Factors selecting for avoidance of drilled shells by the hermit crab Pagurus longicarpus

Periwinkle shells (Littorina littorea) that have been drilled by moon snails are abundant at Nahant, MA, and yet hermit crabs ( Pagurus longicarpus) are rarely found living in them. In the laboratory, hermit crabs will occupy drilled shells only if no other options are provided to them; they will, in fact, choose intact shells that are too small for them over drilled shells of appropriate size. This paper documents the selective forces that might account for this marked avoidance of drilled shells by hermit crabs. We show that drilled shells increase the vulnerability of the hermit crabs to decapod predators (e,g,, the green crab, Carcinus maenas), in part by allowing predators to enlarge the drill hole and also by reducing the force needed for predators to crack the shells open. Living in drilled shells also made the hermit crabs in our study more vulnerable to eviction by conspecifics and more vulnerable to low-salinity stress. Thus, moon snail predation on periwinkles, rather than supplying the Nahant hermit crab population with suitable empty snail shells, actually serves to destroy a potential resource for the hermit crabs and apparently to heighten competition among hermit crabs for intact shells. (C) 2001 Elsevier Science B,V. All rights reserved.