Quantity of sporocysts and seasonality of two Rhipidocotyle species (Digenea: Bucephalidae) in Anodonta piscinalis (Mollusca: Bivalvia).

The prevalence of Rhipidocotyle campanula in Lake Kuivasjärvi, northern Finland, was 5.7% (n = 1486) during 1989. The prevalences of R. campanula and R. fennica in Lake Saravesi, central Finland, during 1987 to 1989 were 1.0 and 33.2% (n = 1157), respectively. No clear seasonality in prevalence was found in either species, although miracidia of these digeneans are present only in late summer and autumn. The proportion of infected clams in which the gonad was full of sporocyst tubules was on average 90% for R. campanula and 30% for R. fennica. Clams with only few sporocyst tubules in their gonad were observed most frequently in spring. Old; empty sporocyst ribbons of R. fennica first appeared in the clams during August after the beginning of the cercarial emergence and disappeared slowly thereafter. However, in both species viable sporocyst ribbons with early developmental stages of cercariae were found throughout the year. The emergence of R. fennica cercariae began during July and continued throughout September in Lake Saravesi. The duration of cercariae emergence was found to be up to 72 days in the laboratory. R. campanula cerariae in the northern Lake Kuivasjärvi emerged from mid-June until mid-August.