Abstract

Signal crayfish fed on two species of thin-shelled freshwater snails, Lymnaea peregra and L. stagnalis, using the chelate walking legs, maxillipeds, and mandibles; the chelipeds were not used to break the snail shells. Individual crayfish varying from 16-61 mm carapace length were offered a regularly replenished choice of snails within the range 3-35 mm shell length. All the smaller crayfish (16-44 mm) chose to eat some size classes of snails significantly more frequently than both smaller and larger classes (P<0.001 in all but one case). Preferred size class was positively correlated to crayfish size. When choice was progressively reduced by preferred sizes not being replaced, smaller crayfish eventually ate both smaller and larger snails. Two larger crayfish (55 & 61 mm) showed no preferences and ate all snail sizes offered. We suggest that the thin shells of these snails offer little defence against crayfish predation and that, when size selection occurs, mechanical and biological factors other than shell strength deter crayfish from selecting the largest snails.

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