Intra-specific competition drives variation in the fundamental and realized niches of the hermit crab, <I>Pagurus criniticornis</I>

Abstract

The difference between preferred and available resources within natural environments can be viewed as the disparity between fundamental and realized niches. We employ this concept to examine intra-specific sexual variation in the shell-size relationships of the hermit crab Pagurus criniticornis (Dana, 1852) from two treatments: (1) a natural unbiased population (i.e., realized niche); and (2) a group subjected to free-choice experiments that were able to select a preferred shell from a selection of high-quality shells (i.e., fundamental niche). Crab-shell relationships for both treatments suggest that males and females select shells proportional to their size. The relationships between crab size and preferred shell weight (i.e., fundamental niche) derived from free-choice experiments enabled the calculation of a shell adequacy index (SAI) for all individuals from both treatments. A strong interaction between SAI values across sex and treatment (♂Free choice = ♀Free choice = ♂Population > ♀Population) suggested that non-ovigerous females may experience a considerable niche disparity in their natural habitats while ovigerous females tend to inhabit shells with a greater prevalence of damaged shell apices. Results suggest a competitive hierarchy whereby males out- compete females for access to high-quality shells. Despite niche disparities, P. criniticornis typically occupied shells of good condition (low damage and encrustation), suggesting that resource limitation may not be as severe as that reported for other hermit crab populations. Our data provide insights into intra-specific competition that reflect the ability of individuals to win and hold high-quality resources within resource-limiting environments.