The coconut crab (Birgus latro)

Although B. latro is a derived type of hermit crab, only the juveniles use salvaged snail shells to protect their soft abdomens, and adolescents sometimes use broken coconut shells for that purpose. Unlike other hermit crabs, the adult coconut crabs do not carry shells, but instead harden their abdominal terga by depositing chitin and chalk. Not being constrained by the physical confines of living in a shell allows this species to grow much larger than other hermit crabs in the family Coenobitidae.[20] Like most true crabs, B. latro bends its tail underneath its body for protection.[17] The hardened abdomen protects the coconut crab and reduces water loss on land, but must be moulted periodically. Adults moult annually, and dig a burrow up to 1 m (3 ft 3 in) long in which to hide while vulnerable.[18] It remains in the burrow for 3–16 weeks, depending on the size of the animal.[18] Depending on the animal's size, 1–3 weeks are needed for the exoskeleton to harden after moulting, during which time the animal's body is soft and vulnerable, and it stays hidden for protection.[21]