Fiddler crab claws work as a deflection antipredator defence

Conspicuous coloration in body parts that can be autotomized, diverting predator attacks from vital to non-vital regions, is called deflection. Fiddler crabs typically have a cryptic or conspicuous carapace (vital area), while the claw (non-vital) is often conspicuous and used for social communication. Here, we tested whether the conspicuous claws of fiddler crabs divert predator attacks away from their carapaces, enhancing survival. To do this, we used a robotic crab model that replicated the colours and reproductive waving display of the two-toned fiddler crab, Gelasimus vomeris. Models were placed in the field to be attacked by Australian brush-turkeys, Alectura lathami. We analysed whether the first attack was directed at the claw or the carapace with differently coloured models. Our results show that robot crab models with conspicuous claws drew half of the attacks to the claw, whereas models with non-conspicuous claws were attacked predominately on the carapace. This suggests that the claws of the fiddler crabs effectively attract attacks away from the carapace, functioning as a deflection mechanism. This is the first study demonstrating a claw-deflection strategy in crustaceans, indicating that the claw not only plays a role in intraspecific signalling but also mitigates associated predation risks.