Pathology survey of the short-neck clam Ruditapes philippinarum occurring on sandy tidal flats along the coast of Ariake Bay, Kyushu, Japan

The pathological condition of the short-neck clam Ruditapes philippinarum was surveyed along the coast of Kumamoto, Japan, in June 2004. DNA sequences of the non-transcribed spacer region and internal transcribed spacer region flanking 5.8S rRNA identified Perkinsus olseni among the clams. Ray's fluid thioglycollate medium assay indicated that 96.7% of the clams surveyed from the Kiguchi River tidal flat (native clams, Stn KR-N) and 96.7% of the clams surveyed from the Midori River tidal flat (Stn MR) were infected with P. olseni with an infection intensity of 464,278 and 199,937 Perkinsus cells/gram tissue wet weight (gWW), respectively. In contrast, 66.7% of the clams imported from China and stored along the Kiguchi River tidal flat (Stn KR-I) and 20.2% of clams from the Arao tidal flat (Stn AT) were infected with P. olseni with an infection intensity of 37,547 and 3382 Perkinsus cells/gWW, respectively. Brown ring disease was detected in the clam population from Stn KR-I at a prevalence of 90.0%. Polymerase chain reaction and the 16S rRNA sequence suggested that the agents of brown ring disease observed at Stn KR-I were Vibrio tapetis-like bacteria. Sporocysts and metacercariae of unidentified trematodes were also observed in the gonads and mantle of the clams from Stn KR-I, Stn MR, and Stn AT, at prevalences of 7.1-42.9%. Metacestodes (larval tapeworms) were found in the foot and digestive gland at a prevalence of 52.5%, 30.0%, and 14.3% in clams from Stns MR, AT, and KR-N, respectively. Histology also showed massive hemocyte infiltration and inflammation among clams heavily infected with P. olseni. Castration of the follicle was typical among clams infected with the trematode. The data indicate that most of the clams along the coast of Kumamoto are infected with various pathogens at various rates of infection, and these pathogens could have negative effects on the clam population in the long term.