Correlated evolution of conspicuous coloration and burrowing in crayfish

Conspicuous colors have fascinated biologists for centuries, leading to research on the evolution and functional significance of color traits. In many cases, research suggests that many conspicuous colors are adaptive and serve some function in sexual or aposematic signaling. In other cases, a lack of evidence for the adaptive value of conspicuous colors troubles biologists, such as within organisms that live underground and are rarely exposed to the surface. Here, we use phylogenetic comparative methods to investigate color evolution throughout freshwater crayfishes that vary in burrowing ability. Within the taxa we analyzed, conspicuous colors have evolved independently over 50 times; and these conspicuous colors are more common in semi-terrestrial crayfishes that construct extensive burrows. The intuitive, but not evolutionary-justified assumption when presented these results is to assume that these colors are adaptive. But contrary to this intuition, we discuss the hypothesis that coloration in crayfish is neutral. Supporting these ideas, small population sizes and reduced gene flow within semi-terrestrial burrowing crayfishes may lead to fixation in color-phenotype mutations. Overall, our work brings into question the traditional view of animal coloration as a perfectly adapted phenotype.