Males of the hermit crab Pagurus filholi show precopulatory guarding behavior in which a male grasps the shell of a mature female with his left cheliped before copulation. Hermit crabs were most common in rocky intertidal areas with cobbles and boulders, while many guarding pairs were observed on fronds of brown algae such as Sargassum confusum in Hakodate Bay, Japan. We examined three hypotheses explaining why the guarding pairs were most common on algae; (1) aggregation place for mature males and females to find mates, (2) avoidance of male-male combat, and (3) avoidance of predators. If solitary males and females climbed up algae, then many guarding pairs were observed after pairing, but only guarding pairs climbed up the algae after removing all crabs. Experiments in aquariums showed that the disturbance rate for guarding pairs due to male-male competition was lower on the algae than in boulder and rocky flat areas, and few disturbances were observed by predatory crabs in all habitat types. These results suggest that the guarding males climb up the fronds of algae to sequester guarded females from rival males and avoid male-male combat. This behavior could be considered as a male counter tactic against indirect female choice mediated by sex pheromones in which females release sex pheromones while guarded, attracting many rival males and inducing male-male competition.
Males of the hermit crab Pagurus filholi perform assessment behavior toward females, as a preliminary step of precopulatory guarding, during the reproductive season. It is known that such behavior is elicited by female sex pheromones, but the compounds involved have never been characterized in this species. Several experiments were conducted to develop a reliable bioassay along with purification procedures to identify potential compounds with pheromonal activity in Pagurus filholi. We developed a bioassay protocol to assess pheromonal activity by using an empty shell with cotton containing either artificial seawater (control) or test water. We measured and compared the time duration of male assessment behavior toward each shell if the test water contained female sex pheromones. Ultra-filtering of seawater samples potentially containing pheromones showed that the compound was <1 kDa in molecular weight. Males showed precopulatory assessment behavior toward “female conditioned” water samples treated with open column purification and eluted with MeOH, suggesting that compounds triggering male behavior were low polar molecules. Molecules with pheromonal activity were not volatile after freeze drying, effective even after heating to 90 °C, and remained active in seawater at 12 °C even after 6 days from sample collection, which suggests a rather stable characteristic of the female sex pheromones of this species.
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