Shell abandonment by a hermit crab is an effective antipredator behavior against a portunid crab

When hermit crabs recognize the risk of predation, they generally hide inside their shells or flee from the area, and little is known about their escape behaviors after being captured by a predator. We discovered that the hermit crab Pagurus filholi abandons its shell and attempts to flee when captured by the predatory crab Charybdis japonica, and we examined the effectiveness of this behavior in laboratory experiments. We fed captive C. japonica with P. filholi occupying gastropod shells. When the C. japonica could not break the shell even a little, it gave up and the P. filholi neither abandoned its shell nor was eaten. However, when the predator succeeded in partially breaking the shell, the hermit crab often abandoned the shell, and when it did so, it often survived. Once the predator had completely broken the shell open (implying that the shell has been broken to the extent that it no longer has any structural integrity), all hermit crabs that remained back in the shell (22/22) were consumed, and those that abandoned the shell were also often consumed (3/5). Therefore, hermit crabs do not need to abandon their shells if a predator is unable to break the shell even slightly, but if a predator begins to break the shell, they have a better chance of survival if they abandon their shells at an early stage.