Ideal Home: Hermit Crab

, 23 February 2021
Ideal Home: Hermit Crab
South claw hermit crab © Christopher Glanfield

By Sarah Ward

Living Seas Officer

Hermit crabs are a group of marine crustaceans which have a soft abdomen, creating the need to make use of external resources to protect themselves from predators and their environment. A quick online image search will bring up hermit crabs using all sorts of things as a make-shift home, including (alas!) man-made items such as plastic debris and metal cans, however generally they tend to prefer the empty shells of sea snails.

Finding a suitable shell can be quite the challenge for a small crab in a big ocean, and this life constraint has caused a number of quite unique behaviours to have evolved in hermit crabs, allowing them to find their ideal home.

Hermit crabs are able to detect chemical changes in their nearby surroundings: they will naturally be attracted to the pheromones which are released by a dying sea snail. When the snail dies, their soft, fleshy body will decay quickly, leaving behind the hard shell. Having sniffed out the dying snail, the hermit crab will be ready and waiting to inspect the shell and decide if it wants to move in. If it already has a shell, the crab will make a decision as to whether the new shell is superior to the current one, sussing out the new shell for any damage, size and fit. If a shell is too big, it’ll be too heavy to carry around; too small and it won’t allow the crab to fully hide inside. Some species have a clear preference for particular species of shell; for example, the south-claw hermit crab, which we find here in Sussex, tends to opt for netted dog whelk shells.

A high-quality shell can sometimes be hard to come by for hermit crabs, so they are not shy of picking a fight with a fellow hermit crab if they take a fancy to their shell. A fight is initiated by the attacker mounting its opponent and ‘rapping’ their shell with its claw. It’s a battle of stamina – whoever outlasts the other will take ownership of the prized shell.

Hermit crabs can be found on rocky intertidal shores in Sussex, so do look out for them if you’re out rock pooling!

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