Abstract
Pea crabs of the family Pinnotheridae are widely known as symbionts or parasites of bivalve mollusks; however, their occurrence and ecological impacts remain poorly documented for many economically important host species. Here, we report the first confirmed host record of pinnotherid crabs inhabiting the mantle cavity of the Japanese scallop Mizuhopecten yessoensis from Mutsu Bay, northern Japan, and quantitatively assess their effects on host growth and somatic condition. A total of 881 scallops, including juvenile and subadult individuals, were examined between March 2024 and November 2025. Pinnotherid crabs were detected across multiple localities, with parasitism prevalence of 32.8 % in juvenile scallops and 27.3 % in subadults, showing no significant difference between developmental stages. Mature females, including ovigerous individuals, were observed within host scallops, indicating successful reproduction of the crabs inside this newly documented host. Despite similar prevalence across stages, the impacts of parasitism were strongly stage dependent. Parasitized juvenile scallops exhibited significantly reduced shell length and soft tissue index compared with non-parasitized individuals, whereas no significant effects were detected in subadults. Results from size-adjusted statistical models indicated that parasite load significantly reduced somatic condition independent of host shell length. Quantile regression further revealed a strong host-parasite size constraint, with maximum crab size increasing with host size. These results indicate that pinnotherid crabs function as true parasites in M. yessoensis, particularly during early life stages, imposing measurable physiological costs. Our findings highlight parasitism as a previously overlooked biotic factor that may influence early-life performance and resilience in a major aquaculture species under environmental stress.