EFFECTS OF PARASITES ON LIFE-HISTORY OF THE FRESH-WATER 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 trematode s. The river is ice-covered for ca. 5 months each year. The clam popul ation was found to be infected by three species of digeneans: Bunodera luciopercae was the dominant parasite with prevalences of 34-35 % thr oughout the year; Palaeorchis crassus (ca. 7.5 %) and Phyllodistomum e longatum (ca. 5 %) were present mainly in the summer. Only 1.5 % of th e clams had joint infections by two parasites. The lowest percentage o f total trematode infection (ca. 10 %) was found at a host shell lengt h 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. a mnicum matured and laid eggs in early August, at the age of ca. 13 mon ths. The eggs developed into embryos in marsupia on the inner gills. T he 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 emb ryos increased with increasing parent size up to a maximum of 29. The infected clams did not reproduce. Annually the actual production of em bryos was ca. 50 % of the potential embryo numbers without digenean in fection. Probably the parasite-induced decrease in host fitness is eve n higher in terms of reproductive biomass production, since the large clams castrated by parasites are expected to have had a potential to p roduce not only more but larger embryos. Both semelparous and iteropar ous populations of P. amnicum have been described. The focal populatio n in Finland had an extended life cycle of 3 years probably because of seasonally low water temperatures, but they reproduced only once in t heir life because of parasitic castration of large clams. The reproduc tive effort (number of embryos per parent) in their first (and only) r eproduction is as high as with other semelparous populations and much higher than that of another population with extended life cycle but it eroparous reproduction.