Journal of Experimental Marine Biology and Ecology Ž . 262 2001 75–89 www.elsevier.nlrlocaterjembe Factors selecting for avoidance of drilled shells by the hermit crab Pagurus longicarpus Jan A. Pechenik a, ) , Jennifer Hsieh a , Shannon Owara b , Peter Wong b , Durwood Marshall c , Steven Untersee a , Wei Li a a Biology Department, Tufts UniÕersity, Medford, MA, 02155, USA b Mechanical Engineering, Tufts UniÕersity, Medford, MA, 02155, USA c Statistical and Research Computing Specialist, Tufts Academic Technology, Tufts UniÕersity, Medford, MA, 02155, USA Received 8 February 2001; received in revised form 21 March 2001; accepted 28 April 2001 Abstract Ž . Periwinkle shells Littorina littorea that have been drilled by moon snails are abundant at Ž . 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 Ž . 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. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Hermit crabs; Pagurus; Shell selection behavior 1. Introduction Hermit crabs lack a protective carapace; the members of most species gain protection Ž by occupying gastropod shells after the original inhabitant has died or been killed e.g., ) Corresponding author. Tel.: q 1-617-627-3199; fax: q 1-617-627-3805. Ž . E-mail address: jan.pechenik@tufts.edu J.A. Pechenik . 0022-0981r01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. Ž . PII: S0022-0981 01 00284-2