I have a new favorite animal, and it's a fish. Not just any old fish, this one lives deep underground in caves in Mexico, and not only is it blind, it has lost its eyes altogether.

The Mexican tetra is a small and boring-looking animal, but appearances are deceptive. This fish is famous among evolutionary biologists, physiologists and sleep scientists for its hidden talents.

The Mexican tetra comes in two forms, a "normal" form that lives in rivers and streams, and a blind form that lives in caves. Charles Darwin wrote about blind cave fish in "Origin of Species": "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 affected other changes, such as an increase in the length of antennae or palpi, as compensation for blindness."