Postmortem changes in strength of gastropod shells: evolutionary implications for hermit crabs, snails, and their mutual predators

Abstract

Calliostoma ligatumshells inhabited by hermit crabs were weaker than shells inhabited by snails collected at the same locality. When shells ofC. ligatumwere loaded repetitively to 80% of their predicted failure load, hermit crab-inhabited shells showed an immediate drop in shell strength followed by a progressive further loss of shell strength over the next 18 days. SnailinhabitedC. ligatumshells exhibited a decrease in strength after 9 days, but returned to initial values within 18 days of loading.Hermit crabs thus bear shells significantly weaker than they were when borne by the gastropods that produced them. Reported similarities in vulnerability of gastropods and hermit crabs to shell-crushing predators may be artifacts of the metric (critical size) used to compare vulnerabilities. Hermit crabs probably were a significant factor in the diversification of durophagous predators in the Mesozoic, supplying a prey base identical in size and shape to gastropods but with significantly lower resistance to crushing. The unpredictability of strength in hermit-crab inhabited shells may maintain the apparently inefficient indiscriminate attacks common among durophagous predators.