Prevalence of ontogenetic changes in colour brightness among benthic invertebrates and their association with microhabitat shifts

This study examined 15 species of intertidal invertebrates to determine the prevalence of ontogenetic changes in body colour brightness, the influence of diet and sunlight on body colouration, and the relationship between shifts in brightness and in microhabitat use. Most species did not undergo substantial changes in brightness or pigmentation. Some degree of change could be detected by image analysis software in 11 of the 15 species, but changes in brightness sufficient to be detected by a human observer occurred in only 6 species, and changes in pigmentation were apparent in only 3 species. In a controlled experiment, shell brightness and pigmentation of hatchings of the snail Nucella ostrina were found to be phenotypically plastic, changing in response to exposure to sunlight but not to diets of mussels or barnacles. Ontogenetic shifts in brightness were not phylogenetically constrained, occurring in the 3 phyla represented in our study (Mollusca, Annelida, Arthropoda). Shifts in brightness and pigmentation to increasingly darker shells or exoskeletons were found to occur in all species that shifted from cryptic to exposed microhabitats during ontogeny; the size at which colour brightness shift occurs is closely matched with the size at which microhabitat shift occurs in 3 species for which such information is available. However, little or no ontogenetic shift in brightness was detected in motile species with partial or no microhabitat shift. The ontogenetic timing of colour shifts may therefore serve as a marker of a transition between ecologically distinct phases of life.