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MOVEMENTS OF THE SYMBIOTIC CRAB LIOPETROLISTHES MITRA BETWEEN ITS HOST SEA URCHIN TETRAPYGUS NIGER

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 I 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. Many organisms form aggregations on resources that are patchily distributed. The composition of these aggregations may be highly organized or unstructured, depending on the species under consideration. Aggregations of many marine invertebrates in resource patches are characterized by a high and rapid turnover (e.g., Taylor, 1998). Fluctuations within aggregations may be particularly strong when resource patches are abundant and costs for moving between patches are low. For example Taylor (1998) concluded that amphipods crawled directly from one alga onto another suggesting that costs and risks of moving between patches are low. In particular, when only specific requirements (e.g., feeding) can be fulfilled in one patch, organisms may switch between patches that allow them to fulfill different essential requirements. Thus, resource characteristics (function, distribution, size, and longevity) have a strong influence on resource utilization behavior and consequently on the intraspecific aggregation pattern of the organisms that utilize these resources. Organisms with an obligate symbiotic life style are restricted in their distribution and abundance by the availability of suitable hosts. Factors such as host morphology, size, and distribution patterns are likely to affect symbiont abundance per host individual (intensity), percentage of hosts occupied (prevalence), and the partner fidelity of these associations and thereby intraspecific interactions among symbiotic organisms (Tsuchiya and Yonaha, 1992). Crustaceans live on/in a variety of invertebrate organisms of highly di