Infection characteristics of Himasthla elongata cercariae in cockles as a function of water current

Digenean trematodes are widespread parasites of marine fauna. The first intermediate host of Himasthla elongata (Echinostomatidae) is the periwinkle Littorina littorea. The second host is less specific, as it includes Various bivalve species. The definitive host is a waterbird. The free-living cercaria acts to ensure transmission from the first and second intermediate hosts. We examined the impact of water flow (still and running water) and host size (6-8, 8-10, 10-12 mm shell length classes) 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 replicate) acquired infections and 60% of the added cercariae were recovered as metacercariae in cockle tissue (mainly foot and siphons). Infection intensity increased significantly with cockle size. Considering the 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 potential dispersal of H. elongata is at least a few hundred metres.