The exchange of shells between individual hermit crabs usually results in an increase in the shell fit of both crabs. It appears that information about the internal volume of the other crab's shell is utilized by the non-initiating crab in determining whether to exchange or not. Experiments
with the diogenid Calcinus tibicen showed that the negotiation model of resource exchange predicts the outcome of shell exchanges in that species when volume is the measure of shell size utilized. Rearing crabs in environments which either increased or decreased the frequency of shell
exchange experience indicated that crabs learn to utilize the information about an opponent's shell. That is, crabs without shell exchange experience do not behave according to the predictions of the negotiations model of exchange. It is hypothesized that the population level effects of interactions
between crab species which utilize shells in common will vary from competitive to mutualistic. The category of ecological process involved will depend upon the capture efficiency of the crabs. This in turn will be related to the relative size of the snail populations. The fact that crabs learn
to negotiate can speed up the dynamic interactions between crab species.
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