Hermit crabs don't grow their own shells, so to protect themselves they have to salvage shells abandoned by other sea life, usually from sea snails. But really they'll use whatever they can find, which often includes glass bottles, cans or shotgun shells. People who keep hermit crabs as pets also have the option of providing them with artificial shells, which can be made from recycled materials.
As a crab grows, it must often seek out new shells that provide a better fit. In this way, these cute crustaceans are constantly recycling dwellings that would otherwise go to waste.