Hair-trigger autotomy in porcelain crabs is a highly effective escape strategy

The benefits of autotomy, the voluntary shedding of limbs, have been adequately demonstrated in vertebrates but are poorly studied in invertebrates. We provide strong experimental evidence for an antipredatory benefit to autotomy in two porcelain crabs (Petrolisthes cinctipes and P manimaculis). Since the outcome of autotomy depends critically on the species of predator and prey involved, we first surveyed field populations of porcelain crabs to identify ecologically relevant predators to use in subsequent experiments. We then examined the escape tactics of the porcelain crabs in response to the four potential predators we identified, all larger predatory crabs. Cheliped autotomy was induced by three of the predator species (Cancer antenna?,ius, Hemigrapsus nudus, Pachygrapsus crassipes); the fourth did not attack porcelain crabs. Autotomy occurred in response to 34% of all attacks, and in 67% of attacks in which the prey was held at some point by only the cheliped. Autotomy was a highly effective escape mechanism against these predators; 58 of 59 crabs that autotomized escaped, usually while the predator stopped pursuit to cat the shed cheliped. Reliance on autotomy as a primary mechanism for escaping predators may be particularly common in small crabs that cannot adequately defend themselves by other means and in suspension-feeding crabs that do not need their chelipeds for feeding.