The fiddler crab Uca mjoebergi uses ultraviolet cues in mate choice but not aggressive interactions

The perception of ultraviolet light and its role in communication has received a great deal of attention in recent years. Although ultraviolet light is no different to other wavelengths in many respects, its role as a signal does deserve special consideration, if only because it is frequently overlooked by human observers who are unable to see it. Studies on the role of ultraviolet cues in inter- and intrasexual interactions are still rare, and generally focused on vertebrates with relatively complex visual systems. Here we show for the first time that crustaceans, with a relatively simple, probably dichromatic, visual system, also use ultraviolet cues in mate choice. We found that the enlarged claws of male Uca mjoebergi reflect ultraviolet wavelengths of light. Mate-searching females preferred ultraviolet-reflecting males over those whose reflectance was blocked with sunscreen. Conspecific males showed no such preference when deciding which rival to attack, suggesting that ultraviolet cues are not involved in signalling competitive ability.