Digenean parasites of the bivalve mollusc Pisidium amnicum in a small river in eastern Finland.

The host-parasite relationship between digeneans and a semelparous population of the mollusc. Pisidium amnicum Müller in a small river in eastern Finland was studied during 1992/1993. The parasite prevalence of the population was high. The total prevalence was 45.6% in 1992 (n = 790) and 47.5% in 1993 (n = 160). The dominant digenean, Bunodera luciopercae (34.2% in 1992, 35.0% in 1993), had highest prevalences in July/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 increased as clams aged, while the other species were most common in middle-sized clams. Apparently B. luciopercae rediae dominate over P. elongatum, which has only sporocyst stages, while P. crassus, which has large rediae, is more deleterious to the clam and induces host mortality. Double infections were significantly less common (1.2%) than might be expected by chance. All parasites castrated their hosts; no clam containing both parasites and embryos was found. Semelparity of the population is apparently caused by parasitic castration.