Testing the role of male-male competition in the evolution of sexual dimorphism: a comparison between two species of porcelain crabs

Abstract

Theory predicts marked sexual dimorphism in terms of body size and body structures used as weapons (e.g. chelipeds) in gonochoric species with intense male sexual competition for receptive females and reduced or no sexual dimorphism in species where competition among males is trivial. We tested this hypothesis using a pair of closely-related species of symbiotic porcelain crabs as a model. In one species that inhabits sea anemones solitarily, competition among males for receptive females is unimportant. In a second species that dwells as dense aggregations on sea urchins, male-male competition for sexual partners is recurrent. We expected considerable sexual dimorphism in body size and weaponry in the urchin-dwelling crab and reduced sexual dimorphism in the anemone-dwelling crab. In agreement with expectations, in the urchin-dwelling crab, male body size was, on average, larger than that of females and males invested considerably more to cheliped length than females. Also supporting theoretical considerations, in the anemone-dwelling crab, sexual dimorphism in terms of body size was not detected and differences between the sexes in investment to cheliped length were minor. Interestingly, chelipeds were more developed both in males and females of the anemone-dwelling crab than in the urchindwelling crab as a result of the importance of these structures for monopolization of their naturally scarce anemone hosts. Another difference between the studied species was the existence of two clearly distinguishable ontogenetic phases in males of the urchin-dwelling crab but not in males of the anemone-dwelling crab. Whether the two different male morphs display different male reproductive strategies in the urchin-dwelling crab remains to be addressed. Other conditions that might additionally explain the observed differences in sexual dimorphism (e.g. female mate choice) between the studied species remain to be explored.

Population distribution, host-switching, and chemical sensing in the symbiotic shrimp Lysmata pederseni: implications for its mating system in a changing reef seascape

“…Therefore, it is difficult to explain the existence of solitary hermaphrodites brooding eggs in the studied population if males and/or hermaphrodites were not switching among hosts in search of sexual partners. The existence of solitary hermaphrodites brooding embryos in L. pederseni agrees with reports for other symbiotic species with gonochoric sexual systems in which males move among hosts in search of receptive females (Baeza et al 2001;Baeza and Asorey 2012). Our results also contrast with those reported for other gonochoric and socially monogamous symbiotic species in which females do not brood embryos when found solitarily in/on their hosts (Baeza et al 2013).…”

Relative growth and morphological sexual maturity size of the freshwater crab Trichodactylus borellianus (Crustacea, Decapoda, Trichodactylidae) in the Middle Paraná River, Argentina

“…These results show that it is necessary to inquire about the sexual and mating system of this species in order to evaluate differences in measurements and trends found in this study (Baeza and Azorey 2012). On the other hand, as behavioral observations in the natural environment are difficult, it is recommended to raise crabs in the laboratory to evaluate mate selection and mating possibilities of different sizes.…”

Growth of the coconut crab Birgus latro estimated from mark-recapture using passive integrated transponder (PIT) tags

“…Females can become larger, or much larger, than males in some crustaceans (e.g. Bergström 1992, Stewart & Kennelly 2000, Béguer et al 2011, Ahamed & Ohtomi 2012, Baeza & Asorey 2012 and most fishes (Pauly 1994). The faster growth in females is considered to be a life history strategy, enabling females to attain a larger size and increase egg production (Berglund 1981).…”

“…Differences in growth patterns between sexes (sexual size dimorphism) are associated with not only the energetic costs but also sexual selection. Male sexual competition is considered an important evolutionary force driving sexual dimorphism in body size (Baeza & Asorey 2012). When male sexual competition for receptive females is intense, males are expected to attain larger body sizes than females, and this increasing investment in body size by males augments their resource holding potential and, thus, their capacity to access and defend receptive females from rival males (Baeza & Thiel 2007).…”