Effects of parasites on life history of the freshwater bivalve, Pisidium amnicum, in Eastern Finland

The life history of a freshwater clam, Pisidium amnicum was studied in 1981-1983 and in 1992-1993 in a small river in Eastern Finland, with special focus on reproduction and infection rate by digenean trematodes. The river is ice-covered for ca. 5 months each year. The clam population was found to be infected by three species of digeneans: Bunodera luciopercae was the dominant parasite with prevalences of 34-35 % throughout the year; Palaeorchis crassus (ca. 7.5 %) and Phyllodistomum elongatum (ca. 5 %) were present mainly in the summer. Only 1.5 % of the clams had joint infections by two parasites. The lowest percentage of total trematode infection (ca. 10 %) was found at a host shell length of 5 mm. The percentage increased steeply with increasing host size, and reached 90 % at 7.5 mm shell length (the maximum is 9.3 mm). P. amnicum matured and laid eggs in early August, at the age of ca. 13 months. The eggs developed into embryos in marsupia on the inner gills. The shelled embryos were liberated next June-July at a length of 2 mm. The minimum size of a parent giving birth is 4.5 mm. The number of embryos increased with increasing parent size up to a maximum of 29. The infected clams did not reproduce. Annually the actual production of embryos was ca. 50 % of the potential embryo numbers without digenean infection. Probably the parasite-induced decrease in host fitness is even higher in terms of reproductive biomass production, since the large clams castrated by parasites are expected to have had a potential to produce not only more but larger embryos. Both semelparous and iteroparous populations of P. amnicum have been described. The focal population in Finland had an extended life cycle of 3 years probably because of seasonally low water temperatures, but they reproduced only once in their life because of parasitic castration of large clams. The reproductive effort (number of embryos per parent) in their first (and only) reproduction is as high as with other semelparous populations and much higher than that of another population with extended life cycle but iteroparous reproduction.