Mortality of Spats of Manila Clam Ruditapes philippinarum Experimentally Challenged with the Protozoan Parasite Perkinsus olseni

In a previous field survey of Perkinsus olseni in Manila clams in a tidal flat contaminated with the parasite, the parasite was not detected in spats smaller than 2 mm in shell length collected between April and June. ​To understand the reason, we undertook an experimental challenge, exposing clam spats to the zoospores of parasite. ​The spats became infected and began to die when mean infection intensity reached about 106 cells/g soft tissue. ​Our results, in conjunction with previously reported filtration rates of Manila clams, suggest the absence of infected spats was attributable to clams' low filtration and low zoospore densities.