INFECTION CHARACTERISTICS OF HIMASTHLA-ELONGATA CERCARIAE IN COCKLES AS A FUNCTION OF WATER CURRENT

Digenean trematodes are widespread parasites of marine fauna. The firs t intermediate host of Himasthla elongata (Echinostomatidae) is the pe riwinkle Littorina littorea. The second host is less specific, as it i ncludes Various bivalve species. The definitive host is a waterbird. T he free-living cercaria acts to ensure transmission from the first and second intermediate hosts. We examined the impact of water flow (stil l and running water) and host size (6-8, 8-10, 10-12 mm shell length c lasses) on the settling success of H. elongata cercariae in the second intermediate host by utilizing the common cockle Cerastoderma edule. Under 2 scenarios (free-stream velocity of 0 and 6 cm s(-1)) more than 90% of the experimental population of cockles (90 individuals per rep licate) acquired infections and 60% of the added cercariae were recove red as metacercariae in cockle tissue (mainly foot and siphons). Infec tion intensity increased significantly with cockle size. Considering t he filtering capacity of cockles, the dimension of the flume, and the flow velocity, a passive infection mechanism is proposed based on the suspension feeding activity of the host organism. The scale of potenti al dispersal of H. elongata is at least a few hundred metres.