Fish of Cave Darkness

In many, but not all, independently evolved cave-fish populations, eyes and pigmentation are reduced. Darkness weakens the advantage of vision, but the evolutionary explanation is not simply that selection stopped maintaining “costly eyes”: researchers discuss relaxed selection, mutation accumulation, and natural selection favoring eye reduction, including through energetic savings. In some cave forms of the Mexican tetra, studies have documented changes in taste systems and lateral-line sensitivity that help fish locate food and detect water movement. This does not mean every cave species necessarily has keener smell and taste; sensory changes differ across lineages and environments.