Bivalve transcriptomics reveal pathogen sequences and a powerful immune response of the Mediterranean mussel (Mytilus galloprovincialis)

Bivalves have colonized the interface between land and sea for the last 500 million years. Although bivalves lack an adaptive immune system, they are extraordinarily well adapted to adverse environmental conditions. Bivalves are valuable aquaculture resources worldwide and are used as sentinels for monitoring pollution. In this work, the immune transcriptomes of mussels (Mytilus galloprovincialis and edulis) and clam (Ruditapes decussatus) were sequenced. For comparative purposes, an already published transcriptome dataset of Ruditapes philippinarum was also included in the analyses. The 454 pyrosequencing of stimulated hemocytes resulted in more than 400,000 reads for each transcriptome. The percentage of annotated sequences ranges from 50% for mussels to 30–40% for clams. Considering the 28,061 non-redundant sequences from the four transcriptomes, the four species share 785 genes. Moreover, sequences related to different putative pathogens were found in the four bivalves. A high number of bivalve herpesvirus ORFs were found, which confirms the value of NGSs as tools to detect and quantify pathogen RNA. Based on an examination of the immune-enriched transcriptomes of these four species, we can conclude that bivalves present an immune system that differs from its conventional characterization as a simple innate immune response against invading pathogens. Enrichment analyses showed that species in the Mytilus genus, especially M. galloprovincialis, possesses a significantly higher number of sequences related to immune processes and killing molecules than species in the Ruditapes genus. This could be related to the broader ecological niche occupied by mussels and the scarcity of reported mussel mass mortalities compared to the high number of mass mortalities reported for clams.