Colour Polymorphism in Coconut Crabs Is Under Relaxed Selection From Conspecifics?

Colour polymorphisms may result from frequency dependent selection, different lighting environments, genetic drift or heterozygote advantage, or may simply be under relaxed selection. Nocturnal coconut crabs have a sex and age independent red/blue colour polymorphism, live in the same habitat and have few predators. To determine whether coloration is involved in intraspecific communication, I performed field choice experiments using photographs of crabs of different colour and size to test whether crabs use these cues in decision making. Neither the colour nor size of crabs in photographs influenced coconut crabs' behaviour, but separate observations of live crabs showed that disputes over food were settled on the basis of the relative size of live conspecifics, not their coloration. This study adds to a small body of literature suggesting that the coloration of certain crustacea is under relaxed predator and conspecific selection and may have little adaptive significance.