Behavioral Ecology and Fitness Theory in a Tropical Hermit Crab

Certain aspects of the behavioral ecology of the tropical hermit crab, Calibanarius abidigitus, can be analyzed more fully with the use of fitness set theory. Building on the premise that an optimum shell weight/body weight ratio (termed "weight index") confers maximum fitness to individual crabs, two independent estimates of fitness, clutch size and percentage aggressive dominance, were plotted against a wide range of weight indexes. These measures were made for individuals of C. albidigitus inhabiting two environments (represented as two species of gastropod shells) and behavioral and reproductive fitness sets were constructed. The dissimilarities of the predicted optimum weight indexes for the two fitness sets, combined with the fact that suitable gastropod shell present a limiting resource, indicate a behavioral strategy by which larger shells than necessary for maximum reproductive fitness are utilized. This strategy, termed the optimal ratios strategy, has definite advantages to individual hermit crabs and has probably evolved in response to severe competition for shells.