Less choosy or different preference? Impact of hypoxia on hermit crab shell assessment and selection.

Hermit crabs that rely on gastropod shells for protection have to assess a number of shell features, each of which can bring different, and sometimes conflicting, advantages to their bearer. We examined how environmentally induced stress, in the form of hypoxia, can alter the relative benefits of different shell features and result in the selection of different shells by the hermit crab Pagurus bernhardus. Hermit crabs under hypoxic conditions spent significantly less time investigating shells before entering them. Despite this apparently superficial assessment, they chose shells that were well suited to hypoxic conditions. After 24 h, they were found in shells that were significantly lighter than those chosen by hermit crabs under normal oxygen. This shift in preference was achieved at the expense of the internal spaciousness of the shell, which may have deleterious implications for predation and reproduction. This cost was more evident for larger hermit crabs since there was no relationship between hermit crab size and shell characteristics for hermit crabs in hypoxia. Under oxygen stress, hermit crabs therefore alter their shell preference in favour of lighter, and thus smaller, shells, probably reflecting a greater concern for energy minimization than protection from predation or reproduction. Copyright 1998 The Association for the Study of Animal Behaviour.