Rare shell colours in bivalves reveal multiple evolutionary pathways to blue and green colouration

Green and blue are rare colours in bivalve shells. Where such colours occur, it is unclear whether they result from shared pigments across taxa, as similar appearances may arise from different compounds and are not necessarily homologous. Identifying their biochemical basis is therefore crucial to understanding the evolution and possible metabolic costs of these unusual traits. We used Raman spectroscopy to analyse resin-embedded cross-sections of valves from 15 bivalve species with green or blue shells. Species represented diversity in shell colour in terms of both phylogeny and morphological traits, including colour location (organic periostracum, organic layers in the calcareous shell or inorganic calcareous matrix). Green colour consistently occurred in organic layers rather than calcareous shell, while blue colour occurs in both calcareous shell and organic layers. Blue colour appears to be due to carotenoid-based pigments, similar to other pigments observed in bivalve shells. Green colour, however, is due either to novel pigments, not previously identified in mollusc shells, which are only weakly Raman active or nanostructures that produce structural colour in the absence of green pigments. Within bivalves, to date structural colour has been reported only in one genus, underscoring its evolutionary significance.