The social behaviour of symbiotic organisms is influenced by the density and distribution pattern of hosts. Herein we examined the host-use behaviour of the anemone-dwelling crab Allopetrolisthes spinifrons (Porcellanidae) in which adults usually live as solitary individuals on their hosts. Adults of this crab use two different sea anemone species, one intermediate-sized species that can be found at relatively high densities in the intertidal zone and another large one that occurs at significantly lower densities in the shallow subtidal zone along exposed rocky shores of the south-east Pacific. Mark–recapture experiments demonstrated that crabs in subtidal waters (low abundance of hosts) remained for long time periods on the same hosts while crabs in the intertidal environment (high abundance of hosts) frequently changed hosts. There were no differences in host fidelity between male and female crabs. In an immigration experiment in the intertidal zone, significantly more juveniles immigrated than had been present originally, indicating that host-use behaviour is age dependent. The sex ratio between resident and immigrated individuals did not change. In an additional experiment hosts were planted at two different densities (high and low) both in the shallow subtidal and the intertidal zone to test whether host fidelity of crabs depends on distance between hosts. At the subtidal site, about half the crabs remained on their sea anemones for 14 days while at the intertidal site most crabs disappeared within 1 day in both density treatments. At both sites crabs were seen changing hosts in high density treatments, but no such events could be witnessed at low densities of hosts. Although the results are not fully conclusive, they suggest that host movements are affected by host densities. This relationship may be mediated by host (anemone species, size, distance) and site-specific (predation pressure, exposure time) factors. In general, the present study indicates that host characteristics influence host fidelity of symbiotic organisms and thereby, their social behaviour.
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