Hermit crabs live in secondhand homes, and a shortage turns deadly
A hermit crab's soft, curled abdomen has no armour of its own, so it borrows a snail's empty shell and carries it everywhere. As the crab grows it must find a bigger one. That constant house-hunting shapes how hermit crabs fight, trade, cooperate and sometimes die in plastic rubbish.
Around 800 species rely on a portable shelter, most often a snail shell, though some use bivalve or tusk shells, hollow wood or stone. The tip of the abdomen clamps onto the shell's central spiral. Larvae begin symmetrical and free-swimming, then metamorphose into lopsided bottom-dwellers that go looking for a shell. Despite the name, they are not true crabs; they sit with squat lobsters and porcelain crabs in a separate group, Anomura. King crabs turn out to be hermit crabs that abandoned shells, and in 2023 their family was returned to the superfamily after being removed in 2007.
The habit is ancient. Fossil hermit crabs are found inside snail shells from the Late Cretaceous, while earlier ones lived in ammonite shells, including a specimen from the Speeton Clay of Yorkshire and a Jurassic find from Russia. The oldest known member, Schobertella hoelderi, comes from the earliest Jurassic of Germany.
Good shells are scarce, so competition is fierce. Crabs of similar size may fight or kill for a favoured one, while those stuck in undersized shells grow more slowly. Many species also swap shells in elaborate exchanges. Land hermit crabs, in the family Coenobitidae, mostly inherit shells that earlier owners have hollowed out chemically and physically to make them lighter; big crabs moved into ordinary unmodified shells in experiments usually die. Some such homes last for generations.
When shells run out, crabs make do with cans, glass or plastic. A 2024 Polish study found 10 of 16 land species using artificial homes, including light bulbs. Slippery plastic can trap a crab inside, and the scent of its dead body lures others hoping for a vacated shell, creating a lethal chain. Larger marine species take a friendlier approach, carrying sea anemones on their shells to deter predators in exchange for scraps. At the extreme, the Indos crab can reach coconut size and live 12 to 70 years, and the shell-less coconut crab is the largest land invertebrate on Earth.
Source: Hermit crab
