Predicting territorial behavior in symbiotic crabs using host characteristics: a comparative study and proposal of a model

Many studies on the social behavior of symbiont species have inferred that besides environmental factors (predation pressure, competition) host-related characteristics (i.e., host abundance and distribution, size, and morphology) have a major influence on the evolution of symbiont behavior. However, in many cases little information is available on host populations and few comparative studies have been conducted to test predictions. Herein, we compare the basic ecology of the sea anemone Phymactis clematis and the sea urchin Tetrapygus niger, each being host to a different porcellanid crab species, Allopetrolisthes spinifrons and Liopetrolisthes mitra, respectively. We first provide a description of host abundance and distribution and then test predictions concerning the behavioral ecology of the two crab symbionts to understand how and to what extent host characteristics affect the behavior of symbionts (i.e., host-resource monopolization). In the field, distances between host individuals (similar to15 cm) were similar for both sea anemones and sea urchins. However, the sea anemones were significantly less abundant than sea urchins. Furthermore, sea anemones represent a relatively small and morphologically simple microhabitat for A. spinifrons crabs, whereas sea urchins represent a relatively large and morphologically complex microhabitat for L. mitra crabs. The population distribution and size-specific association pattern of the two symbiotic crabs differed substantially, with A. spinifrons leading a solitary and L. mitra a gregarious life-style. Adults of A. spinifrons only cohabited with sexually immature individuals on their host, whereas adult L. mitra frequently occurred together with other adult crabs of both sexes on their host. These differences suggest strong territorial behavior for A. spinifrons but not for L. mitra, which is supported by results from other experimental studies on the intraspecific interactions of the two crab symbionts. The presence or absence of host-resource monopolization behaviors in these crab species appears to be an adaptive behavioral response depending on features of the corresponding host species - particularly host abundance, morphological complexity, and relative size. Based on these results, we developed a conceptual model to predict the probability for a symbiont to monopolize a host successfully, depending on host characteristics. The model predicts that the probability of host-resource monopolization behavior of a symbiont decreases with increasing (1) host abundance, (2) host complexity, and (3) relative host size. The present study is a first attempt to understand how host-resource monopolization behaviors have evolved and developed in symbiotic crustaceans and other marine invertebrates.