Why does a hermit crab change its shell?
Lacking a protective exoskeleton, a hermit crab uses the vacated shell of a mollusc as a temporary safe house, but must find a series of bigger homes as it grows. Finding a shell exactly the right size can prove difficult, and can result in bizarre house-swap chains.
If a crab locates one that’s not the right size, it will wait nearby until another crab looking for a new shell arrives. When that crab sheds its old home and takes the empty shell, our first crab moves in to that newly vacated one, casting off its own, which is then adopted by a smaller crab... and so on. The number of house- swaps each crab undertakes in its life varies depending on water temperature, habitat and species.
Polly Pullar
How do hermit crabs pick the right shell?
Taking over empty shells for protection is a neat trick. However, for hermit crabs, the quest for the perfect home is a lifelong preoccupation full of compromise. Swapping shells is a risky, yet necessary, procedure for a constantly growing crab, and finding a good fit is more of an art than an exact science.
This hasn’t stopped researchers observing how the European hermit crab surveys a prospective shell. A crab will use its stalked compound eyes to gauge a shell’s size, weight and even colour, and it also has a good feel, using its legs and antennae, to estimate the shell’s volume, shape, condition and manoeuvrability, and whether it is already occupied. Gathering and processing all this information is a remarkable cognitive feat. Recently scientists discovered that if hermit crabs ingest microplastics, their decision-making is impaired, disrupting this essential survival behaviour.
Gillian Burke
What if they can't find big enough shell?
If they find themselves in a shell that is too small, and if nothing larger is available, common hermits have the rare ability to decrease in size from moult to moult.
Can hermit crabs leave their shells?
These soft-bodied crustaceans are so dependent on their shells for protection that they will only venture out to copulate or to upgrade to better accommodation.
Where do hermit crabs live?
Only smaller specimens, inhabiting periwinkle shells, tend to occur in rockpools. Larger ones, which can reach 8cm in length and require more spacious accommodation such a whelk shells, usually live below the low-tide mark.
Are hermit crabs real crabs?
Hermit crabs are not true crabs, from which they split about 200 million years ago, but are more closely related to squat lobsters, which are not themselves true lobsters.
What do hermit crabs eat?
Hermit crabs are opportunistic omnivores, who will scavenge, kill, browse and graze whatever plant or animal matter is available. They are readily cannibalistic and can filter microscopic food from the water using their bristly mouthparts.
Why do they encourage anemones?
Common hermits encourage anemones to attach to their shell, and will even transfer them from shell to shell when they move house. The anemones' stinging tentacles may deter predators.
Which hermit crab claw is the biggest?
Unlike many fiddler crabs, which may be left- or right-handed, a common hermit's right-hand claw, or cheliped, is always the larger of the two.
Illustration by Dan Cole/The Art Agency
