There’s an extraordinary sight in this clip from the BBC’s Life Story, narrated by Sir David Attenborough: A line of hermit crabs self-arranges from smallest to largest. They’re ready and waiting to exchange their shells—but why? Attenborough explains that “as a crab grows its shell becomes an ever-tighter fit so eventually the crabs need to move into a bigger one.” And so these small decapod crustaceans have organized, hopefully ensuring that each one will quickly find and move into a slightly larger mobile home.
