THE ROLE OF SHELL STRENGTH IN SELECTIVE FORAGING BY CRAYFISH FOR GASTROPOD PREY

1. Four gastropods common in Wisconsin lakes, Amnicola limosa, Gyraulu s parvus, Physella gyrina and Helisoma anceps, were exposed to predati on by three crayfish congeners, Orconectes rusticus, O. propinquus and O. virilis in the laboratory to determine prey preference. 2. There w ere no differences in prey choice among the crayfish congeners, but th ere were clear differences in electivity for the different snail prey. 3. Crayfish had higher electivities for the thin-shelled, piano-spira l pulmonate Gyraulus parvus, despite its lower abundance in samples, t han the thicker-shelled prosobranch Amnicola limosa. Electivity for an other piano-spiral pulmonate, Helisoma anceps, was low, evidently beca use of its relatively thick shell and larger size. Amnicola limosa and the thin-shelled pulmonate Physella gyrina (present at roughly the sa me relative abundance as Gyraulus parvus) were neither selected nor av oided. 4. Crayfish electivity appears to be a function of the resistan ce of shells to chipping by crayfish mandibles. Although different spe cies are preferred, this is similar to the previously described select ion of thin-shelled species by sunfish.