Predation Risk and Avoidance Behavior in Two Freshwater Snails.

We examined the predator avoidance behaviors of two common freshwater snails, Physella virgata and Planorbella trivolvis, to the crayfish Procambarus simulans. In response to crayfish predation, the snails crawled above the waterline for several hours, then returned to the water. A significant size-dependent relationship existed between crawlout (vertical migration above the waterline) and vulnerability to predation. All observed size classes of P. virgata, and small P. trivolvis, were vulnerable and crawled out in response to crayfish predation. Large, invulnerable P. trivolvis did not display any overt avoidance behavior, but relied instead on strong shell architecture for defense. We suggest that, in these species, crawling above the waterline reduces the probability of an encounter between vulnerable thin-shelled snails and crayfish. This behavior is an adaptive response to predation.