Phylogenetic distribution of shell colour in Bivalvia (Mollusca)

In order to learn more about the evolution of shell colour in molluscs, 20 characters related to shell colour were recorded for 81 bivalve clades using the entire collection of dry shells at the Natural History Museum in London (> 44000 lots) and plotted onto a published phylogeny. Statistical tests for phylogenetic signal show that coloured shells are not distributed evenly across the class. The phylogenetic distribution of colour is statistically significant, as are the distributions of individual shell and periostracal colours. Blue and green shells are rare, with non-iridescent blue and green coloration occurring on the outer side of the valve more commonly in the periostracum than in the shell matrix and on the inside of valves in species that produce proteinaceous sheets overlying calcareous shell. These findings suggest that these colours in bivalves are attributable to pigments that are more easily incorporated into organic material than into calcareous shell. Ancestral state reconstructions show that the ancestral bivalves likely had a coloured shell. Although similar colours can arise from different pigments or from structural elements, the broad-scale phylogenetic distribution of colour in Bivalvia, along with the co-occurrence of 'sets' of colours, probably reflects, in part, the distribution of classes of pigments. This confirms the idea that the major classes of pigments found in molluscan shells are evolutionarily ancient and continue to contribute to shell coloration, despite recent genomic evidence suggesting that protein moieties associated with pigments might be evolutionarily diverse.