The hermit crabs form a sort of conga line ordered from largest to smallest crab. As the largest crab enters its new home, the next crab in line takes the vacated shell, leaving an open shell for the crab behind him. The shell swapping continues down the line until everyone has upgraded. This chain reaction is called a vacancy chain, and it’s an ingenious way for the creatures to survive while sharing limited resources.

Consider the hermit crab.