Carotenoid-Independent Shell Coloration in the Pacific Oyster, Crassostrea gigas: Divergence from the Molluscan...

Carotenoids are widely regarded as the main pigments driving yellow to orange shell coloration in mollusks, yet their role remains poorly understood in the economically important Pacific oyster Crassostrea gigas with abundant shell color morphs. To fill this research gap, we integrated Raman spectroscopy, HPLC detection, dietary supplementation trials, and gene expression profiling across four selectively bred strains with black (SB), white (SW), gold (SG), and orange shells (SO). Raman spectra revealed distinct pigment compositional profiles among different shell color morphs but displayed no characteristic carotenoid peaks within the 1500-1550 and 1150-1170 cm⁻¹ wavenumber ranges. Although HPLC detected only trace amounts of fucoxanthin in golden shells, carotenoid concentrations in the mantle showed no correlation with shell color and fluctuated markedly under different environmental conditions. Furthermore, a 28-day dietary supplementation trial verified that carotenoids were neither incorporated into shell structures nor did they contribute to mantle pigment deposition. Notably, key genes involved in carotenoid metabolism, including CgAPO, CgBCO, and CgSR-BI, were significantly upregulated in the hepatopancreas after dietary carotenoid feeding but were not expressed in mantle tissues, implying their physiological role in metabolic utilization rather than pigment allocation for shell coloration. Collectively, these findings challenge the conventional view that carotenoids dominate molluscan shell pigmentation, demonstrating that C. gigas deviates from the canonical carotenoid-based coloration paradigm, offering valuable insights for the genetic improvement of oyster color traits and advancing our understanding of molluscan pigmentation.