Infectivity of Himasthla spp. (Trematoda) in cockle (Cerastoderma edule) spat

The common cockle Cerastoderma edule is intermediate host to several specie s of digenean trematodes. However, little is known about the factors influe ncing the settlement of trematode larvae in cockles as well as their effect s on the host. Aspects of the transmission ecology, behaviour and effects o n juvenile host specimens of trematode larvae belonging to the genus Himast hla and utilizing mudsnails Hydrobia ulvae as their first intermediate host and various waterbirds as their definitive host have been studied. Cercari ae of Himasthla spp. (more than 90% of the metacercariae were identified as H. continua post-experimentally) exhibited a host size-dependent selection within the host size range from 2 to 6 mm. Infection efficiency was reduce d by the presence of substratum allowing cockles to burrow, suggesting that the foot and visceral region may be attacked by cercariae. No increased mo rtality due to parasite infections could be demonstrated within 5d after in festation. The metacercariae within experimentally infected cockles were pr imarily located in the connective tissue within the foot and visceral regio n. Although some cockles were completely filled with metacercariae only sma ll functional effects were detected, such as a slightly prolonged burrowing time of infected specimens.