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 urchin-dwelling 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. © 2011 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, 105, 548–558.