Prevalence and Intensity of Perkinsus sp. Infection in Mizuhopecten yessoensis and Its Impact on the Immune Status of Bivalves

Despite the economic importance of <i>Mizuhopecten yessoensis</i>, little is known about their parasites and immunity. This study, the first to examine the prevalence and intensity of <i>Perkinsus</i> across three age groups of scallops from four locations in the Sea of Japan, revealed that the gills, mantle, and digestive glands of one-year-old specimens from mariculture farms are heavily colonized. The cases of infection were notably higher in older specimens (mostly hemolymph and shell), suggesting that they act as carriers of <i>Perkinsus</i>. An immunological analysis indicated that when the pathogen is found only in the hemolymph and mantle, there is an increase in plasma protein concentrations, which likely plays a crucial role in resisting infection. However, when hypnospores were present in the mantle and gills, a decrease in reactive oxygen species and granulocytes occurred, accompanied by an increase in hemoblasts and agranulocytes. Phagocytic activity increased only when the pathogen appeared in the digestive gland. This evidence highlights the heightened vulnerability of young scallops, emphasizing the necessity for preventive measures against infection. The current troubling epidemiological situation regarding scallop diseases in the region suggests a rise in epizootics, raising doubts about the sustainability of the scallop farming industry unless timely interventions are implemented.