eventually covering them completely, and their vision deteriorates until they are completely blind.
The Mexican tetra comes in two forms: the blind cave fish and a surface-dwelling form with full vision. It is thought that blind cave fish may have evolved from their seeing surface-dwelling counterparts after either becoming trapped or voluntarily moving into cave systems. There is still much debate over whether the two forms are in fact different species and they are the focus of much scientific study as researchers attempt to understand evolution. In evolutionary terms, blind cave fish came about very recently.
Although they are blind, blind cave fish are able to detect light using their pineal gland (a gland in the brain which controls the sleep cycle and can detect light in some species). If the blind cave fish detects light using this gland, they will move away from it, instinctively seeking the darkness of the cave systems they dwell in.