Ontogenetic changes in shell preferences and resource partitioning by the hermit crabs Pagurus hirsutiusculus and P. granosimanus

Ontogenetic shifts in resource use can serve to reduce mortality and intraspecific competition, but can also increase a population's likelihood of extinction. We examined the ontogeny of shell type (i.e. snail species) preferences as well as shell type usage in the field in 2 sympatric species of intertidal hermit crabs, Pagurus hirsutiusculus and P. granosimanus, on Vancouver Island, Canada. An experiment examining shell type preferences by 4 hermit crab size classes, spanning a broad range of juvenile and adult sizes, found that both species have well-defined shell type preferences throughout benthic life; shell type occupancy at 7 field sites was largely consistent with shell type preferences. In addition, shell type preferences in both species changed during ontogeny, in terms of the preferred shell types but also in the diversity of shell types that were used, which decreased with increasing crab size. Thus, as a cohort of hermit crabs grows from newly settled juveniles to full-sized adults, the availability of suitable shells will depend on different snail species; shell availability at different stages of ontogeny would thus be controlled by independent mechanisms. In addition, similar-sized hermit crabs of the 2 species mostly ignored the shell types used by the other species, a resource partitioning that would facilitate coexistence. If shell availability limits the local abundance of these hermit crab species, as previously suggested, our findings indicate they would be limited by different resources (i.e. shells produced by different snail species). The lack of correlation between the abundance estimates of the 2 species at our 7 field sites supports the hypothesis that the local abundance of P. hirsutiusculus and P. granosimanus is controlled by independent factors.