Abstract

The Manila clam (Ruditapes philippinarum), commonly found on mudflats and sandy beaches along the coastal Yellow Sea, is frequently infected by the protozoan parasite Perkinsus olseni. The intensity and prevalence of P. olseni infection in Manila clams vary spatially and seasonally, often peaking during the post-spawning period. In this study, we investigated the infection intensity and prevalence of P. olseni in Manila clams collected in October 2013, when most clams had completed spawning. Using Ray’s fluid thioglycollate medium assay (RFTM), we found that, except for clams from Pohang and Imwon on the east coast, P. olseni infection was widespread among clams from tidal flats and sandy beaches on the west coast, with prevalence rates ranging from 87% to 100%. The infection intensity varied widely, from no infection (Pohang and Imwon) to over a million cells per gram of gill tissue at most sampling sites. Clams from sites on the west coast, such as Seonjae, Hwangdo, and Gomso, exhibited infection levels of 1.0 to 4.3 × 106 cells/g gill, while those from the south coast, including Wanddo, Yeosu, Tongyeong, and Masan, showed levels of 1.1 to 4.8 × 106 cells/g gill. Histological analysis revealed a similar range of infection intensity, from 0 (no infection in Pohang and Imwon) to 3.6 (Hwangdo). The high infection intensities and prevalence observed in the clams during this study may be partly attributed to elevated water temperatures and physiological stress during the post-spawning period, a pattern previously noted in clams from tidal flats along the west coast.