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

Abstract

Several studies have suggested that shell colour may be phylogenetically distributed within the phylum Mollusca, but this pattern is confounded by our ignorance of the homology of colour and lack of understanding about the identity of most molluscan pigments. We use resonance Raman spectroscopy to address this problem by examining bivalve pigments producing a range of colours and compare spectra from taxa at different phylogenetic levels. The spectra of most shell pigments exhibited a skeletal signature typical of partially methylated polyenes, possibly modified carotenoids, with the strongest peaks occurring between 1,501–1,540 cm−1 and 1,117–1,144 cm−1 due to the C═C (ν1) and C–C (ν2) stretching modes, respectively. Neither pigment class nor mineral structure differentiated Imparidentia and Pteriomorphia. Spectral acquisitions for purple pigments for two species of Asaphis suggest that identical or nearly identical pigments are shared within this genus, and some red pigments from distantly related species have similar spectra. Conversely, two species with brown shells have distinctly different pigments, highlighting the difficulty in determining the homology of colour even within a single class of pigments. Curiously, we were unable to detect any Raman activity for green‐coloured shell or pigment peaks for the yellow area of Codakia paytenorum, suggesting that these colours are due to structural elements or a pigment that is quite different from those observed in other taxa examined to date. Our results are consistent with the idea that classes of pigments are evolutionarily ancient but heritable modifications may be specific to clades.

The Diversity of Linear Conjugated Polyenes and Colours in Nature: Raman Spectroscopy as a Diagnostic Tool

“…Linear conjugated polyenes are molecules that contain more than two double bonds located on neighboring carbons arranged in an aliphatic carbon chain. This structural arrangement encompasses chromatic biomolecules (biochromes) such as carotenoids (Figures 1 and 2), [1,2] polyunsaturated aldehydes (Figure 3) [3–9] and polyolefinic fatty acids [10] (Figure 4). The linear extension of the conjugated double bonds in these molecules is the main feature that determines the spectroscopic properties of these light‐absorbing molecules, which have an electronic absorption in the blue‐green visible region of the electromagnetic spectrum between 400 and 500 nm.…”

“…Carotenoids are present widely in different vegetables and fruits, [2] in bird's feathers, [22] algae, [23] fungi, lichens, bacteria, cyanobacteria, [23,24] sponges, [25,26] corals, [27–31] molluscs, [26] mollusc shells, [9,32,33] crustaceans, echinoderms, tunicates and fish [26] . The vast majority of carotenoids derive from phytoene (C 40 ) which is precursor of the linear lycopene, simple hydrocarbon carotenes and xanthophylls (Figure 1).…”

“…Conjugated polyenals containing from 6 to 9 C=C unsaturations were identified from parrots’ feathers [5,6,8,42–45] and conjugated polyenals containing from 6 to 14 unsaturations are found in shells, [7,9,33,46] pearls [11,47,48] produced by molluscs and octocorals (Figure 3). [7,29,30,49–56] …”

“…Unlike their carotenoid analogues, psittacofulvins are biosynthesized by animals and they are found keratinised within the feather follicles [43,67] . Intriguingly, molecules with similar spectral features have also been identified by Raman spectroscopy in marine and fresh water invertebrates inserted into calcareous matrices (CaCO 3 polymorphs) of shells, [7,9,32,46] pearls, [11,47,48] and corals [7,29,30,49,51–54,56] . In these organisms, the conjugated length of the double bonds ranges from 6 to 14 unsaturations.…”

Recent advances in linear and nonlinear Raman spectroscopy. Part XIV

“…Their results are consistent with the idea that classes of pigments are evolutionarily ancient but heritable modifications may be specific to clades. [119] 14 | ART AND ARCHAEOLOGY…”