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, we investigate whether among-lake differences in shell strength relate to water chemistry or predator abundance and whether shell strength is inducible owing to predation risk from crayfish. Amnicola limosa shells were stronger in lakes with abundant crayfish predators. An experiment and a general understanding of prosobranch evolution suggest that this may result from selection rather than induction. The experiment indicated a weakening of shells of slow-growing A. limosa in the presence of crayfish. This may have resulted from resource depression caused by a strong behavioral response that reduced feeding time. Physa skinneri shell strength was correlated with lake calcium concentrations, and a weak trend with calcium was apparent for Helisoma anceps. Decreased P. skinneri shell strength in low-calcium lakes may result from retention of scarce calcium by the body at the expense of allocation to the shell. Populations of H. anceps differed in rates of shell strength increase with body size, suggesting that rates of shell strength accumulation with ontogeny vary among populations. Shell strength increased more rapidly in lakes with abundant predators than in lakes with few predators.