Interspecific signalling competition between two hood-building fiddler crab species, Uca latimanus and U. musica musica

Competition between species can include signal competition in addition to competition for space and resources. Fiddler crabs (Uca spp.) are an excellent system for examining visual signal competition given their use of visual displays and burrow structures such as sand hoods. Where fiddler crab species overlap in habitat, females may face challenges as signal receivers to avoid heterospecifics and simultaneously discriminate among conspecific males. We investigated the consequences of competition between two species, U. latimanus and U. musica, which both build hoods and intermingle in the same habitat. Males of U. latimanus were larger than U. musica and had claws that were relatively shorter but potentially more powerful, suggesting that U. latimanus may be a superior competitor in direct combat. The hoods of the two species differed significantly in average size and shape, and in areas where they had heterospecific neighbours, U. musica males built hoods that were more distinct from heterospecific hoods, whereas U. latimanus built hoods that were more similar to heterospecific hoods. Wandering male U. musica were less likely to approach heterospecific than conspecific males, whereas U. latimanus were not. Wandering females of both species approached significantly more conspecific than heterospecific males, although some U. musica females entered heterospecific burrows. Uca latimanus females were significantly more attracted to conspecifics with hoods than without hoods; and although U. musica females showed the same tendency, the trend was only marginally significant. The high density of heterospecific males may reduce the ability of female U. musica to discriminate among conspecific males.