The behavior of symbiotic organisms is strongly affected by host ecology. It has been hypothesized that symbionts that inhabit densely aggregated hosts show little territorial behavior and move frequently between host individuals. Herein we tested this hypothesis for the porcellanid
crab Liopetrolisthes mitra that occurs in aggregations of several crabs on sea urchins Tetrapygus niger. The hosts of these crabs are often in direct physical contact to each other and the structure of symbiont aggregations suggests that these are unstable, possibly due to frequent
movements of crabs. Herein, we examined the mobility of crabs both in laboratory and field experiments. Many crabs that were placed at natural densities on one sea urchin moved to crab-free sea urchins within 12 h over night. Both sexes and all sizes of crabs moved between different sea urchin
individuals, but adult males moved more frequently than adult females. However, when placed on a sea urchin with a receptive female, male crabs reduced their movements. In the field, the first crabs appeared on experimentally crab-free sea urchins within 1 d, and within 3 d after the start
of the experiment these sea urchins had been fully recolonized by crabs. Crabs on experimental sea urchins were significantly smaller than on control sea urchins, but no significant differences in sex-ratio were found after 3 or 5 d. Our results suggest that the mating system of L. mitra
is similar to that of many free-living crabs in which males roam in search of receptive females. We hypothesize that this behavioral similarity is due to the fact that individual sea urchin hosts are difficult to guard by crustacean associates due to their structural complexity and their aggregation
pattern. A comparison with published reports on other urchin-dwelling crabs suggests that host abundance and distribution exert a strong effect on symbiont movements and host-use patterns.
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