EFFECT OF SIZE-DEPENDENT MUSKRAT (ONDATRA-ZIBETHICA) PREDATION ON THESPATIAL-DISTRIBUTION OF A FRESH-WATER CLAM, ANODONTA-PISCINALIS NILSSON (UNIONIDAE, BIVALVIA)

We studied the effect of central-place foraging by muskrats on the spa tial distribution of freshwater clam Anodonta piscinalis. We also anal ysed the prey-size preference of muskrats. We collected A, piscinalis shells from four muskrat middens representing different prey populatio ns 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 dist ance 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 th ree 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 counte rbalance the evolutionary response to predation.