What would you do with a soft tail?
A hermit crab slips its soft abdomen into an empty shell. The crab is protected while nestled deep inside, sometimes nestled too deep to see. The delicate abdomen is gently curved to fit snuggly in the shell; trying to pull the crab out could rip it in half.
As the hermit crab grows, it needs larger shells. A large hermit crab in a too-small shell won’t be able to withdraw all the way into the shell, leaving it exposed to predators and thieving relatives. A small hermit crab in a very large shell may be able to withdraw so far that you wouldn’t be able to see it—even a shell that looks empty may not be.
Leaving shells on the beach leaves more hermit crab housing!