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Anatomy Of A Hermit Crab

Are you intrigued by the anatomy of a hermit crab? Terms like Chelipeds, Setae, and Cephalothorax might have come your way and sparked curiosity about their meanings and functions. In this comprehensive guide, we’ll demystify the complex world of hermit crab anatomy.

We’ll delve into whether these fascinating creatures possess a heart, a brain, or a mouth, and explore all the internal and external parts that make up a hermit crab.

Parts You Can See

We’ll start with the parts you can see on the outside of your hermit crab, even the parts that are always covered by the acquired shell they use. The outer parts of a hermit crab include:

  • Antennae
  • Antennule
  • Compound eyes
  • Exoskeleton
  • Cephalothorax 
  • Maxillipeds
  • Chelipeds
  • Pereiopods
  • Carapace
  • Setae
  • Abdomen (tail)
  • Gills
  • Pleopods
  • Uropods
  • Gonopores
  • Telson

Now, let’s go over all these parts, and what purpose they serve.


Close up of hermit crab antennas
Close up of hermit crab antennas

These little wiggly, black appendages are sensory organs just like what insects have. Hermit crabs have two pairs of antennae. The longer pair are sometimes called “feelers” because if you watch them closely, the hermit crab will use that pair to tap and touch things in front of them.

The smaller pair is used to smell and taste, and they are called the antennules. If you are trying to hand feed your hermit crabs you gently touch the food to these little feelers so they can tell it’s something good to eat.

Imagine wiping your forehead with that hamburger before putting it into your mouth so you can taste it. That might make for some messy eating!

Compound Eyes

Eyes of a hermit crab
Eyes of a hermit crab

The little stalks with the black, blue, or other colored bulbs are the eyes on hermit crabs. They don’t have eyelids, but they have a hard, protective covering to protect them and keep sand out.

They are compound eyes because they have hundreds of tiny, faceted lenses to help them notice the smallest movements. Have you wondered how they are able to pull themselves into their shells so fast? It’s because they notice the slightest movements and quickly react.

These compound eyes are very similar to insect eyes, and allow them to see nearly all over at once, but they can’t see below themselves.


Hermit crab without a shell
Hermit crab without a shell

The exoskeleton is the hard, outer covering of hermit crabs. Shrimp, lobsters, crawfish, and insects all have exoskeletons to protect their inner organs and give them shape. These organisms don’t have bones.

If they didn’t have an exoskeleton they would be soft and squishy like an octopus or jellyfish.

Exoskeletons don’t grow with the animal, even though the insides do continue to get bigger. This outer covering has to be shed occasionally, when this happens for hermit crabs, they will burrow into the ground, and stay there until they shed the old exoskeleton.

The new covering is soft and flexible, and during this time the crab is very vulnerable to injury and predation, so they stay hidden until it hardens and offers protection again.


The cephalothorax is the head and back of the hermit crab. These are the first two segments of the body that contain most of the internal organs. They are both covered by the hard exoskeleton.


These are the mouthparts on hermit crabs. They are very hard to see because they are so small, but there are 3 pairs of tiny arm-like projections that pull food into the crab’s mouth. Crabs don’t have teeth to chew food, instead, they break it up into tiny pieces and swallow it.

In crabs’ stomachs, there are bumps and nodules similar to teeth that break down food.


Chelipeds are fancy names for the crab’s claws. Most hermit crabs have a larger claw they use to defend themselves, fight off rivals, or pull smaller hermit crabs out of shells they want. Hermit crabs also use that big claw to seal off the opening in their shell and protect themselves.

The smaller claw is used to grab food and put it into their mouth parts. It’s still called a cheliped.


These are all the legs of the hermit crab. Including the chelipeds or claws, hermit crabs have 5 pairs of legs. Their first pair of legs are the claw waving appendages, the next 2 pairs are used for walking and movement.

The rest of the legs tend to stay inside the shell, but they do have uses. The 4th pair of shortened, stubby legs move the crab into the shell and help hold on tight. The 5th and final pair of tiny legs are used to keep the gills clean. This pair of legs have miniature pinschers that remove excrement, or poop from the shell.


The carapace is part of the exoskeleton. It’s just a name for the hard plates that cover the head and back, AKA the cephalothorax. It’s much like a turtle, whose upper shell is called a carapace.


See those tiny hairs on the outside of the exoskeleton? Those are sensory organs called setae, and they are not real hairs. The setae are actually tiny strands of chitin, the hard material that creates the exoskeleton.

These hair-like projections can wear down, and break off. When a hermit crab molts, the setae are regenerated. If you have ever owned a hermit crab, you may have noticed that they look “hairier” after molting, that’s because all the setae are new and haven’t broken off.

These setae have multiple purposes. These filaments help the crab feel, help with grooming, and they absorb water when the crab soaks.

Abdomen (tail)

The soft, curled back end of the hermit crab is the abdomen. Sometimes it’s referred to as the tail, but hermit crabs don’t technically have a tail.

The abdomen is still covered by an exoskeleton, it’s just soft and pliable. That’s why hermit crabs need abandoned shells, to protect that soft backside.


Hermit crabs don’t have lungs, they breathe through gills. These are modified to work on dry land, but there are some hermit crabs that stay in the water and will suffocate if taken out of the water, just like fish.

All hermit crabs are born in the water, but land hermit crabs leave the water and live their adult lives on land. They still need a high level of humidity to breathe properly. If the air is too dry, the gills can’t exchange oxygen and carbon dioxide properly, which leads to suffocation.


These are only found in female hermit crabs. They are leg-like appendages that she will attach her eggs to. Using a secretion that acts like a glue, the female hermit crab will stick the eggs to the pleopods until they are ready to be released into the water.


This is the little curled projection at the end of the abdomen that helps hold the hermit crab in the shell.


On the 3rd pair of legs, on female hermit crabs, you may see a pair of tiny holes very close to the body. These are gonopores or sexual organs of a female hermit crab.

Male hermit crabs will deposit their spermatophore (packets of sperm) into these holes during mating.


The telson is the end tip of the hermit crab’s abdomen or “tail.” It is also called the anus, you know, the part where solid waste comes out.

What’s On The Inside Of A Hermit Crab?

You can see the outer parts of a hermit crab most of the time. Many parts stay inside their shell, and you may only get a small, quick glimpse when they switch shells.

What’s on the inside of a hermit crab? Do they have the same organs as we do? Let’s find out, shall we? These are the major parts located inside a hermit crab:

  • Esophagus-Food travels from the mouth down the esophagus to the stomach.
  • Foregut-One section of the stomach
  • Midgut diverticulum-This is a pouch part of the middle section of the stomach.
  • Midgut gland-Also called the hepatopancreas, this organ functions as a liver, 

pancreas, and gallbladder. It can store and absorb digested nutrients. 

  • Posterior midgut cecum-This is the beginning of what is basically the large intestine.
  • Hindgut-The large intestine, or colon organ in hermit crabs. It’s where the final bit of nutrients are absorbed, while the waste gets shoved to the anus.
  • Anus-The opening where waste is excreted.
  • Antennal gland-These glands are sometimes called “green glands.” You’d think that since they are attached to the antennae, these glands would have something to do with sensation, touch, smell, taste, or the like.

Nope, this is where hermit crabs pee. Yes, pee, as in urinate. Hermit crabs have glands in their heads that function like kidneys and secrete waste. So many inappropriate jokes I can’t put down here… Moving on!

  • Thoracic ganglionic mass-Hermit crabs don’t have brains. Instead, they have bundles of nerve tissue that function together like a primitive brain. These masses include the thoracic ganglionic mass and the…
  • Supraesophageal ganglion-The second brain-like mass that’s located near the bottom of the hermit crab.
  • Ventral nerve cord-This organ is similar to the spinal cord on a vertebrate. Since crabs don’t have bones, they don’t have vertebrae to protect the spinal cord, instead, it travels from the supraesophageal ganglion to the end of the hermit crab.
  • Heart-Hermit crabs don’t have a true heart that circulates the blood throughout their system. Instead, the vessels constrict to move blood to and from sinuses or cavities that surround the organs.

These arteries include the antennal, cephalic, optic, ventral thoracic, sternal, and hepatic arteries, other blood vessels include the ventral aortic branch and the posterior aorta.

  • Vas deferens-These are the tubes that travel from the male crab’s testes and carry the spermatophores to the exit.
  • Testis-Male hermit crab’s sex organs.
  • Gonopores-The holes in female hermit crabs where the male deposits his sperm sacs during mating.


Do hermit crabs feel pain?

Yes, hermit crabs can feel pain, and research suggests they can retain the memory of pain and seek to avoid those situations. Crabs can sense heat, chemicals, and caustic materials and they will stay away from these things.

It’s safe to say that they can feel damage to their shells and experience pain from injuries.

How do hermit crabs give birth?

Female hermit crabs will protect fertilized eggs inside their shell. They are attached to small, thin appendages called pleopods. When the eggs are about to hatch, the female will go to the ocean, partially pull her soft abdomen out of the shell, then she will scrape them free and release the eggs into the water.

Do hermit crabs leave their shell to mate?

Hermit crabs’ sexual organs are located farther back on their bodies, and are protected by their shells. When they mate, hermit crabs will go into seawater, partially emerge from their shells, and begin the mating process.

A male has a packet of sperm called a spermatophore that he deposits into the female gonopores.

Are hermit crabs born with shells?

When tiny hermit crabs hatch from eggs, they are fully aquatic and resemble miniature shrimp. As they grow, they will continuously molt until they look like adult hermit crabs, only very small. Then they will search for an appropriate shell, usually an empty snail shell.

So, no, hermit crabs are not born with shells. They have to find protective shells, even when they are babies.

What happens if you touch a molting hermit crab?

Hermit crabs will burrow into the substrate when they have to molt. During this time they are extremely soft and vulnerable, and they will stay burrowed until the new exoskeleton is sufficiently hardened.

During this time, you shouldn’t disturb, or try to touch the little crab. You could cause extremities to break off, or cause a tear in their cephalothorax. Injuries of this magnitude are most likely fatal.


There you have it, the anatomy of a hermit crab. As you can see, hermit crabs have several differences inside and outside compared to most mammals. What may seem weird to us—I know I wouldn’t want to urinate from my eyebrows, or my hairline—it’s perfectly natural and works for them.

Hermit crabs are really unique creatures, and if they are properly cared for, they can be a great pet that lives with you for a long time.

If you found this guide interesting or informative, please leave us a message below! We’d love to hear from you, and maybe you can connect with other hermie owners who have great advice to share.

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