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

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 CC (nu(1)) or CC (nu(2)) 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.