A Unique Specimen
Credit: Richard Borowsky

This cavefish was collected from a cave called Caballo Moro in northeastern Mexico. Although its eyes are useless and grown over, this cavefish has retained the darker coloring of its surface-dwelling Mexican tetra relatives.

"This cave fish is from an unusual population that seems to be evolutionarily young and has not achieved full reduction of eyes and pigmentation," study researcher Richard Borowsky told LiveScience.

Borowsky also noted that the Caballo Moro cave is unusual because it has a collapsed roof that lets in some sunlight. The present of light in the cave's water may explain why the cavefish possesses dark pigmentation, he said.