In a perfect world there would be an abundance of shells from which to choose … but alas, suitable shells are hard to come by. And while the portion of crab that we can see looks pretty tough, the vulnerable soft abdomen curled up in the shell cannot be exposed for very long.

Their solution is ingenious. When a crab happens upon a shell that’s too big, the crab sits and waits, sometimes up to eight hours. More crabs show up to check out the potential new digs, and for those for whom the shell is too big, they line up and wait. And they line up in size order. Up to 20 crabs, lined up biggest to smallest, waiting for just-the-right–size crab to appear. And when the Goldilocks crab shows up, it sheds its shell and takes up the new one, and each crab of successive size hops out of their old shell and into the larger one ahead of it. Known as a vacancy chain, it’s a term originally used by social scientists to describe a way to trade resources.

<p><a href=”″>Crabs</a&gt; from <a href=”″>John Brown</a> on <a href=””>Vimeo</a&gt;.</p>