Hermit crabs have soft, vulnerable abdomens and need to wear shells to protect themselves. A hermit crab without a shell eventually falls victim to predators or even other hermit crabs.
Hermit crabs grow throughout their lives and wear many different shells at different life stages. Pet hermit crabs should have a variety of shell choices available since they are picky about finding a shell that fits well. Hermit crabs that cannot find a replacement shell readily may try to hide behind a piece of coconut fiber or other debris. Pet crabs prefer shells with round openings and smooth internal surfaces.