…As they grow, hermit crabs must move into larger shells, so they are always on the lookout for a more spacious dwelling. And an undamaged shell is preferable to a broken one, even if the shells are the same size. Knowing this, the researchers decided to dramatically change the available hermit crab real estate on Carrie Bow Cay. They placed 20 beautifully intact shells that were a little too big for most hermit crabs at various spots around the island and watched what happened.
When a lone crab encountered one of the beautiful new shells, it immediately inspected the shelter with its legs and antennae and scooted out of its current home to try on the new shelter for size. If the new shell was a good fit, the crab claimed it. Classic hermit crab behavior. But if the new shell was too big, the crab did not scuttle away disappointed—instead, it stood by its discovery for anywhere between 15 minutes and 8 hours, waiting. This was unusual. Eventually other crabs showed up, each one trying on the shell. If the shell was also too big for the newcomers, they hung around too, sometimes forming groups as large as 20. The crabs did not gather in a random arrangement, however. Rather, they clamped onto one another in a conga line stretching from the largest to smallest animal—a behavior the biologists dubbed “piggybacking.”
Only one thing could break up the chain of crabs: a Goldilocks hermit crab for whom the shell introduced by Lewis and Rotjan was just right. As soon as such a crab claimed its new home, all the crabs in queue swiftly exchanged shells in sequence. The largest crab at the front of the line seized the Goldilocks crab’s abandoned shell. The second largest crab stole into the first’s old shell. And so on.
No one had ever documented such well-orchestrated shell swapping before, but similar behavior was not unknown. In 1986, Ivan Chase of Stony Brook University made the first observations of hermit crabs exchanging shells in a “vacancy chain”—a term originally coined by social scientists to describe the ways that people trade coveted resources like apartments and jobs. When one person leaves, another moves in. Since then, several researchers—including Lewis and Rotjan—have studied the behavior in different hermit crab species. Some preliminary evidence suggests that other animals use vacancy chains too, including clown fish, lobsters, octopuses and some birds. As Chase explains in the June issue of Scientific American, vacancy chains are an excellent way to distribute resources: Unlike more typical competition, a single vacancy chain benefits everyone involved—each individual gets an upgrade. So it makes sense that hermit crabs and other animals have evolved sophisticated social behaviors to make the most of vacancy chains.