Same Species, Different Appearance
Credit: Masato Yoshizawa | Univ. of Maryland

One theory of the A. mexicanus' varying populations is that, historically, at least two groups of the fish lived in the rivers of Sierra de El Abra, Mexico. One group colonized the caves and became extinct on the surface. The other group then restocked the rivers, but also invaded the caves, joining the original cave-dwelling population, according to the researchers.