Mexican cavefish spend their entire lives in the dark. With no need for vision, many of them lost functional eyes. In more than 30 varieties of Mexican cavefish, the eyes stop developing as embryos grow into larvae. Although they are still the same species as a sighted, surface-dwelling fish (Astyanax mexicanus), the cave-dwelling varieties carry genetic mutations that scientists have yet to fully unravel.
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Source: Phys.org