Correlation between body pigment reduction and eye reduction in subterranean fish and crustaceans
Loss of eye structures and body pigmentation are well documented heritable traits in subterranean organisms. Lamarck and Darwin determined that eye loss in cave organisms must be from disuse and not from natural selection. Since then, evolutionary speleologists have debated whether natural selection or neutral mutation has caused eye loss and de-pigmentation. Here we consider the evolution of eye and pigment reduction in subterranean organisms from a perspective of time rather than cause. While eyes and pigment are advantageous in well-lit environments, eyes maintain advantages in much darker environments than pigment does. This leads to the question of whether pigment reduction occurs before eye reduction in cave organisms. A comprehensive review of eye and pigment phenotypes in 366 subterranean vertebrate and invertebrate species suggests that pigment reduction does occur before eye reduction. In both invertebrate (Fisher’s p = 1.27x10-25) and vertebrate (Fisher’s p = 1.36x10-47) subterranean aquatic organisms body pigment tended to be further reduced relative to eye development.