Safety factors of tropical versus temperate limpet shells: multiple selection pressures on a single structure.

Abstract

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. On average, shell strength and foot tenacity for tropical E Pacific limpets were twice those for the temperate limpets; average safety factors for the 2 groups were about equal. This comparatively narrow range of safety factors was due to a highly significant association of greater shell strengths with greater foot tenacities. Selection has presumably acted to closely link the mechanical performances of these two rather independent structures, the shell and the foot. 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 appears to have influenced the precision with which the shells of these species have become adapted to a single selection pressure. -from Author