Why does a hermit crab change its shell?
ALacking a protective exoskeleton, a hermit crab uses the vacated shell of a mollusc as a temporary safe house, but must find a series of bigger homes as it grows. Finding a shell exactly the right size can prove difficult, and can result in bizarre house-swap chains. If a crab locates one that’s not the right size, it will wait nearby until another crab looking for a new shell arrives. When that crab sheds its old home and takes the empty shell, our first crab moves in to that newly vacated one, casting off its own, which is then adopted by a smaller crab... and so on. The number of houseswaps each crab undertakes in its life varies depending on water temperature, habitat and species.
Hermit crab: serial houseswapper.