One early morning in June of 1986, I waded into a shallow tide pool on Long Island, squatted on a plastic milk crate and dropped an empty snail shell into the water. In a few minutes a small hermit crab skittered toward the shell, probed the opening with its claws to measure the size of the interior space and rotated the spiral casing several times to look for holes. Almost quicker than I could follow, the crab pulled itself out of its old refuge and thrust its vulnerable abdomen into the snail shell I had dropped. Satisfied with the exchange, the animal strolled away, leaving its previous, smaller shell behind. A few minutes later another hermit crab discovered the first one’s discarded dwelling and, after the same inspection ritual, scuttled away with its newfound lodging. About 10 minutes later a third crab found the second’s old home and claimed its prize, abandoning a small shell with a large hole.