DIGENEAN PARASITES OF THE BIVALVE MOLLUSK PISIDIUM-AMNICUM IN A SMALLRIVER IN EASTERN FINLAND

The host-parasite relationship between digeneans and a semelparous pop ulation of the mollusc Pisidium amnicum Muller in a small liver in eas tern Finland was studied during 1992/1993. The parasite prevalence of the population was high. The total prevalence was 45.6% in 1992 (n = 7 90) and 47.5 % in 1993 (n = 160). The dominant digenean, Bunodera luci opercae (34.2% in 1992, 35.0% in 1993), had highest prevalences in Jul y/August and in winter. Two other species, Palaeorchis crassus (7.8 % in 1992, 7.5 % in 1993) and Phyllodistomum elongatum (4.7 % and 5.0 %) , were rare during the winter. The prevalence of B. luciopercae increa sed as clams aged, while the other species were most common in middle- sized clams. Apparently B. luciopercae rediae dominate over P. elongat um, which has only sporocyst stages, while P. crassus, which has large rediae, is more deleterious to the clam and induces host mortality. D ouble infections were significantly less common (1.2 %) than might be expected by chance. All parasites castrated their hosts; no clam conta ining both parasites and embryos was found. Semelparity of the populat ion is apparently caused by parasitic castration.