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.
It may seem strange, but this was one of the happiest moments in my life as a researcher. For nearly 10 years I had been wondering whether hermit crabs take up residence in one another’s vacated shells. I finally had my confirmation. I was the first person to observe an animal making use of what sociologists and economists call a “vacancy chain”—an organized method of exchanging resources in which every individual benefits by claiming a more desirable possession abandoned by another individual. Even though hermit crabs have relatively simple brains and nervous systems, they have evolved sophisticated social behaviors to make the most of vacancy chains.