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Behavior

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Behaviour

Hermit crab shell-switching patterns have been used as case studies in vacancy chain theory (Lewis and Rotjan 2009), a body of theory originally developed by sociologists to understand how vacancies involving discrete, reusable, and limited resources such as apartments, jobs, and cars are transferred within human populations. When an individual gets a new resource, the vacancy thus created can propagate down the socioeconomic order through a series of interdependent events, with the result that many individuals can benefit “downstream” through the acquisition of new physical resources or social positions. Rotjan et al. (2010) describe 2 types of hermit crab vacancy chains, synchronous and asynchronous. These 2 types of vacancy chains are both social and stand in direct contrast to solitary shell interactions involving a single crab and a single shell. Synchronous vacancy chains occur after several crabs adjacent to an available vacant shell have queued in decreasing size order; as soon as the largest crab switches into the vacant shell, a rapid series of sequential shell switches takes place. In contrast, in asynchronous vacancy chains individual crabs encountering a suitable vacant shell will switch and their discarded shells will later be discovered and occupied by other crabs. Thus, asynchronous vacancy chains do not involve social interactions or queue formation, and sequential shell switches take place over considerably longer time periods. In both cases, vacancy chains are terminated when the last shell discarded is of such low quality (too small or damaged) that all crabs reject it. In field observations, Rotjan et al. (2010) found that after investigation of a vacant shell that was too large, hermit crabs would remain near (within 50 cm) the shell rather than moving away immediately: crab waiting times ranged from several minutes to more than an hour, and up to 20 waiters at a time were present near the empty shell. Crabs exhibited waiting behavior at 55% (6 of 11) of stations with large vacant shells and at 100% (9 of 9) of stations with medium vacant shells. Waiters were observed at all stations where synchronous vacancy chains eventually occurred. (Rotjan et al. 2010 and references therein)

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© Shapiro, Leo

Source: EOL Rapid Response Team

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