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 under-stand 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 10 6 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.