Color variation and visual modeling provide no support for adaptive coloration in a blue crayfish

The adaptationist dogma among research on animal colors has recently come into question, as several studies have suggested that bright, conspicuous colors in animals are not always adaptive, but evolutionarily neutral. However, demonstrating that a color trait is evolutionarily neutral is complicated—there is no consensus as to what qualifies as sufficient evidence for a color trait to be considered neutral. Our study discusses these issues while investigating the function of coloration in a conspicuously colored blue crayfish, Cambarus monongalensis, which inhabits semi-terrestrial burrows and rarely interacts with the surface. By correlating sex and size with color variables, we found that three body regions of C. monongalensis do not vary in color with respect to sex or size, thereby rejecting two of our adaptive hypotheses. Additionally, visual models of crayfish and predators suggest that crayfish may not be able to distinguish body colors across the body and against backgrounds, but that predators can distinguish these colors. By integrating these results with prior studies, we discuss how bright colors in crayfish may be an ideal system to investigate the neutral theory of color evolution. Ultimately, such research may challenge the adaptationist dogma in research on animal color and other phenotypes.