20 August 2012:

While at Cape Cod I was fascinated by the wide variety of hermit crabs in the tidal pools. Each crab has a mobile home. The tiniest wear cone-shaped whelk shells. Larger ones wear round snail shells. They’re all in marvelous shapes and sizes. 

Hermit crab housing is not a steady state. The ideal shell has room for the crab to grow and allows him to retract in the face of danger. When the shell’s too small the crab is vulnerable.

As he grows, an individual hermit crab is forced to acquire a series of larger shells but a right-sized shell is not always easy to find.  His quest is most successful when he joins a house hunting social group.

Though their name is “hermit” these crabs work together when shells are not extremely scarce. Their cooperation was not well understood until researchers from Tufts University and the New England Aquarium teamed up to study the Social context of shell acquisition in Coenobita clypeatus hermit crabs, published in April 2010.

According to researcher Randi Rotjan, “Hermit crabs are really picky about real estate because they’re constantly getting thrown back into the housing market.”

When a hermit crab needs a new home he keeps his eye out for any larger shell.  When he finds one that’s empty, but too big, he waits next to it.  He won’t use this shell but a larger crab will … and that crab will be in a smaller shell … and that smaller shell might be just the right size.  So he waits.

Pretty soon this lone hermit crab has attracted a variety of others who are also in the housing market.  They mill about, waiting.  The smaller ones piggyback on the larger ones and ride around like papooses.  They don’t want to miss their chance.

Eventually all the crabs are lined up by size in a synchronous vacancy chain.  The crab who wants the large empty shell is in place and bang!  “The chain fires off in seconds, just like a line of dominoes,” says Rotjan.  Everyone moves in at once.

Big move-in events are not unique to hermit crabs.  This is Move-In week at Carlow, Chatham, Duquesne, Point Park, Carnegie Mellon, and the University of Pittsburgh.

We’re about to see a lot of synchronous vacancy chains in Pittsburgh … but they won’t fire off in seconds.  😉

p.s. Click here to read more in Scientific American.

(photo by Paolo Costa Baldi, license: GFDL/CC-BY-SA 3.0, via Wikimedia Commons.  Click on the image to see the original.)