Raman spectroscopic insights into rare blue and green shell colouration in bivalve molluscs

Abstract

Abstract 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. Identifying their biochemical basis is therefore a crucial first step towards understanding the evolution and possible metabolic costs of these unusual traits. Using Raman spectroscopy, we analysed 15 bivalve species with green or blue shells to identify the pigments responsible. Species represented bivalve diversity of shell colour in terms of both phylogeny and morphological traits, including colour location (periostracum, organic layers in the calcareous shell, or calcareous shell). Green colour consistently occurred as discrete layers within the organic periostracum or organic layers interleaved within the calcareous shell. Blue colour was similarly found as layers in the periostracum but conversely, as diffuse colour in the inorganic calcareous shell. We did not observe blue colour in organic layers within the calcareous shell. Blue pigmentation is likely carotenoid-based, similar to pigments reported in other bivalves. In contrast, green colour is produced via two distinct pathways: either from previously unidentified, weakly Raman-active pigments or from photonic nanostructures producing structural colour. Within bivalves, to date green structural shell colour (excluding nacre) has been reported in only one genus, but the absence of any Raman-active pigments in ten divergent species raises the question of whether structural colour may be more prevalent in this clade than currently recognised.