Invertebrates
Hermit Crab
Paguroidea · also called hermit crab, soldier crab
The hermit crab solved a problem — a soft, spiral belly with nothing to protect it — by moving into someone else’s shell. Its whole life is shaped by that borrowed home: outgrowing it, searching for a bigger one, defending it, and, when a good shell turns up, lining up with other crabs to swap houses in a single coordinated chain.
Overview
Hermit crabs are a superfamily of about 1,100 species found in every ocean — tide pools, reefs, the deep sea — and on tropical coasts. They are not true crabs: they have a soft, coiled abdomen and only two visible pairs of walking legs, the rear ones reduced to grip the inside of a shell. Most are scavenging omnivores and important shore and reef cleaners.
The borrowed shell
A hermit crab lives inside an empty sea-snail shell, holding on with hook-like rear appendages and often plugging the entrance with an enlarged claw when it withdraws. As it grows it must find a larger shell, and in many places the supply of shells limits how big and how numerous the crabs can be.
Vacancy chains
When a new, larger shell appears, hermit crabs gather and queue largest-to-smallest. The biggest crab moves in, and each crab down the line takes the shell just vacated — a whole line rehoused in seconds. It is a textbook “vacancy chain.”
Partners and passengers
Land hermit crabs breathe air (and include the young stage of the coconut crab). Many marine species carry sea anemones on their shells — protection and camouflage for the crab, mobility and scraps for the anemone — and move the anemone when they change shells.
Status
Hermit crabs are not assessed as a group. They are abundant, but coastal species are hit by shell removal by beachcombers and, badly, by plastic litter — crabs climb into bottle caps and containers, cannot get out, and die, and each death attracts more crabs to the same trap.
Hermit Crab: common questions
Why does a hermit crab live in a snail shell instead of growing its own?
Unlike true crabs, a hermit crab's abdomen is soft, elongated, and curled into a spiral, with no hard covering — it evolved that way to fit inside a coiled gastropod shell. The shell is portable armour — it protects the vulnerable body and, in many species, seals the entrance with an enlarged claw when the crab withdraws — without the cost of the crab growing and repeatedly moulting its own heavy shell over the whole body. The trade-off is dependence on a supply of empty snail shells of the right size and shape, which is a real constraint — in places where shells are scarce, hermit crabs are smaller, more crowded, and will fight over shells or occupy poor substitutes.
What is a "vacancy chain" and do hermit crabs really queue?
Yes, and it is a well-documented behaviour. When a large empty shell (or a shell newly vacated by a dead snail) turns up, hermit crabs are attracted to it, often within minutes, by the smell of the dead snail or of an injured crab. They assemble around it and, remarkably, form a line ordered by body size, each crab holding the one behind. The largest crab that fits inspects and enters the new shell; the moment it does, the next crab takes the shell it just left, and so on down the line, so a single new shell can rehome a dozen crabs in a few seconds. It is one of the clearest examples of a "vacancy chain" — a resource-redistribution pattern also seen in things like human housing and job markets.
Are hermit crabs good pets?
Land hermit crabs are sold cheaply as low-maintenance pets, which is misleading. They are social animals that do poorly kept singly, they need a warm, humid tank with both fresh and salt water, deep substrate to moult in (they bury for weeks and must not be disturbed), and a selection of appropriately sized empty shells to change into. Crucially, they cannot be captive-bred at any scale — their larvae need the open ocean — so essentially every pet land hermit crab is wild-caught, often roughly, and painted shells sold with them can poison the crab. Many die within months in captivity. They can live for decades if kept well, which few owners manage.
How does plastic pollution kill hermit crabs?
Hermit crabs investigate small containers as potential shells, and on beaches littered with plastic they climb into bottle caps, film canisters, and broken containers. If a crab gets stuck, or if the container is smooth-walled, it cannot climb out and dies. Worse, a dying hermit crab releases a chemical signal that in nature means "empty shell available here" — so its death attracts more hermit crabs to the same trap, which also become stuck and die, drawing yet more. Surveys of remote islands have found single plastic containers holding dozens of dead hermit crabs, and researchers estimate that hundreds of thousands of crabs are killed this way each year on some island groups.