Why are corals colorful?

Abstract: The diversity of coral colours contributes to the visually arresting appearance of coral reefs. The involved colour polymorphisms are often due to the environmentally regulated expression of multicopy genes encoding pigments from the green fluorescent protein (GFP) family. High-level expression of several representatives of these protein pigments reduces photo-damage of the coral’s symbiotic algae. However, trade-offs associated with their expression can have detrimental effects on the fitness of symbiotic corals in low light environments. We have shown that the extreme light conditions at the upper and lower end of the depth distribution range of symbiotic reef corals expose colour morphs to opposing selective pressure, thereby promoting the evolution and persistence of intraspecific colour polymorphisms. Furthermore, we suggest that the neutral impact of the pigmentation on coral fitness under ambient light conditions together with the multicopy nature of the colour gene polymorphism has been a major driver of the evolution of the staggering diversity of GFP-like proteins. This diversity gave rise to multiple functions of these pigments in habitats with different environmental conditions including the recently discovered wavelength transformation in deep water habitats by which the host improves the light climate for its symbiotic algae.