Colour dimorphism in labrid fishes as an adaptation to life on coral reefs

Conspicuous colouration displayed by animals that express sexual colour
dimorphism is generally explained as an adaptation to sexual selection,
yet the interactions and relative effects of selective forces influencing
colour dimorphism are largely unknown. Qualitatively, colour dimorphism
appears more pronounced in marine fishes that live on coral reefs where
traits associated with strong sexual selection are purportedly more
common. Using phylogenetic comparative analysis, we show that wrasses and
parrotfishes exclusive to coral reefs are the most colour-dimorphic, but
surprisingly, the effect of habitat is not influenced by traits associated
with strong sexual selection. Rather, habitat-specific selective forces,
including clear water and structural refuge, promote the evolution of
pronounced colour dimorphism that manifests colours less likely to be
displayed in other habitats. Our results demonstrate that environmental
context ultimately determines the evolution of conspicuous colouration in
colour-dimorphic labrid fishes, despite other influential selective
forces.