« Whenever a land hermit crab is lucky enough to come across an empty shell (sometimes because a behavioural ecologist put it there) and if no one else is around, it will stop, take a closer look and probably try on the new shell for size. If it likes what it finds it will keep the new home and continue on its way. However, if the shell is too big the crab won’t pass on by, but will sit quietly next to it, sometimes for as long as 24 hours. In that time other crabs will probably amble past and wonder what’s going on. Then a spontaneous hermit party breaks out. Don’t get too excited, though, because the main thing that happens when hermit crabs get together is they start forming queues.
A gaggle of hermit crabs clustered around a big empty shell will sort themselves out into a size-ordered line with the biggest at one end, leading to the smallest at the other. This orderly formation is called a vacancy chain, and people form them too, of jobs and houses. The crabs work out who goes where by clambering around and feeling up each other’s shells. Sometimes, if there are lots of hermits in the area, several queues will form around a single, large vacant shell and then things get a bit more interesting: a tug-of- war ensues. The biggest crabs will wrestle over the coveted empty shell while the little ones further down the line will shift queues like supermarket shoppers speculating on which checkout will move fastest.
Eventually, one queue will win control of the empty shell and, in a flurry of claws, everybody in the successful line moves house. Each crab slips out of its old shell and into the newly abandoned shell of the crab one place ahead of it in the queue. They all get a new shell, one size bigger, and quickly scuttle off, once again going their separate ways. Behavioural ecologists have worked out that forming vacancy chains provides benefits for all the crabs involved; adding just one new shell can efficiently provide new homes, of just the right sizes, for a whole gang of hermits. »
— Helen Scales, Spirals in Time: The Secret Life and Curious Afterlife of Seashells
