Meeting Abstract
Naturalists have long sought to explain the drivers of conspicuous colour evolution. Wrasses and parrotfishes display conspicuous coloration during the terminal phase, which transitions from a duller initial phase with growth. The degree of colour difference between phases, dichromatism, ranges from extreme to none, and appears more pronounced in species that live on coral reefs. Here traits that are thought to promote the evolution of dichromatism via sexual selection, including protogynous sex change and polygynous mating, are purportedly more common. Support for this hypothesis is limited to qualitative assessments and the claims have not been subject to rigorous comparative analysis incorporating evolutionary history. Here we use phylogenetic comparative methods to quantify the interaction and relative effects of selective pressures driving the evolution of dichromatism. We show that wrasses and parrotfishes exclusive to coral reefs are the most dichromatic, but surprisingly, this effect is not influenced by sexual ontogeny or mating system. While selective pressures acting on both initial and terminal phases might affect the magnitude of dichromatism, they vary with habitat only among terminal phase fishes that are more likely to display blue colours on coral reefs. Habitat-specific conditions including clear water may underlie the adaptation of specialized visual signals and perception; however, we find that depth is not a contributing factor. Fishes on coral reefs are more likely to display dichromatism on permanently-visible body regions, suggesting the structural refuge of coral habitats may mitigate any increased risk of predation to conspicuously coloured fishes. Our results show that environmental conditions ultimately shape the selective forces underlying the evolution of pronounced dichromatism.
