The role of carapace colour in species recognition in a fiddler crab

Species recognition can be interpreted as the identification of sexual compatibility among individuals, which is related to the assessment of partner quality. This compatibility can be assessed using visual signals of conspecifics such as body coloration. Fiddler crabs constitute a group with a global distribution, comprising over 100 species with diverse colours. They perceive these body colours and use them for intersexual communication, primarily through signalling of the hypertrophied claw of males. However, few species have been investigated regarding the role of body coloration as a species-specific signal, and it remains unknown whether other body regions, such as the carapace, convey information about individual quality or species identity. Hence, this study aimed to examine whether carapace coloration of two sympatric fiddler crab species (Leptuca leptodactyla and Leptuca cumulanta) affects species recognition. To do this, we used individual females of L. leptodactyla that were subjected to a binomial choice of males with claws and carapaces reflecting different colours, simulating conspecifics (L. leptodactyla) or heterospecifics (L. cumulanta). We observed that females were unable to distinguish conspecifics from heterospecifics using only claw colour. On the other hand, we discovered that the white carapace coloration of L. leptodactyla was crucial for females to identify conspecific males. This is the first study to demonstrate that carapace coloration of male fiddler crabs functions in species recognition, reinforcing the idea that body traits beyond sexual ornaments can influence animal communication, affecting species identity and/or mate selection.