Hermit crabs: The social animal that will help its peers come out of their shells (by force)

You know what a hermit crab looks like. Those beady eyes just slightly sticking up from the shell of a snail make this critter one of the cutest animals you can find on the P.E.I. shoreline (move over, plovers).

But did you know that besides being the cutest, they’re also the coolest?

“These animals are super fascinating,” says Jeff Clements, an aquatic biologist with Fisheries and Oceans Canada and expert on all things fly under the sea. “There’s all kinds of other super cool behaviours that they do.”

Hermit crabs are members of a group of crabs known as pagurids — animals that live in the empty shells of snails.

You can find hermit crabs in many different habitats, including sandy or muddy beaches and along rocky shores, especially those that have tide pools.

“They like to stay wet…. You may see them roaming around on the dry shore, but you’re more likely to find these if you’re walking ankle-deep in water,” Clements said.

They eat pretty much anything they can get their hands — er, claws — on. That includes detritus (the poop and bits of dead body parts from other organisms; see the lugworm), plants and other animals.

“Because they’re so versatile, hermit crabs can actively predate on some animals if they can manage to capture and kill them,” Clements said.

“You’ll often find them scavenging on the carcasses of dead animals that are floating around on the bottom of the sediment there, such as crabs, and they’ll eat the bits of dead plants and animals that are floating about.”

If you come across one, you’re also likely running into a hundred of so of its best friends. Hermit crabs are pretty sociable creatures, with complex mating rituals and other social behaviours that make the animals “pretty neat,” in the words of our aquatic biologist.

So let’s talk about shells.

Are you seriously wearing that?

You can find three types of hermit crabs on P.E.I. beaches: the Acadian hermit crab, the hairy hermit crab, and the long-wristed hermit crab. They’re often found in periwinkle or moon snail shells, but they can live in many different types.

Clements said these species are difficult to tell apart, with only a few distinguishing features on their heads and claws.

But that doesn’t mean they’re all the same, for hermit crabs are also unwavering stylists.

“There’s always going to be hermit crabs that are in shells that they don’t like — one that’s a bit too small, one that’s a bit too big,” Clements said. “If a hermit crab comes across an empty shell, it’ll assess it by looking at it and feeling it, and if it likes that shell more than the one that it’s in, it’ll trade up.

Why do they do this? These fickle fashionistas will never be caught dead sporting a shell that’s loose-fitting, or one that’s too revealing.

“If the shell is too big, it’s heavy and it takes a lot of energy to move it around,” Clements said. “If it’s too small, it can be hard to fit into, and it can leave soft parts of the body exposed and vulnerable.”

Sometimes hermit crabs perform this clothes swap — house swap? — in pairs, Clements said. When one crab finds a bigger shell, it can give its old shell to a friend as a hand-me-down.

“[The shells] create in some situations… a kind of chain reaction within the social group,” he said.

Other times, the switch is not that amicable.

Getting crabby

In hermit crab world, shells are such a prized commodity that they’re worth fighting to the death for.

Well, maybe not to the death. But it’s still pretty violent by hermit crab standards.

“If [a hermit crab] sees a shell that another hermit crab has, but it wants that shell, it will go over and basically knock on that shell really hard with its claws to try and get the hermit crab that’s in the shell it wants out, so it can basically steal its home,” Clements said.

Hermit crabs fight hard and often. Not just for shells, but also for other limited resources such as food and mates.

With all this competition, they have to be clever and pick their fights.

Because their body is hidden from view by their shells, hermit crabs have to gauge the size of their opponent by looking at their claws.

Clements said some of the smaller hermit crabs actually spend energy making their claws bigger to trick their opponent into thinking they’re big and scary. (Sure.)

And if they’re putting so much care into their outfit, why not accessorize?

“Sometimes, they’ll decorate their shelves with algae or sea anemones to camouflage or add protection.”

Be nice now

Hermit crabs, like most of the animals CBC P.E.I. has profiled on our Beach Finds series this summer, are threatened by climate change.

“They’re cold-blooded animals, which means that the temperature of their body internally is dependent on the temperature of the body outside,” Clements said. “There’s evidence that increasing temperatures can affect their physiology, and so as ocean temperatures get warmer, there’s some concern that effects may happen there.”

Because of their shell-based social behaviours, ocean acidification — which makes it hard for animals like moon snails to keep their shells — may also impact the little critters, Clements said.

This is one potential reason why we might want to leave shells on the beach. We might want to leave those resources for the hermit crabs rather than putting them on a kitchen table.— Jeff Clements

So, if you see a hermit crab, be kind.

“Hermit crabs generally are pretty harmless. If you pick them up, they’re going to retract into their shell and hide for the most part,” Clements said, though he added others may indeed try to pinch you.

“As I mentioned, shells are really, really important to hermit crabs…. This is one potential reason why we might want to leave shells on the beach. We might want to leave those resources for the hermit crabs rather than putting them on a kitchen table.”