Avoidance of drilled gastropod shells by the hermit crab Pagurus longicarpus at Nahant, Massachusetts

Most hermit crabs depend on empty gastropod shells for shelter; competition for appropriate shells is often severe. This study determined whether shel ls that have been drilled by naticid gastropods are suitable for occupancy by the hermit crab Pagurus longicarpus. Differences in the characteristics of empty shells and those occupied by hermit crabs were assessed at two adj acent field sites in Nahant, Massachusetts. Drilling damage was far more fr equent in empty gastropod shells than in shells occupied by hermit crabs, s uggesting that individuals of P. longicarpus avoid drilled shells. They did not appear to avoid shells with other forms of damage. Laboratory experime nts confirmed that these hermit crabs preferentially chose intact shells ov er drilled shells, even when the intact shells offered were most suitable f or crabs half the weight of those tested. Final shell choices were generall y made within 1 h. The hermit crabs apparently discriminated between intact and drilled shells based on tactile cues, since crabs kept in the dark sho wed the same preference for intact shells. The hermit crabs strongly avoide d, to nearly the same extent, artificially drilled shells, naturally drille d shells, and shells with holes artificially drilled on the opposite side o f the shell from where they would normally be located. Possible selective f orces causing P. longicarpus to show such strong behavioral avoidance of dr illed shells include increased vulnerability of crabs in drilled shells to osmotic stress, predation, and eviction by conspecifics. (C) 2000 Elsevier Science B.V. All rights reserved.