Negative Impacts of Perkinsus olseni Infection in Manila Clam Ruditapes philippinarum Observed from Tidal Flats in Anmyeondo Island on the West Coast of Korea During Post-Spawning Period

The high load of protozoan parasites in marine bivalves often leads to mass mortalities of the hosts. On the west coast of Korea in the Yellow Sea, the protozoan parasite Perkinsus olseni has been identified as the agent causing mass mortality of Manila clam Ruditapes philippinarum . During August and September 2004, mass mortality of clam occurred at Hwangdo (HD) tidal flat in Anmyeondo Island on the west coast, resulting in a 50% reduction in the clam landings. Shortly after the mortality event, we examined pathology, and the fitness of the survived clams from HD to elucidate the impacts of P. olseni infection. Histology revealed that clams collected from HD in October 2004 were infected by P. olseni. In histology, P. olseni could be observed from all types of tissues of clams from HD, and severe inflammation was observed in the gills. Ray's fluid thioglycollate medium assay (RFTM) indicated that the infection intensity in clams from HD (1.738 × 10 6 cells/g gills in October and 1.476 × 10 6 cells/g gills in December) was significantly higher than the levels in clams from the neighboring tidal flats (0.001 to 0.622 × 10 6 cells/g gills, P < 0.05). Condition index (CI) and the total carbohydrate levels in clams from HD in October were significantly lower than those values in clams from other tidal flats ( P < 0.05). In October, a negative correlation was observed between P. olseni infection intensity and CI in clams from HD, suggested that a high load of P. olseni causes substantial impacts on the host condition.