Watch Orderly Hermit Crabs Line Up Biggest To Smallest, Swap Shells In 'Conga Line'

Even hermit crabs, it turns out, aren't safe from a housing crisis.

In the above clip from the BBC show, "Life Story," several hermit crabs on a small Caribbean island off the coast of Belize are shown in their quest for suitable shelter, which involves lining up according to size and swapping shells in an orderly fashion.

Hermit crabs rely on shells (and flotsam, in some cases) to protect their soft abdomens from predators and the elements. They are always on the lookout for shells that might better suit their size and situation since, as the video states, "To be left without a shell is a death sentence."

Amazingly, the crabs have developed this very cooperative, if self-serving, way of swapping shells -- a sociological behavior scientists call a "vacancy chain." Mark Laidre, then a post-doctoral fellow at the University of California, Berkeley, published a 2012 study that found that hermit crabs congregate around a smaller hermit crab, form a "conga line" smallest to largest, and then take turns moving into a larger shell.

This behavior was witnessed firsthand by two researchers from Tufts University who experimented with Caribbean hermits on an island in Belize called Carrie Bow Cay. By providing pristine shells to hermit crabs on a beach, they watched as crabs organized their house-swapping ritual.

Scientific American describes how after a lone crab inspected and passed on a shell that was too big, it patiently waited nearby.

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."

Sometimes, it doesn't work out well for everybody. In the BBC clip above, one little crab ends up in a shell with a hole in it, worse off than when it first joined the conga line.

Moral of the story? Never trust a desperate hermit crab in the middle of a growth spurt.