SAFETY FACTORS OF TROPICAL VERSUS TEMPERATE LIMPET SHELLS: MULTIPLE SELECTION PRESSURES ON A SINGLE STRUCTURE.

A comparison of the safety factors of tropical and temperate limpet shells in the eastern Pacific yielded two results of significance. A safety factor was defined as shell strength/maximum tenacity, where maximum tenacity (force required to detach foot) determines the maximum prying force that a crab or bird predator can exert on the shell. 1) On average, shell strength and foot tenacity for the tropical limpets were twice those for the temperate limpets. In contrast, the average safety factors for the two groups were approximately equal. This comparatively narrow range of safety factors was due to a highly significant association of greater shell strengths with greater foot tenacities. The implication of this result is that selection has acted to closely link the mechanical performances of these two rather independent structures, the shell and the foot. 2) The presence of an additional class of predators which feed on the tropical limpets was reflected in the safety factors of their shells. Whereas the shells of both tropical and temperate limpets are exposed to predator-induced prying forces, the shells of the tropical group are also exposed to lateral crushing forces generated by fish predators. This additional selection pressure was associated with several deviations from a regression of safety factor versus variability in shell strength which had been calculated previously for the temperate limpets. As predicted, the magnitudes of these deviations were correlated with the degree of exposure to this additional selection pressure. Hence, the presence of more than one selection pressure appears to have influenced the precision with which the shells of these species have become adapted to a single selection pressure. The use of safety factor analysis provides a very useful methodology for identifying additional selection pressures or adaptive constraints on biological structures.