Spatial distribution of the protozoan parasite Perkinsus sp. found in the Manila clams, Ruditapes philippinarum, in Korea.
Epizootic mortalities of Manila clams, Ruditapes philippinarum, have been reported along the west and south coast of Korea for the past several years. This is the first report on the spatial distribution of the infection, and infection intensity of Perkinsus sp. in Korean waters. Ray's fluid thioglycollate medium technique (RFTM) and Choi's 2 M NaOH digestion technique were used in detection and quantification of Perkinsus sp. Perkinsus sp. was detected in clams collected from 18 of 22 sites investigated along the west and the south coast of Korea [date not given]. Perkinsus sp. cells were found mostly in connective tissues of gills, mantle, gonad and digestive gland while they were rarely observed in the foot and siphon tissues. The trophozoites were often enclosed in amorphous granules resulting from the cellular defence reaction of the clams. White nodules were often observed on the surface of heavily infected clam tissues as a result of inflammatory responses. Among the infected clams, infection prevalence ranged from 23% to 100%. Average infection intensity at each sampling site varied from 12 to 3 924 309 Perkinsus cells/individual or 6 to 873 000 cells/g tissue. Infection intensity and prevalence was much higher among the clams collected from commercial clam beds located on tidal flats along the west and south coast. In contrast, clams collected from sand beaches on the east coast and Weido and Kimnyong in Cheju Island were not infected by Perkinsus. It was believed that spatial distribution of Perkinsus sp. is in some way controlled by temperature, salinity and substrate type.