Prevalence of a Perkinsus Protozoan in the Clam Ruditapes philippinarum in Japan.

Infection of a protozoan Perkinsus (Apicomplexa) on the clam Ruditapes philippinarum has been reported from various localities of Japan since 1998. However, no ecological data are available heretofore on this parasitic organism. This is the first report on the spatial distribution of the prevalence of Perkinsus sp. in Japanese waters. A Perkinsus sp. specific polymerase chain reaction (PCR) method and a Ray's fluid thioglycollate medium technique were used for detection of Perkinsus sp. Clams infected with Perkinsus sp. were found from 63 of 74 study sites all over Japan except for the eastern coast of Hokkaido, prevalence ranging from 3.1 to 100%. Moreover, all the clams imported from neighbor countries are noted to be infected with Perkinsus sp. The prevalence and intensity of infection were greater in clams inhabiting commercial digging places and fishinggrounds than in undisturbed or non-fishing grounds. In contrast, the prevalence was relatively low in clams occurring in estuaries and in clams smaller than 15 mm in shell length. Perkinsus infection has also been detected in other bivalves, such as Ruditapes variegata and Protothaca jedoensis, collected from areas near to shell digging beds of R. philippinarum.