Carotenoids are biologically active pigments that are well-known to enhance the defense and immunity of the vertebrate system. However, in invertebrates, the role of carotenoids in immunity is not clear. Therefore, this study aims to review the scientific evidence for the role of carotenoids in invertebrate immunization. From the analysis of published literatures and recent studies from our laboratory, it is obvious that carotenoids are involved in invertebrate immunity in two ways. On the one hand, carotenoids can act as antioxidant enzymes to remove singlet oxygen, superoxide anion radicals, and hydroxyl radicals, thereby reducing SOD activity and reducing the cost of immunity. In some organisms, carotenoids have been shown to promote SOD activity by up-regulating the expression of the ZnCuSOD gene. Carotenoids, on the other hand, play a role in the expression and regulation of many genes involved in invertebrate immunity, including thioredoxins (TRX), peptidoglycan recognition receptor proteins (PGRPs), ferritins, prophenoloxidase (ProPO), vitellogenin (Vg), toll-like receptor (TLRs), heat shock proteins (HSPs), and CuZnSOD gene. The information in this review is very useful for updating our understanding of the progress of carotenoid research in invertebrate immunology and to help identify topics for future topics.

The noble scallop Chlamys nobilis Reeve displays polymorphism in shell and muscle colors. Previous research showed that the orange scallops with orange shell and muscle had a significantly higher carotenoid content than the brown ones with brown shell and white muscle. There is currently a need to identify candidate genes associated with carotenoid-based coloration. In the present study, 454 GS-FLX sequencing of noble scallop transcriptome yielded 1,181,060 clean sequence reads, which were assembled into 49,717 isotigs, leaving 110,158 reads as the singletons. Of the 159,875 unique sequences, 11.84% isotigs and 9.35% singletons were annotated. Moreover, 3,844 SSRs and over 120,000 high confidence variants (SNPs and INDELs) were identified. Especially, one class B scavenge receptor termed SRB-like-3 was discovered to express only in orange scallops and absent in brown ones, suggesting a significant association with high carotenoid content. Down-regulation of SRB-like-3 mRNA by RNA interference remarkably decreased blood carotenoid, providing compelling evidence that SRB-like-3 is an ideal candidate gene controlling carotenoid deposition and determining orange coloration. Transcriptome analysis of noble scallop reveals a novel scavenger receptor significantly associated with orange scallop rich in carotenoid content. Our findings pave the way for further functional elucidation of this gene and molecular basis of carotenoid deposition in orange scallop.