This is an amusing video from the BBC1 showing a Hermit Crab 'swap meet'.
As hermit crabs grow, they need larger shells. Since suitable intact shells are sometimes limited, competition often occurs. The availability of empty shells depends on the relative abundance of gastropods and hermit crabs, matched for size. If the crabs vary significantly in size, the occurrence of fights over the shells will decrease or even become nonexistent.
The ultimate eco-friendly home! Now if we could do this as humans.