How do hermit crabs make a new shell?

The shell that the hermit crab adopts and wears and changes many times throughout his decades long lifetime helps him in several ways.

The hermit crab has a very delicate abdomen that is protected by the shell. Without this shell, his abdomen could become punctured very easily and the crab would die. The hermit crab also carries water in his shell to keep the abdomen moist. If it were to dry out, again, the hermit crab would die. The crab balances the salinity of the water within his shell by accessing fresh or salt water as needed.

The shell is also his shelter, and protects him from predators. The ideal fitting shell will allow the hermit crab to pull fully inside and to block off the opening with his large pincer.