Shell Exchange in Hawaiian Hermit Crabs

Shell exchange behavior of intertidal Hawaiian hermit crabs was observed in the laboratory. Outcomes of 255 shell-related interactions were analyzed to test the predictive powers of two models of resource exchange . In the case of intraspecific interactions, the negotiations model (which predicts that exchanges should occur only when both crabs will gain in shell value) was superior to an aggression model of resource exchange . In the case of interspecific interactions, the negotiations model accurately predicted outcome of CalcinusCalcinus interactions, but poorly predicted outcome of Calcinus-Clibanarius interactions. HERMIT CRABSNEED GASTROPOD shells for protection, and one means of acquiring them is by exchange of shells with other crabs (Hazlett 1966). Shells can be a limiting resource for crabs (Hazlett 1981), and the behavior patterns associated with the exchange of this resource have often been viewed as fights (Hazlett 1967, 1972, Bach et al. 1976, Dowds and Elwood 1983, 1985). Indeed, an earlier paper on Hawaiian hermit crabs (Hazlett 1970) viewed shell fights as a mechanism of ecological competition. The idea that exchanges of shells occur primarily when both crabs gain in resource value has been termed negotiations (Hazlett 1978), and this type of behavioral interaction can result in ecological processes that differ qualitatively from competition (Hazlett 1987, Vandermeer et al. 1985). These ecological processes can occur both intraspecifically (Abrams 1982a) and interspecifically (Hazlett 1983). Because most of the results that support the negotiations model of resource exchange were obtained from observations of Caribbean species of hermit crabs, it is of interest to examine the resource exchange patterns of Pacific species to test the generality of the 1 Contribution no. 807 from the Hawai i Inst itute of Marin e Biology. Manuscript accepted 31 January 1990. 2 Department of Biology, University of Michigan , Ann Arbor, Michigan 48109. model. Before the development of the negotiations idea as an alternative model of resource exchange, an earlier study ofHawaiian species (Hazlett 1970) emphasized the aggressive/ competitive nature ofhennit crab interactions. MATERIALS AND METHODS Crabs used in the study were collected at a variety of locations around the island of Oahu, Hawaii. The crabs were placed in water tables at the Hawaii Institute of Marine Biology, Coconut Island, Oahu. The crabs were not experimentally treated in any way and were interacting with each other in an environment that was similar to a 0.5 x 1.5 m tide pool. Observations were made during daylight hours, using natural lighting, during the months of January through May of 1988. Crabs were periodically fed with algae and detritus on rocks placed in the water table. Species composition and the relative proportions of different species changed over the 5 months of observation as new specimens were continually added to the water table . Thus no quantitative statements about the relative amounts of interversus intraspecific interactions can be made. The observer sat near the water table and continually scanned the water table for social behaviors (the results of reproductive interactions are reported elsewhere [Hazlett 1989]). A shell-related interaction was recorded when-