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.

How hermit crabs choose their shells?

It is clear that hermit crabs do not enter shells at random, but select shells according to shape, shell opening and weight. A variety of mechanisms such as shell rapping, olefactory signals, and visual ques have been shown to transfer information about the quality of a shell.

Do hermit crabs fight over shells?

When a hermit crab wants to take another crab's shell, it attacks by tapping on the shell vigorously and with great energy. During such shell fighting, attackers will try to evict opponents in a manoeuvre that seems to require high muscle strength.