The hermit crab Dardanus pedunculatus and the sea anemone Calliactis tricolor engage in a mutualistic relationship in the tropical reef ecosystem of Mo’orea, French Polynesia. This mutualism is shaped by the pressures on D. pedunculatus individuals to acquire actively and compete for C. tricolor in the wild. D. pedunculatus exhibits a consistent trend in behavior when competing for and placing C. tricolor on its gastropod shell. Since C. tricolor seems to be a valuable resource to D. pedunculatus, this species engages in intraspecific competition. Experiments were designed to examine the behaviors of D. pedunculatus under laboratory conditions when competing for and placing their symbiont. D. pedunculatus individuals steal C. tricolor from one another, with larger D. pedunculatus dominating in these encounters. In contests between two D. pedunculatus individuals, competition is also based on size dominance. The size of the cheliped of the left-handed D. pedunculatus is the most strongly correlated to winning a fight for C. tricolor. The placement of C. tricolor follows a distinctive symmetrical pattern on the shell of D. pedunculatus. By equally distributing C. tricolor, D. pedunculatus could possibly be protecting itself from attack coming at any angle. These behaviors stem from the benefits that both organisms derive from their union. Understanding the behavioral patterns linked to this symbiotic relationship helps to examine the broader web of interactions that build the complexity of reef ecosystems.