What makes a good home for hermits? Assessing gastropod shell density and relative strength

ABSTRACT

The survival and reproductive success of hermit crabs is intrinsically linked to the quality of their domicile shells. Because damaged or eroded shells can result in greater predation, evaluating shell structure may aid our understanding of population dynamics. We assessed the structural attributes of Cerithium atratum shells through assessments of (a) density using a novel approach involving computed tomography and (b) tolerance to compressive force. Our goal was to investigate factors that may influence decision making in hermit crabs, specifically those that balance the degree of protection afforded by a shell (i.e. density and strength) with the energetic costs of carrying such resources. We compared the density and relative strength (i.e. using compression tests) of shells inhabited by live gastropods, hermit crabs (Pagurus criniticornis) and those found empty in the environment. Results failed to show any relationship between density and shell size, but there was a notable effect of shell density among treatment groups (gastropod/empty/hermit crab). There was also a predictable effect of shell size on maximum compressive force, which was consistent among occupants. Our results suggest that hermit crabs integrate multiple sources of information, selecting homes that while less dense (i.e. reducing the energy costs of carrying these resources), still offer sufficient resistance to compressive forces (e.g. such as those inflicted by shell-breaking predators). Lastly, we show that shell size generally reflects shell strength, thus explaining the motivation of hermit crabs to search for and indeed fight over the larger homes.

RESPONSIBLE EDITOR:

• Ingo Wehrtmann

KEYWORDS:

• Compression assay
• computed tomography
• decision-making
• shell density
• shell strength
• subjective resource value

Acknowledgements

We would like to thank Marcelo Favero from the Serviço de Imagenologia of Hospital Universitário and the technical team of the Departamento de Engenharia Metalúrgica e de Materiais of the Escola Politécnica, both departments of Universidade de São Paulo, whose collaboration was essential for the research. We would also like to thank the two anonymous reviewers for their suggestions and comments to our manuscript.

Disclosure statement

No potential conflict of interest was reported by the authors.

ORCID

Daniel Gorman http://orcid.org/0000-0002-6768-2638

Additional information

Funding

This work was supported by the Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) under grants to MNR (no. 2010/20550-7) and DG (no. 2013/07576-5).