Shell selection and utilization in a terrestrial hermit crab, Coenobita compressus (H. Milne Edwards)

Shell utilization and shell selection studied in the tropical terrestrial hermit crab, Coenobita compressus (H. Milne Edwards). Three major shell related variables are examined; shell size, shell species, and shell condition. Nerita scabricosta (Lamarck) is the most commonly occupied shell, and it is also preferred over other shell species. Coenobita in the field are usually found in smaller than the preferred size of shell, and the difference between utilized and preferred shells is most pronounced in relatively small individuals. Shell size preference differs significantly between similarly sized crabs collected at different sites. There is also a preference for shells which have previously been used by other Coenobita individuals. This preference is due to shell modification by Coenobita, which increases the effective size of the shell. It is argued that the fitness of most sexually mature individuals is probably unaffected by the supply of new shells, since these are too small to be utilized. It is possible that the interaction of adult Coenobita for shells may be better described as mutualism than as competition. This illustrates the fact that a significant difference between utilized and preferred resources need not imply competition for those resources. Results indicate that the nature of intraspecific competition and population regulation in at least this terrestrial hermit crab is significantly different from these processes in marine hermit crabs.