The effect of past experience on the size of shells selected by hermit crabs

The relationship between crab size and shell size selected, in a free-choice situation, was tested for crabs maintained under several conditions . Individuals of Calcinus tibicen that were held and presented with an excess of shells for 7 days prior to testing selected smaller shells than control crabs (either those maintained for 7 days with no excess shells available or those experiencing natural field conditions) . This shift occurred whether the excess shell type was the same or different from the shell type tested after experience. In a similar set of experiments, individuals of C . laevimanus were held for 31 days and although the trends in shell choice were similar to those for C. tibicen, the differences were not significant . Hermit crabs often use an empty gastropod snail shell to protect their soft abdomen . The size and species of shells occupied has numerous ramifications for the biology of hermit crabs, including growth, survivorship and reproductive potential (Hazlett 1981, 1989 ; Borjesson & Szelistowski 1989; Wilber 1989). Because empty gastropod shells are not usually abundant in the intertidal zone (Hazlett 1981), hermit crabs are exposed to varying degrees of resource availability in terms of shell choice. Occupying a shell that is too large could negatively effect growth and fecundity of the crab and its ability to protect itself from predators (Vance 1972 ; Bertness 1981) . However, given the low probability of finding the right size of empty shell, following an increase in size via growth by a crab, crabs may accept or even select a shell that is slightly too large if it becomes available (Childress 1972) . The discrepancy in shell size selected should he a function of the crab's recent experience with resource availability . Therefore, crabs should modify their preference for shell size based on the availability of empty shells . In the following experiments, I address the question of how short-term exposure to differing shell resource levels influences the size of shell selected by hermit crabs. In particular, do crabs select smaller shells if environmental cues indicate an excess of this resource? And does this environmental influence work across shell species? METHODS I conducted observations on the behaviour of Calcinus laevimanus at Coconut Island, Oahu, Hawaii, in 1988 and on Calcinus tibicen (shipped from the Florida Keys) at Ann Arbor, Michigan, in 1990 and 1991 . The general procedure for both species for each replicate was to provide 30 individuals free access to 200 shells of a given species for 24 h (see Wilber 1990) after the crabs had been in one of four prior conditions . These conditions were: (1) recently collected in the field, (2) held in the laboratory in an aquarium with no excess shells available for either 7 (C. tibicen) or 31 (C . laevimanus) days, (3) held in the laboratory in an aquarium with excess shells of the shell species to be tested for 7 or 31 days, or (4) held in the laboratory in an aquarium with excess shells of a different species of gastropod for 7 or 31 days . Calcinus laevimanus was tested with Trochus intexus and Turbo sanwichensis; C. tibicen was tested with Nerita sp. and Cerithium sp. shells . After crabs had been in the free-access situation for 24 h, I removed them and measured several parameters . I measured cephalothorax length and allowed the shells to dry for 2-4 days, at which time shell weight and internal volume were determined . Shell volume was determined by weighing the shell before and after fine dry sand was added to the dry shell . Knowing the density of the sand I was able to calculate the internal volume of the shell . The data were analysed using an analysis of covariance (ANCOVA). A significant interaction between 0003-3472/92/070203 +03 $03 .00/0 © 1992 The Association for the Study of Animal Behaviour 203