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Journal of Experimental Marine Biology and Ecology 243 2000 169–184
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Effects of shell fit on the biology of the hermit crab Pagurus longicarpus Say
Jennifer E. Angel
Department of Biology , Tufts University, Medford, MA 02155, USA
Received 19 February 1999; received in revised form 5 July 1999; accepted 27 July 1999
Abstract
Most hermit crabs have a specialized lifestyle that requires them to occupy gastropod shells. The size of shells that hermit crabs inhabit relative to their body size affects their growth, survival
and fecundity. In this study, the effects of shell fit on individual growth rate, risk of predation, feeding rate, and activity level were examined in the laboratory for hermit crabs Pagurus
longicarpus collected from Nahant, MA, USA. Feeding rate and general activity level of hermit crabs confined to tightly fitting shells and hermit crabs occupying shells of preferred size were not
significantly different. Hermit crabs confined to tightly fitting shells grew at significantly slower rates, and were significantly more susceptible to predation by a common North Atlantic rock crab,
Cancer irroratus. While this study provides further evidence of the negative effects of tight shell fit on growth and survivorship of hermit crabs, a mechanistic explanation for the decreased growth
rate of hermit crabs in tightly fitting shells is still sought.
2000 Elsevier Science B.V. All rights
reserved.
Keywords : Behavior; Growth rate; Hermit crab; Predation risk; Shell adequacy
1. Introduction
The dependence of hermit crabs on the shells they inhabit has long been a subject of fascination to ecologists. Ideally, a hermit crab continually switches to larger shells as it
grows, thereby maintaining a shelter that adequately protects it from predators and provides enough space for a female hermit crab’s brood Childress, 1972; Hazlett,
1981. The ability of hermit crabs to seek out and occupy shells that provide optimal fit
Present address: c o Jan A. Pechenik, Department of Biology, Dana Laboratory, Tufts University, Medford, MA 02155, USA. Tel.:
11-617-627-3199; fax: 11-617-627-3805. E-mail address
: jangelstanfordalumni.org J.E. Angel 0022-0981 00 – see front matter
2000 Elsevier Science B.V. All rights reserved.
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.E. Angel J. Exp. Mar. Biol. Ecol. 243 2000 169 –184
is well documented Scully, 1979; McClintock, 1985; Wada et al., 1997. In reality, there are rarely enough shells of the right sizes for all the hermit crabs in a population
e.g., Vance, 1972a; Scully, 1979. Such shell limitation forces many hermit crabs to occupy shells that are the wrong size for them; often the shells are too small.
Shells of inadequate size can increase the risk of both desiccation and predation. Mortality from desiccation at low tide increased for pagurid hermit crabs in shells that
were too small for them Taylor, 1981. The hermit crab Pagurus granosimanus was significantly more vulnerable to predation by the brachyuran crab Cancer gracilis when
the hermit crab was unable to retreat completely into its shell Vance, 1972b. Whether inadequate shell fit increases risk of predation in other pagurid species has not been
examined.
When confined to shells that were too small for them, the temperate hermit crabs Pagurus bernhardus Markham, 1968, Pagurus pollicaris Fotheringham, 1976a,
Pagurus longicarpus Fotheringham, 1976a; Blackstone, 1985, and Clibanarius vittatus Fotheringham, 1976b grew more slowly. Nothing is known of the mechanisms
regulating the reduced growth rate observed in hermit crabs with tight shell fit. Possible mechanisms that could mediate growth rate declines include increased metabolic
activity, decreased feeding activity, or decreased food assimilation efficiency.
While hunting or scavenging food, lobsters and true crabs must often leave shelter and expose themselves to potential predators. In contrast, the hermit crab, a scavenger and
filter feeder, carries shelter with it at all times Hazlett, 1981. However, a hermit crab’s chances of avoiding predation are only as good as its shell protection. This mobile
protection is probably less effective if a hermit crab has outgrown its shell, and is unable to withdraw its body fully inside. Thus, for a hermit crab in a shell-limited environment,
maximal growth best achieved by maximal net energy intake can be at odds with the need for protection. Because there is strong evidence of a behavioral component to the
hermit crab’s assessment and improvement of shell fit Elwood and Neil, 1992; Hazlett, 1996; Wada et al., 1997, it is likely that a quantifiable behavioral change contributes to
any changes in growth rate the hermit crab experiences.
In the summer of 1997, approximately a quarter of the individuals of P . longicarpus at
Nahant occupied tightly fitting shells personal observation, making research of effects of tight shell fit relevant. In this study, it was first determined whether growth rate
decreased for hermit crabs confined to tightly fitting shells. Then, ultimate and proximate mechanisms that might influence growth rate were examined.
2. Methods
