Vacancy chains

I’m fascinated by how vacancy chains unfold in nature.

Wikipedia describes both synchronous and asynchronous vacancy chains in hermit crabs. During a group exchange, crabs form a queue from biggest to smallest around an empty shell. When a crab of the right size takes that shell, its former home becomes available to the next crab:

… the crabs then all rapidly exchange shells, in sequence, until they all have a larger shell, allowing them to grow without restriction.