Molluscan shell colour

Abstract

The phylum Mollusca is highly speciose, and is the largest phylum in the marine realm. The great majority of molluscs are shelled, including nearly all bivalves, most gastropods and some cephalopods. The fabulous and diverse colours and patterns of molluscan shells are widely recognised and have been appreciated for hundreds of years by collectors and scientists alike. They serve taxonomists as characters that can be used to recognise and distinguish species, however their function for the animal is sometimes less clear and has been the focus of many ecological and evolutionary studies. Despite these studies, almost nothing is known about the evolution of colour in molluscan shells. This review summarises for the first time major findings of disparate studies relevant to the evolution of shell colour in Mollusca and discusses the importance of colour, including the effects of visual and non-visual selection, diet and abiotic factors. I also summarise the evidence for the heritability of shell colour in some taxa and recent efforts to understand the molecular mechanisms underpinning synthesis of shell colours. I describe some of the main shell pigments found in Mollusca (carotenoids, melanin and tetrapyrroles, including porphyrins and bile pigments), and their durability in the fossil record. Finally I suggest that pigments appear to be distributed in a phylogenetically relevant manner and that the synthesis of colour is likely to be energetically costly.

Colour in bivalve shells: Using resonance Raman spectroscopy to compare pigments at different phylogenetic levels

“…It seems likely that the polyenes in molluscan shell pigments are based on carotenoid precursors, which are known to colour soft tissues and are correlated with colour in shells . As most animals are incapable of producing carotenoids de novo, molluscan carotenoids are likely to be dietary in origin, whereas some modifications may be genetically controlled . As such, the differently coloured shells may reflect both environment (dietary intake of different carotenoids) and genetics (heritable modifications of identical or differing carotenoid precursors).…”

“…Colour and pattern are often associated in the natural world with camouflage, warning, and sexual selection . Understanding how new biological colours have arisen is a fundamental challenge to our understanding of evolutionary ecology and developmental biology, but despite over a century of interest, the evolution of colour in molluscan shells is only just beginning to be explored in detail . Of particular interest is the observation that colour and the distribution of fluorescence associated with some pigmentation both seem to be distributed in a phylogenetically significant manner .…”

Introductory Chapter: Melanin, a Versatile Guardian

“…Certain organisms repurposed melanin for additional protective functions: ink of cephalopods, octopus, squid and cutleish, blackened by melanin, which provides protection from predators [3]. In mollusks other than cephalopods, melanin, together with other chromophores, serves to produce color paterns in their shells [4]. In insects, melanin is used even more resourcefully, not only for pigmentation of the exoskeleton but also for cuticle hardening, wound healing and in their innate immune responses [5].…”

Colorful seashells: Identification of haem pathway genes associated with the synthesis of porphyrin shell color in marine snails

“…Significant advances have been made toward the characterization of pigments and their biosynthetic pathways for plants, vertebrates, and certain invertebrate groups (e.g., Braasch, Schartl, & Volff, ; Grotewold, ; Hodges & Derieg, ; Joron et al., ; Nijhout, ; Wittkopp & Beldade, ; Wittkopp, Williams, Selegue, & Carroll, ); however, the molecular pathways leading to shell pigmentation have not been completely elucidated for any mollusk (Mann & Jackson, ). The phylum Mollusca is highly diverse, species rich, ecologically important, and abounding in colorful exemplars, so our lack of understanding about pigment evolution in this clade is a serious gap in our knowledge of how color has evolved in the natural world (Williams, ).…”

“…The congruence of colors arising from different pigments suggests that there may be selective pressures leading to convergent evolution in these taxa (Williams et al., ). Apart from Clanculus , uroporphyrin pigments are also responsible for coloration of soft tissues and shells of other (mostly marine) mollusks (reviewed in Williams, ), the integument of some annelids (Fox, ), and turaco bird feathers (Nicholas & Rimington, ).…”