Hermit crabs use abandoned seashells as mobile homes. As a crab grows, its shell becomes an ever tighter fit. So, eventually, a crab needs to move in to a bigger one.
Suitable empty shells are few and far between. . .
And this one is far too spacious for this crab. But instead of continuing its search, this small crab settles down to wait. Other crabs, also looking to upgrade to a bigger home, soon gather whenever a shell washes ashore.
Unfortunately, the new shell is too big for them, too. Despite that, the gathering crabs begin to measure each other up, and then they do something really rather extraordinary. They arrange themselves into an orderly queue.
The biggest at the front, smallest at the back. They're lining up with one aim, to exchange properties. But none of the crabs can make a move, because the chain is not yet complete.
They're all waiting for the right sized crab to come along. And this is it. The latest arrival pushes to the head of the queue.
At first, the chain falls apart. As it reforms, the small crab finally makes its move. .
. and takes its place at the end of the line. After a close inspection, the big crab moves in.
This triggers a chain-reaction. Each crab hurriedly moves into the shell vacated by the crab ahead of it in the line. Just as the small crab attempts to move into its new home.
. . a fresh arrival muscles in.
To be left without a shell is a death sentence. A naked crab will quickly cook under the intense tropical sun. But, after all the swapping, there is still one shell left.
Sadly, it's no bigger than this small crab's original shell. And worse, it's got a hole in it. But even a bad shell is better than being homeless.
