Fun Facts About Hermit Crabs

Hermit crabs are fascinating creatures. There are both terrestrial hermit crabs (which are sometimes kept as pets) and aquatic hermit crabs. Both types of crabs breathe using gills. Aquatic hermit crabs get their oxygen from the water, while land hermit crabs need a humid environment to keep their gills moist. Even though you may see a hermit crab on the beach near the ocean, this could still be a marine hermit crab. Even though they may look like appealing pets, don't take a wild crab home with you, as hermit crabs (especially aquatic ones) have very specific requirements they need to survive.

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Hermit Crabs Change Shells

Hermit Crab (Pagurus bernhardus) Climbing on Stipe, Scotland
Hermit Crab (Pagurus bernhardus) Climbing on Stipe, Scotland. Paul Kay / Oxford Scientific / Getty Images

Unlike true crabs, if a hermit crab gets sick of its shell, it can move out. In fact, they have to change shells as they grow. While gastropods like whelks, conch and other snails make their own shells, hermit crabs seek shelter in the shells of gastropods. Hermit crabs can commonly be found inhabiting the empty shells of animals such as periwinkles, whelks and moon snails. They usually don't steal shells that are already occupied. Instead, they'll search for vacant shells.

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Hermit Crab in a Clear Shell

Hermit Crab in Clear Glass Shell
Hermit Crab in Clear Glass Shell. Frank Greenaway/Dorling Kindersley/Getty Images

Hermit crabs are crustaceans, which means they are related to crabs, lobsters and shrimp. Although it has 'crab' in its name, a hermit crab out of its shell looks more similar to a lobster than a crab.

In this cool (but somewhat creepy!) image, you can get an idea of what a hermit crab looks like inside its shell.  Hermit crabs have a soft, vulnerable abdomen that is twisted to wrap around the spire inside the shell of a gastropod. The hermit crab needs this shell for protection.

Because they don't have a hard exoskeleton and need to use another shell for protection, hermit crabs are not considered "true" crabs.

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Molting

Hermit crab digging a hole, in preparation for molting, Red Sea
Hermit crab digging a hole, in preparation for molting, Red Sea. Jeff Rotman/Photolibrary/Getty Images

Like other crustaceans, hermit crabs molt as they grow. This involves shedding their exoskeleton and growing a new one. Hermit crabs have the extra complexity of having to find a new shell when they outgrow their old one.

When a hermit crab is ready to molt, its new skeleton grows under the old one. The old exoskeleton splits and comes off, and the new skeleton takes some time to harden. Because of this, crabs often dig a hole into the sand to provide protection during the vulnerable time of molting.

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How Hermit Crabs Switch Shells

Red Hermit Crab (Petrochirus diogenes) Changing Shells, Cancun, Mexico
Red Hermit Crab (Petrochirus diogenes) Changing Shells, Cancun, Mexico. Luis Javier Sandoval/Oxford Scientific/Getty Images

The red hermit crab shown here is getting ready to switch shells. Hermit crabs are always on the lookout for new shells to accommodate their growing bodies. When a hermit crab sees an ideal shell, it will sidle up very close to it, and check it out with its antennae and claws. If the shell is deemed suitable, the hermit grab will quickly switch its abdomen from one shell to the other. It may even decide to go back to its old shell.

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Hermit Crab Diet

Hermit Crab, Spain
Hermit Crab, Spain. _548901005677/Moment/Getty Images

Hermit crabs have a pair of claws and two pairs of walking legs. They have two eyes on stalks to make it easier to see what's around them. They also have two pairs of antennae, which are used to sense their environment, and 3 pairs of mouthparts.

Hermit crabs are scavengers, eating dead animals and whatever else they can find. Hermit crabs may be covered with short sensory hairs that are used for smell and taste.

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Hermit Crab Friends

Jeweled Anemone Hermit Crab, Philippines
Jeweled Anemone Hermit Crab, Philippines. Gerard Soury/Oxford Scientific/Getty Images

Hermit crabs often have growths of algae or other organisms on their shells. They also have symbiotic relationships with some organisms, such as anemones.

Anemone hermit crabs attach anemones to their shell, and both organisms benefit. The anemone stings potential predators with their stinging cells and stinging threads and also helps hermit crabs blend in with their surroundings. The anemone benefits by eating the leftovers of the crab's meal, and being transported to food sources. 

The anemone crab will even take the anemone(s) with it when it moves to a new shell!

References and Further Information

  • Coulombe, D. 1984. The Seaside Naturalist. Simon & Schuster. 246pp.
  • Marine Science Institute Blog. 2014. Creature Feature: Jeweled Anemone Hermit Crab (Dardanus gemmatus). Accessed August 31, 2015.
  • McLaughlin, P. 2015. Paguridae. In: Lemaitre, R.; McLaughlin, P. (2015) World Paguroidea & Lomisoidea database. Accessed through: World Register of Marine Species. Accessed August 31, 2015.
  • Naturally Crabby. Hermit Crabs from the Beach. Accessed August 31, 2015.
  • Northwestern Hawaiian Islands Multi-Agency Education Project.  Creature Feature: Anemone Hermit Crabs. Accessed August 31, 2015.
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Kennedy, Jennifer. "Fun Facts About Hermit Crabs." ThoughtCo, Nov. 20, 2017, thoughtco.com/fun-facts-about-hermit-crabs-2291854. Kennedy, Jennifer. (2017, November 20). Fun Facts About Hermit Crabs. Retrieved from https://www.thoughtco.com/fun-facts-about-hermit-crabs-2291854 Kennedy, Jennifer. "Fun Facts About Hermit Crabs." ThoughtCo. https://www.thoughtco.com/fun-facts-about-hermit-crabs-2291854 (accessed January 16, 2018).