. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. PREDATOR AVOIDANCE IN FRESHWATER SNAILS 389 each size class (dependent variable). A second ANOVA separately analyzed differences in the number of surviving snails in each size class found above the waterline as the dependent variable. Each snail species was analyzed sep- arately. Because the data were expressed as proportions (proportion of the snails killed and the proportion of the surviving snails above waterline), the data were arc-sine transformed prior to analysis (Sokal and Rohlf, 1981). Results Experiment I: differen
. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. PREDATOR AVOIDANCE IN FRESHWATER SNAILS 389 each size class (dependent variable). A second ANOVA separately analyzed differences in the number of surviving snails in each size class found above the waterline as the dependent variable. Each snail species was analyzed sep- arately. Because the data were expressed as proportions (proportion of the snails killed and the proportion of the surviving snails above waterline), the data were arc-sine transformed prior to analysis (Sokal and Rohlf, 1981). Results Experiment I: differential vulnerability of Physella and Planorbella Handling times increased exponentially with increasing snail size for both species (Fig. 2A). For P. trivolvis, han- dling times increased more rapidly with increasing shell size than did the handling times for P. virgata. For each snail prey, an exponential equation was fitted by least squares non-linear regression to the handling time data of each snail species. The resultant best-fit non-linear regression between shell length (SL) and handling times (HT) for P. virgata was HT = e°28(SLl (n = 279, r = ). and for P. trivolvis: HT = e"4:(SLI (n = 113, r = ). For both species, the best-fit exponential equa- tions fit the data well, explaining 74-75% of the observed variance in the samples. The ingestion probabilities decreased more rapidly with increasing shell size for P. trivolvis than for P. virgata (Fig. 2). Approximately 60% of the smallest P. trivolvis (5-7 mm SL) were not eaten once captured, and few of the larger P. trivolvis (>8 mm SL) were picked up by the crayfish. In contrast, all small P. virgata (<8 mm SL) were eaten, once captured. The difference in vulnerability between the two snail species was significant; P. virgata were more likely to be eaten, once captured, at all size classes (Wilcoxin signed-ranks test, T := 0, n = 5, P < , Siegel, 1956). The snail size at which 50% of p
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