According to Damian Moran, Rowan Softley and Eric J. Warrant, the blind cavefish lost its eyes because the visual part of the brain consumes 15% of all the energy used by the fish. So it’s less that they lost their eyes because they didn’t need them in the dark. It’s that, in the dark, there’s less total energy to go around so energy-expensive systems are selected against, as the most efficient fish win the energy game.
Read more at http://advances.sciencemag.org/content/1/8/e1500363.full