In this BBC clip from the documentary series “Life Story”, the David Attenborough describes us how crabs hermits create between them a queue, to take the appropriate home depending on their size.

Maybe you know that crabs hermits are those that come within a shell, and are always in search for a new home as they grow. But you have to find the right size, because a very large shell will be difficult to transfer. So, have devised their own home Exchange System, so that everyone is happy.