Bivalves play a key role in aquatic ecosystems and are a valuable commercial resource. The prosperity of these aquatic organisms depends mainly on the effectiveness of their immune defense, in which the hemolymph plays a central role. Hemolymph may be used as an effective non-lethal criterion of health. However, the predictive value of hemolymph analysis depends on the comparison between the obtained results and reference data from healthy individuals living in natural aquatic environments. We collected hemolymph from 15 commercially important species from wild populations at stations located in non-impacted and impacted water areas of the Sea of Japan. Of the 11 hemolymph parameters we analyzed, the total hemocyte count, percentage of hemocyte types, phagocytic activity, presence of reactive oxygen species, and protein concentration differed significantly between populations from non-impacted and impacted water areas. The most responsive species to pollution were Magallana gigas, Crenomytilus grayanus, Mizuhopecten yessoensis, and Mactra chinensis. This work is the first to examine a large number of commercially important species simultaneously. The results of this study are the basis for establishing the health status criteria of commercial bivalves for veterinary control in aquaculture and biomonitoring.

Despite the economic importance of Mizuhopecten yessoensis, little is known about their parasites and immunity. This study, the first to examine the prevalence and intensity of Perkinsus 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 Perkinsus. 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.