Seasonal changes of Perkinsus and Cercaria infections in the Manila clam Ruditapes philippinarum from Jeju, Korea

Abstract

Seasonal changes in the infection intensity of Perkinsus and Cercaria were investigated in Manila clams Ruditapes philippinarum collected from a sand beach on the east coast of Jeju, Korea. Perkinsus prevalence and infection intensity were determined from histological preparations of clams. The prevalence varied seasonally; it was lowest in late September 2001 (6.0%) and highest in March 2002 (86.0%), with an annual mean of 32.9%. The monthly mean infection intensity of Perkinsus varied from 0.11 (September 2001) to 2.08 (March 2002), with a mean of 0.63. Tissue inflammation caused by massive hemocyte infiltration was commonly observed in the gills of clams that were heavily infected with Perkinsus. Degenerated oocytes and castrated follicles were observed in clams that were severely infected with Cecaria tapidis during the spawning period, indicating that C. tapidis interferes with the reproductive process of clams. The prevalence of C. tapidis was highest in early August (12.0%), while none of the clams were infected with Cercaria in some months. Although Perkinsus- and Cercaria-related clam mortalities were not observed in this study, the histological findings clearly show that the parasitism impacts clam reproduction, at least during part of the annual reproductive cycle.