High Prevalence of Pea Crab in a Mussel Population (Mytilus edulis) of the Atlantic French Coast

Pea crabs live in association with various marine invertebrates, particularly notable for their parasitic relationship with numerous bivalves of great economic importance. During a genetic study of mussel populations on the French Atlantic coast, these crabs were found in the mantle cavity of individuals in La Tranche-sur-Mer, predominantly affecting Mytilus edulis. Given the history of mass mortality events in this area, the apparent susceptibility of M. edulis to parasitism, and the negative impact of pea crabs on host physiology, this study focused on determining the crab species found and their effects on mussels. The prevalence of infestation in this population was high (64.6%), with 61.3% of infested mussels hosting one crab and 38.7% hosting two crabs. There were no significant differences in infestation rates between male and female bivalves. Genetic analysis (16S and H3) identified the crab as Pinnotheres pisum, of which two morphotypes were observed depending on sex and stage of development. Despite the high prevalence, infested mussels showed no evidence of impaired growth or reproductive development. However, long-term studies are recommended to explore whether subtle or cumulative physiological effects of crab infestations could indirectly contribute to the mass mortality events recorded in this area.