Shell utilization by tropical hermit crabs under strongly contrasting levels of protection and accessibility

Shell utilization was studied in the tropical terrestrial hermit crab, Coenobitacompressus, in two Pacific coastal sites in Costa Rica with strongly contrasting levels of protection: San Miguel Biological Station, Cabo Blanco Absolute Reserve, which is closed to the public, and Playa Carmen, Malpais, which is one of Costa Rica’s most visited tourist beaches. Hermit crabs were collected at random in replicate samples during the day and at night. Samples of empty shells were collected for reference at each site. Shells were identified to species.  Crabs were weighed and an assessment was made ofhow well each crab fit its shell.

The assemblage of empty shells included 62 species; live hermit crabs were found to use 33 species.  Among live crabs, the diversity of shells was the same for Malpais and Cabo Blanco. Daytime samples were more diverse than nighttime samples at each site, while nighttime samples had larger crabs. Species composition of hermit crab shells differed between sites and from day to night. Malpais crabs tended to be too large for the size of their shells, while crabs from Cabo Blanco tended to fit their shells. Neritascabricosta shells were the most common species in both locations and at both sampling times, although this species comprises less than 1% of the available empty shells. There were substantial differences in species composition between live samples and empty shells, indicating a high degree of selectivity on the part of the crabs.