Defensive Behaviour in Intertidal Crabs

Despite a long roster of prey defences against predators now documented by biologists, any given species utilises only some of them, and we have few a priori predictions as to if, when or where they will be employed. In order to uncover general trends in the way in which crabs avoid predation attempts after they have been detected, we examined the antipredator behaviour of nine intertidal decapod species on three continents. We studied five different types of defensive behaviour and seven morphological and environmental factors which might affect such behaviours at different stages of the predatory sequence. In combination with other studies, we suggest that although intertidal crabs invest a great deal in crypticity to avoid detection, once discovered, they remain immobile, raise their chelae or flee but the extent to which these and other defences are exhibited differ markedly across species. We found that luminance contrast with the background, body size, chelae size and immediate habitat in which crabs were found affected antipredator behaviour whereas neither sex, nor chromatic contrast with the background were important. Our findings broaden understanding of defensive behaviour in several decapod species and highlight how species tailor the use of the same defences in different ways according to threat, morphology and local environment.