Assessing predator risk: how leeches affect life history and behaviour of the freshwater snail Physa acuta

Physa acuta, an important consumer in freshwater ecosystems, is known to have morphological, life history and behavioural responses to fish and crayfish as shell-breaking predators. However, less is known about physid responses to their leech predators, which consume snails without damaging the shell and are commonly found in close proximity to P. acuta and their egg masses. Because juvenile physids are more vulnerable to leech predators than adults, we studied the effects that leech predation may have on a physid’s life history and reproductive behaviour. Snails that were reared with predatory-leech cue (fed conspecific snails) experienced a delay in reproduction. Snails also delayed their reproduction in the presence of crushed-snail cue alone. Snails did not exhibit any size or shell-shape differences over time when exposed to leech cue, although snail growth was depressed over time in the crushed-snail treatment. Both the cue from crushed snails and that from leeches caused a depression in the number of noticeable behaviours that snails displayed during mating-behaviour trials. In mating trials, control snails mated more frequently than snails reared in the crushed-snail treatment. This is the first study to show that chemical cues from predatory leeches affect life history strategies in physids via delayed production of viable egg masses.