Hermit crab

The tenant that carries its house on its back

A hermit crab has a soft belly, so it finds an empty shell, climbs in and carries its home on its back. As it grows it needs a bigger shell — on the beach crabs line up and swap houses: the biggest moves into the new shell, the next one takes its old shell, and so on! Some 'plant' a sea anemone on their shell to sting enemies.

Hermit crabs (Paguroidea, over 800 species) have soft, spiral abdomens and live in empty snail shells; Pagurus bernhardus is common along North Atlantic shores. Shells are a scarce resource: in a 'vacancy chain' crabs queue by size around a big empty shell and all move at once (economists study it as a housing-market model). They clean a shell inside and try another if it doesn't fit. Some species live in mutualism with anemones: the anemone defends, and the crab carries it to food scraps — and transfers it when moving house. Hermit crabs living in plastic bottle caps on beaches are a sign of pollution. Land species (Coenobita) breathe with lungs.

Fun facts

• Hermit crabs queue for shells and all move house at once.
• Some 'plant' an anemone on their shell as a guard.
• With no shells around, they even live in plastic bottle caps.