Abstract
Three marine cercariae, Cercaria pectinata, Cercaria tapidis and Cercaria sp. were found in the short-necked clam Tapes philippinarum from Lake Hamana, Japan. These cercariae are the first reported marine cercariae from the area. Of 3, 200 clams examined, 110 infections for C.pectinata, three for C.tapidis and one for Cercaria sp. occurred. C.pectinata is a yellowish, non-oculate, trichocercous, fellodistomid cercaria, developing in large saccular sporocyst in the host gonad. C.tapidis is an oculate cercaria with long tail of five times as body, and Cercaria sp. is a non-oculate, furcocercous, gymnophallid cercaria, and both species develop in sporocyst.
The infection rate by C.pectinata showed a seasonal change, being low(0-1.2%)from May to August and relatively high (2.0-3.6%)from September to March. Histologically, sporocysts in early stage were found in inter-follicular spaces, and were observed to grow in size in situ with the duration of time and to occupy finally the greater part of the gonad;ripe sporocysts with numerous mature cercariae were observed from March to April.
