Hermit Crabs Form Orderly Queue as Roomy Shell Comes on the Market

A large whelk shell came onto the market at low tide on Thursday morning, and within the hour a queue had formed. By noon it stretched most of the way across the rock pool, eleven hermit crabs long, arranged with remarkable discipline from the largest at the front to the smallest at the back. Nobody had organised it. Nobody needed to. Everyone involved said it was simply how these things are done.

The shell itself, described by one early viewer as roomy, south-facing and full of character, had been vacated by its previous occupant some time ago, under circumstances nobody wished to discuss. It was first spotted by a mid-sized crab named in local reports only as the tenant of the chipped periwinkle. He tried it on for size, found it far too big, and did not leave. Instead he sat down beside it and waited, which, he explained, is what you do.

Others arrived steadily through the morning. Several of the smaller crabs attached themselves to the backs of larger ones and rode along, a practice they described as keeping their place and which the larger crabs described, less warmly, as being hitched to. There was some jostling near the middle of the line over whether two crabs of very similar size had arrived in the right order. It was settled by holding claws up against each other, and the loser moved back one place without further complaint.

For several hours nothing happened at all. Then, shortly after two in the afternoon, the largest crab in the pool arrived, inspected the whelk shell, and climbed out of his own and into it in one practised movement. What followed took less than a minute. The crab behind him moved into his old shell. The next crab moved into the one just vacated. All the way down the line, eleven households changed address in a single rippling motion, like a row of dominoes falling in the right direction.

The smallest crab, left at the end with a slightly dented top shell that had been three sizes too tight on its previous owner, called the day a complete success. Everybody went up one size, she said. Nobody lost out. Asked what would happen to the empty shell she had just left behind, she pointed out that there is always someone smaller coming along.

Hermit crabs really do this. Their abdomens are soft and unprotected, so they live inside empty snail shells, and since a shell cannot grow, a growing crab has to keep finding a bigger home. When a new, larger shell turns up, crabs gather around it, and one that finds a shell too big for it will often wait nearby instead of walking away. Sometimes they line up in order of size, from biggest to smallest, and the smaller ones may hold on to larger crabs and ride along. Once the first crab moves in, the whole swap can run down the line within seconds.

Biologists call this a vacancy chain, borrowing a term sociologists use for the way a single empty job or home is passed along from person to person. A new flat is built, a family moves into it, someone else takes the place they left, and so on down the chain. One empty shell on a beach can end up giving a whole string of crabs a better house. It is the same quiet arithmetic behind every removal van you have ever seen, only faster, and with more claws.