A forced association between the slippersnail Crepidula convexa and the hermit crab Pagurus longicarpus?—possible influence from a third party

The suspension-feeding slippersnail Crepidula convexa is commonly associated with hermit crabs (Pagurus longicarpus) living in periwinkle shells (Littorina littorea) at our study site in Nahant, MA, USA. In 15 field surveys conducted at Nahant in 2000, 2001 and 2003, we found that (1) more than 61.8% of individuals of C. convexa resided on shells occupied by hermit crabs, as opposed to the shells of live periwinkles, empty periwinkle shells or other solid substrates; (2) an average of 8.3% of hermit crabs carried at least one individual of C. convexa; and (3) 39.1– 75.0% of hermit crabs carrying C. convexa were carrying ‘‘large’’ individuals (snails with wet weight >10% of the weight of the periwinkle shells they occupied). However, it is unlikely that individuals of C. convexa seek out shells occupied by hermit crabs to colonize, and they showed no preference for empty periwinkle shells over other solid substrates in the laboratory. Moreover, in the laboratory the hermit crabs preferentially occupied intact shells bearing individuals of C. convexa only when the alternatives were shells that had been drilled by naticid snails. Thus, neither party preferentially associates with the other: rather, extensive predation by naticid snails on periwinkles at Nahant appears to limit the availability of suitable shells for the hermit crabs, forcing them to inhabit shells bearing ‘‘large’’ individuals of C. convexa. Individuals of C. convexa may benefit from this inadvertent association with hermit crabs: by facilitating snail dispersal, transport by hermit crabs should reduce the potential for inbreeding, an important consideration for a species that lacks free-living larvae in its life history. D 2004 Elsevier B.V. All rights reserved.