The Evolutionary History and Morphological Divergence of Corallivorous Fishes on Coral Reefs.

Specialized ecological niches can represent evolutionary traps that increase extinction risk when environments change. The consumption of coral tissue, mucus, and skeleton is a rare feeding niche despite the abundance of coral on reefs, which raises questions about the evolutionary consequences of this dietary specialization. In this study, we used phylogenetic comparative methods, micro-CT scanning, and scanning electron microscopy to explore the evolutionary history of corallivory across reef fishes and compare feeding morphology across species within a major clade of corallivorous species (Chaetodontidae) that vary in their coral consumption. We show that obligate corallivory has evolved independently at least 17 times but is an evolutionary "dead end" for most reef fish species. Early origination of facultative corallivory may have contributed to the success of butterflyfishes, the predominant clade of coral-feeding fishes, as they diversified in competitive reef environments. Obligate and facultative butterflyfish species differ primarily in dental morphology while overall cranial traits are largely similar between groups. Coral feeding requires continuous foraging with narrow energetic margins, and climate-driven disturbances occur markedly faster than evolutionary transitions away from corallivory. This temporal mismatch between change at ecological and evolutionary timescales subjects corallivorous fishes to heightened vulnerability as climate-driven disturbances increase in frequency and severity.