Hermit crabs line up in order of size to swap and upgrade shells with one another

"They arrange themselves in an orderly queue, the biggest at the front, the smallest at the back; they're lining up with one aim: to exchange properties. But none of the crabs can make a move because the chain is not yet complete. They're all waiting for the right-sized crab to come along."

The Hermit Crab vacancy chain, as featured in the 'Home' episode of Life Story, on the BBC.

[Video Link via Reddit]