Why small hermit crabs have large shells |
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Authors: | Tom M Spight |
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Institution: | (1) Department of Invertebrate Zoology, California Academy of Sciences, Golden Gate Park, 94118 San Francisco, California, USA |
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Abstract: | Summary Frequent shell exchanges among hermit crabs imply the enigmatic circumstance that large crabs frequently obtain large shells
from smaller crabs. This seeming anomaly is explored as a key to the shell resource system. It is hypothesized to reflect
how, where, and how often shells become available to the crabs. Shells become available infrequently, as snails die, and are
available to the crabs for only a brief time before they become inaccessible. The standing crop of empty shells is almost
always low and is irrelevant to rates of shell turnover in the crab population. Crabs are most likely to encounter shells
of the wrong size, and the chance of encountering a shell of the desired size decreases as a crab grows. Snails and crabs
are usually found on different portions of the shore; thus, crabs must make “foraging trips” for shells. Under this regime
of shell supply, a crab will get a suitable shell the fastest when it accepts any fresh shell that is larger than its initial
shell. It can then trade with other crabs to improve its shell fit. This behavior will make small crabs into a regular source
of large shells for large crabs, and a shell exchange ritual will be strongly favored because both participants will benefit.
Shells are an unusual resource because they are the object of both competitive and mutualistic interactions. This ambiguous
quality is revealed in the intraspecific and interspecific responses of crabs to each other and to shells. |
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