Intraspecific gastropod shell strength variation among north temperate lakes

Defensive morphological traits may vary intraspecifically. Freshwater snail shells are conspicuous defensive structures. In north-central Wisconsin, w e investigate whether among-lake differences in shell strength relate to wa ter chemistry or predator abundance and whether shell strength is inducible owing to predation risk from crayfish. Amnicola limosa shells were stronge r in lakes with abundant crayfish predators. An experiment and a general un derstanding of prosobranch evolution suggest that this may result from sele ction rather than induction. The experiment indicated a weakening of shells of slow-growing A. limosa in the presence of crayfish. This may have resul ted from resource depression caused by a strong behavioral response that re duced feeding time. Physa skinneri shell strength was correlated with lake calcium concentrations, and a weak trend with calcium was apparent for Heli soma anceps. Decreased P. skinneri shell strength in low-calcium lakes may result from retention of scarce calcium by the body at the expense of alloc ation to the shell. Populations of H. anceps differed in rates of shell str ength increase with body size, suggesting that rates of shell strength accu mulation with ontogeny vary among populations. Shell strength increased mor e rapidly in lakes with abundant predators than in lakes with few predators .