Chemical basis of molluscan shell colors revealed with in situ micro‐Raman spectroscopy

Abstract

Molluscan shells exhibit a wide variety of color tones and patterns that are thought to increase fitness in different natural habitats. However, the chemical aspects of shell pigments have not been well understood. We studied the chemical basis of molluscan shell colors via resonance Raman microspectroscopy using a 514.5‐nm laser on six gastropod, 15 bivalve, and one cephalopod species. Polyene specific signals for C═C (ν1) or C─C (ν2) stretching bands were detected principally from the pigmented regions, and those peak positions correlated with the pigment colors. Because Raman shifts of those bands depend on the conjugation length of polyene backbones, which in turn, affect the highest occupied molecular orbital/lowest unoccupied molecular orbital energy gap, our observations suggest that the conjugation length of polyene is a determinant factor of shell color. Further, our observations and data from previous studies seemed to reveal a quantitative relationship between polyene backbone length and shell color. From an analysis of 89 polyene‐based pigments in 45 molluscan species, we found that yellow, orange/red, and purple/brown/black pigments originate from polyenes with conjugation lengths of 7–9, 9–10, and 10–12, respectively. The shell color shifts from yellow, orange, and red to purple as the polyene chain length increases as reported for free carotenoids. The biological mechanisms underlying the spatial regulations of polyene conjugation systems in molluscan shells will be the most important and intriguing issue to be addressed in the future.

Detection of intact polyene pigments in Miocene gastropod shells

“…This shift can be explained by the conjugation length of the polyene backbones, which is higher for brown polyenes than for red polyenes (Ishikawa et al . 2019).…”

“…2006; Ishikawa et al . 2019). By comparison of the Raman band maxima measured in the present work with those of the synthetic polyenes (Schaffer et al .…”

Fluorescent colour patterns in the basal pectinid Pleuronectites from the Middle Triassic of Central Europe: origin, fate and taxonomic implications of fluorescence

“…2006; Ishikawa et al . 2019; Wade et al . 2019), which show strong signals at about 1100 and 1500 cm −1 .…”

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

“…Studies performed with shells of the marine bivalve Arctica islandica indicated that conjugated polyenals are incorporated within the aragonitic crystals and they are located both on the outside and inside of the shell, developing the same layered pattern typical for the growth edge [78] . Shells and pearls from several molluscs are coloured by molecules such as these, but exceptions were found to occur in the shell of Cypraea moneta , [79,80] recent and extinct (fossil) brachiopod species [33] and in the pearls of Pinna nobilis , [48] where carotenes were identified. In general, carotenoids obtained from the diet are constituents of the lipid bilayer of diverse molluscan cell membranes and are present in some organs (gill, mantle, nervous adductor, leg muscle and hepatopancreas) [81] .…”