Effect of size-dependent muskrat (Ondatra zibethica) predation on the spatial distribution of a freshwater clam, Anodonta piscinalis Nilsson (Unionidae, Bivalvia)

We studied the effect of central-place foraging by muskrats on the spatial distribution of freshwater clam Anodonta piscinalis. We also analysed the prey-size preference of muskrats. We collected A. piscinalis shells from four muskrat middens representing different prey populations and sampled the clam populations quantitatively. Muskrats had clear effects on the spatial distribution of the clams. At all study sites the area close to shore had no clams. The width of the empty area was correlated with the number of shells found in the muskrat midden. The density of clams decreased and their mean size increased with the distance from muskrat midden at two of the sites. Muskrats did not prey on clams smaller than 50 mm. Muskrats preferred 60- to 70-mm clams at three of the sites and 85- to 90-mm clams at the fourth. In an analysis conducted using ages, a selection gradient on the growth rate of clams was found for three of the study populations. However, spatial refuge from predation and inconsistency of selection may slow down or counterbalance the evolutionary response to predation.

Age-, size-, and sex-specific infection of Anodonta piscinalis (Bivalvia: Unionidae) with Rhipidocotyle fennica (Digenea: Bucephalidae) and its influence on host reproduction

A Rhipidocotyle fennica infection in Anodonta piscinalis was studied during 1987–1989 in Lake Saravesi, central Finland. The overall prevalence was 32.3% (n = 1157). Only mature clams, ≥ 3 and ≥ 4 years old in the littoral and sublittoral zones, respectively, were infected. According to log-linear models constructed for the mature clams, there was a statistically significant interaction between prevalence and age of the host, between prevalence and habitat, and between age of the host and habitat; prevalences were higher among old specimens and in the littoral zone than among young clams or in the sublittoral zone. Logit models showed that prevalence increased linearly with host size in both habitats. A relationship between clam growth and infection was also found: fast-growing clams were more often infected or infected clams had grown faster. The quantity of sporocyst tubules increased with the age of the host. Prevalence was higher among female clams than among males. It was estimated that over 50% of mature females in the littoral zone were infected. It was also estimated that there was a 31% reduction in the number of glochidium-bearing clams in the littoral zone of Lake Saravesi due to infection with this parasite.