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04. The Anatomy and Lifecycle of the Hermit Crab

The Anatomy and Lifecycle of the Hermit Crab was a large section of study. I had the help of lots of resources to read and research through including a field study to go and see some types of crustaceans.

To read about these exercises and the field study, please go to -

Learning More About Hermit Crabs and Crustaceans.

Lifecycle of Hermit Crab [Video]

posted Oct 3, 2011, 5:23 PM by November T

YouTube Video

The Lifecycle of the Hermit Crab

posted Oct 3, 2011, 4:21 PM by November T   [ updated Oct 3, 2011, 5:22 PM ]

Hermit Crab Ages

In captivity, hermit crabs can live up to 10 – 15 years.

But in their natural habitat, some people say they can live over 40 years.

The Land Hermit Crab Lifecycle

1.       When a male land hermit crab mates with a female, thousands of eggs are laid and held on the pleopods on the left side of her abdomen.

2.       A female hermit crab has her eggs on her for about a month as they mature.

3.    When the eggs are ready to hatch, the female hermit crab goes down to the ocean when there is a high tide. With the tide in, the hermit crab lays her eggs into the ocean – or sometimes flings them into the water. When the eggs come into contact with salt water, they hatch. The tide sweeps them out into the ocean currents.

4.       These new larvae hatchlings are called Zoea. They are tiny. At this tiny stage they float along as plankton, along with tiny shrimp and other crustaceans. A whale could eat them.

5.       Each zoea passes through 4 – 6 stages lasting 40-60 days each stage, as it gets bigger.

6.       Eventually the zoea grows or metamorphosises into a megalopa which looks like a combination of a

Adult Size

An adult Australian Land Hermit Crab can grow as large as a tennis or soft ball.

That’s not as bad as some species which can grow much larger than that.

hermit crab and a lobster. This stage takes about a month.

7.    At the end of the megalopa stage, it looks for its first mollusc shell and begins to spend longer periods out of the water. 

8.       Finally, the old megalopa buries itself in the sand to moult and metamorphosis into a juvenile hermit crab. The metamorphosis changes the crab’s gills so that they can no longer breathe in water, and must stay on the land to breathe air.

9.       A small hermit crab will need to moult to grow into a larger exoskeleton quite often, maybe every month as it gets bigger. New shells will have to be found to fit its abdomen into.

10.   Older land hermit crabs don’t need to moult as often, and really old hermit crabs might only grow a new exoskeleton once every year and a half.


Female Hermit Crab with Eggs. By Alcock in 1906 via Crab Street Journal[i]

Zoea Stages. Scale Measurement is 1.0 mm across. Diagram from BioOne [ii]

Lifecycle of Hermit Crab.
Diagram from Blue Crab lifecycle, Smithsonian Environmental Research Centre.[iii]

Lifecycle of Hermit Crab. Diagram from Pet Lovers Centre petstore online [iv]

[i] Crab Street Journal page [Online] on breeding

[ii] Zoea stages diagram from BioOne [Online]

[iii] Lifecycle of Blue Crab diagram from SERC [Online]

[iv] Lifecycle Diagram from Pet Lovers Centre [online]

Anatomy of Hermit Crab [Video]

posted Oct 3, 2011, 2:28 AM by November T   [ updated Oct 3, 2011, 2:34 AM ]

YouTube Video

Inside the Hermit Crab

posted Oct 3, 2011, 2:22 AM by November T

Circulatory System – crustaceans don’t have a heart. They have an open circulatory system . In this type of system vessels pump the animal’s blood into sinuses or cavities (holes) in the body. It is called an open circulatory system because the blood doesn’t flow in a closed loop like it does in a human’s closed circulatory system – which has a heart, arteries and veins to return the blood to the heart.

Respiratory (Breathing) and Gills – Land Hermit Crabs start off life in the oceans and, like fish, they take oxygen out of the water with gills. But when land hermit crabs move out of the ocean to live on land, the metamorphosis that takes place changes their gills to allow them to breathe in air. The land hermit crab’s gills have to be kept moist to keep breathing. That’s why they live in places with high humidity to keep their gills wet. If their gills dry out, they will suffocate and die. 

Nervous and Sensory System – Unlike humans who have a big brain and lots of nerves to send signals down the body, the hermit crab and other crustaceans have a much more primitive system. They have a primitive ventral nerve cord and ganglia system similar to an earthworm. Hermit Crabs have good vision thanks to their compound eyes on stalks (although can’t look below them) and sensory organs are found on the body. Like true crabs, hermit crabs have bristles and hairs all over their body – but mostly on their walking legs - which act as touch receptors. The long antennae of hermit crabs – longer than other crabs – have smell sensors, used to find their food. Smell detectors are also found on the legs. When a hermit crab walks over food, its legs smell it, and the chelipeds quickly grasp it and move it into the mouth.

Digestive System – Hermit crabs eat using their chelipeds (claws) and the maxillipeds or small mouthparts. The food is passed into their mouths and through their guts. Waste products are either excreted as urine through the antennal glands at the base of their antennae, or as feaces (poo) through their telson (anus) into their shells. They bump their abdomen around in the shell to move the poo out to the outside.

The above information was put together from multiple reading sources –

1.       Sense Organs – Victoria Museum’s website[i]

2.       Circulatory and Nervous System – Hermit Crab ebook (various authors and titles – see Books

3.       Gills  - various sources including Sue Fox : Hermit Crabs - A Complete Pet Owner's Guide - See book.

4.       Diagram below – anatomical diagrams available at Crab Street Journal – see Websites

[i] Southern Australian hermit crab distribution text from Victoria Museum website [Online}


Shells for Hermit Crabs

posted Oct 3, 2011, 2:08 AM by November T   [ updated Oct 9, 2011, 4:26 PM ]

What types of shells do Hermies like?

Interesting Fact

Above is a picture of a hermit crab in a glass shell. This shell was made by a man in Canada who sells glass shells[iii] for this reason.

Glass shells aren’t great for hermies, though, because they have to keep out of sunlight and glass would let that through and they might get too hot or dried out.

But it is interesting to see how a hermit crab fits in the shell.

Hermit crabs like gastropod mollusc shells like snail shells and others. They need to have a curve or coil in the shell so that the hermit crab’s abdomen can fit in to be protected.

The way hermit crabs use discarded snail shells is a type of symbiotic relationship – this is where there is a relationship between two different organisms. This type of symbiosis is called commensalism – where there is a relationship where one organism benefits (the hermit crab) and the other one (the snail) is not affected by this. [i]

Shells can either be found, fought for, or socially shell-swapped. See more about these in the behaviour section below.

The Northern Territories Government has a handout sheet for kids, which shows us what kind of shells hermit crabs like. [ii This Junior Nature Notes fact sheet says that hermit crabs like coiled shells like screw shells, telescope shells, periwinkle shells, baler shells, murex shells, and turban shells.

What is important also about shells is that the opening size is just the right fit for the hermie. The shell may be much bigger, but the opening has to be perfect. The picture below is originally from[iii]

In the oceans or brackish waters, hermit crabs can choose all sorts of shells or other things to live in. There are lots of cases of people noticing walking clumps of coral or shells with anemones on the back of them that look like they are moving around. Hermit crabs have used coral, sea sponges and shells with other creatures on them as houses. Now, with a lot of human pollution ending up in the oceans, there are also hermit crabs who are living in things like plastic cups, tin cans and other rubbish that shouldn’t be in the sea. 


Gastropod - Any of various molluscs of the class Gastropoda, such as the snail, slug, cowrie, or limpet, characteristically having a single, usually coiled shell or no shell at all, a ventral muscular foot for locomotion, and eyes and feelers located on a distinct head.

mollusc US, mollusk [ˈmɒləsk]n (Life Sciences & Allied Applications / Animals) any invertebrate of the phylum Mollusca, having a soft unsegmented body and often a shell, secreted by a fold of skin (the mantle). The group includes the gastropods (snails, slugs, etc.), bivalves (clams, mussels, etc.), and cephalopods (cuttlefish, octopuses, etc.)

Symbiotic Relationship – the hermit crab has a commensalism symbiotic relationship with sea snails – the hermit crabs uses discarded sea shells to live in, and this relationship does not affect the other organism (the snail).

[i] Commensalism and Symbiotic Relationships between hermit crabs and snails – learnt through a downloadable worksheet using hermit crabs as an example from GoodBuddies [online, pdf]

Hermit Crab project pdf from Northern Territories Government Junior Nature Notes

[ii] The Crabbage Patch website has now become the retail store The Hermit Crab Patch which provides an excellent knowledge base for hermit crab enthusiasts.This image is copywrite on the Hermit Crab Patch website, but can be found in several places across the web.

[iii] Glass Shells for Hermit Crabs available at


Autotomy and Why a Hermit Crab Might Lose Its Limbs

posted Oct 3, 2011, 2:01 AM by November T

Just like lizards who can lose their tails, a crab can lose its legs or claws, a process called autotomy. [i]

A hermit crab might lose its limbs for several reasons –

1. The crab may have been in a fight (shell-fight).

2. The crab may be ill.

3. The crab may be stressed (or PPS).

4. The crab might have injured itself from climbing.

Some lost limbs can be grown back (regenerated) again during moulting, but it takes several moults to get it fully back.

PPS = Post Purchase Stress

This term is used in hermit crab circles.

New hermit crabs are taken from their natural habitat (normally in the tropics) and put into small boxes and trucks for shipping, arriving at pet stores to be purchased by us as pets. Sometimes they don’t have the right conditions to look after them, and they can die several weeks after buying them, from PPS.

[i] Fox, Sue (2000) Hermit Crabs : Complete Pet Owner’s Guide. Barrons Books. N.Y.



posted Oct 3, 2011, 1:17 AM by November T   [ updated Oct 3, 2011, 2:00 AM ]

Hermit crabs moult because their hard exoskeleton does not grow with their body. When they are ready to moult they slow down, and often dig into the sand to protect them while they are vulnerable. Their old exoskeleton splits open and their new body comes out . They have to wait for their new skin to harden up for the next ten days or more, and are very vulnerable during this time. After moulting they often need to find a slightly bigger shell to live in also.

Hermit crabs normally eat their old exoskeleton as it contains chitin to help their new exoskeleton harden up.

The conditions for moulting have to be just right – dark, no surrounding hermit crabs which may be a threat while the moulting hermit crab is vulnerable, enough heat and moisture. If the right conditions are available, the hermit crab will secrete a moulting hormone and dig down to bury itself while moulting. If the conditions are not right, a different hormone is secreted and the hermit crab can delay moulting – but only for so long. Some hermit crabs which can’t delay any longer attempt to moult – sometimes on the surface instead of under the sand. Those that can’t moult in time, or moult in stressful conditions can easily die.

During moulting a hermit crab can grow back lost limbs.


Because hermit crabs are vulnerable while their new exoskeleton hardens, moulting can be a very stressful time for a hermit crab. If the hermit crab can’t bury itself in sand while moulting, it will sometimes be forced to moult on top of the sand.

At other times, it may not have access to a shell (or have lost its shell to a shell-fight).

This means the hermit crab is naked – without a shell – and this is called

Diagrams of a Hermit Crab

posted Oct 2, 2011, 11:11 PM by November T

These are labelled diagrams of the outside of a Hermit Crab, showing all the body parts.

Hermit Crab diagram by Aaron Baldwin,

Labelled hermit crab by Aaron Baldw from Geog.UBC.CA
Labelled Hermit Crab via Enchanted Learning

Labelled Hermit Crab from Enchanted Learning

Body Parts of the Hermit Crab

posted Oct 2, 2011, 10:24 PM by November T   [ updated Oct 2, 2011, 11:11 PM ]

Interesting Fact

When the young hermit crab moves out of the ocean and chooses its first shell to live in, the shape of this shell changes the shape of its abdomen, so some hermit crabs have long pointy abdomens, and some have short fat ones.

Abdomen – the abdomen in a hermit crab holds the crab’s digestive and reproductive systems. It is not covered by shell, so is vulnerable and need’s protection. That is why a hermit crab uses shells, and holds its abdomen inside a shell with uropods and its fourth and fifth legs. Antennae - The long sensory organs (feelers) located towards the front of the crab (each of the long pair of feelers is called an antenna; each of the short pair is called an antennule). Antennae are for touch or feeling around.

Antennule -  a small antenna, esp. one of the foremost pair of a crustacean. Antennule are for smelling things.

Carapace – The hard, protective outer shell of the crab. The carapace is made of chitin.

Cepalthorax  - The hermit crab's head, which contains its mouth, eyes and antennae, is fused to its thorax and is known as the cepalthorax.

Did you Know?

A hermit crab’s stomach is around the same size as its eye. That’s why it eats so little every day, so we can’t really see much eating going [v]

Compound Eyes -  eyes comprised of faceted lenses which are especially adept at picking up fine movements. Eyes are located on movable stalks at the top of the cephalothorax. 

Gills  -  A hermit crab's gills are enclosed in the branchial chamber, which functions as a lung. The branchial chamber is on the sides of the thorax, above the crab's legs. A hermit crab breathes through its gills and branchial chamber, which must be kept moist in order to function. If the branchial chamber and gills dry out, the crab will die. Compared to aquatic crabs, land hermit crab’s gills are reduced in size, and if the adults are kept underwater too long, they will drown. [i]

Legs – A crab’s five pairs of legs are used for different purposes.

·         At the end of the first pair – which are called Chelipeds, are the crab’s powerful claws.  A land hermit crab’s left claw is larger than its right claw. The left claw is used for climbing and defense. The right claw is normally used for eating and climbing. When a crab retreats inside its shell , it uses the left claw to block the shell opening, like an armoured door. 

·         The remaining four pairs of legs are called Periopods. A crab’s second and third pairs of legs are used for walking. The second pair of legs also function like the antennules, and can be used by the crab to detect food. However, they are not nearly as sensitive.

·         A hermit crab’s fourth and fifth pairs of legs are very small and never extend outside its shell. The crab uses these stubby legs to hold itself inside the snail shell and to manoeuvre the shell while walking. These specialised legs are covered in microscopic scales which help the crab grip its shell. [ii]

Maxillipeds  (Pronounced maks-il-li-ped)  - also known as feeding appendages, maxillipeds are very small appendages near your crab's mouth. They are like tiny little hands that take food from the chelipeds and feed the foodstuffs within the very small mouth. Maxillipeds are also used to groom and you will often see them when you handfeed your hermit crab.

Pleopod - The pleopods are small appendages located on the left side of the crab's abdomen. A female crab attaches her eggs to the fine setae on her pleopods using a gluelike substance. Male land hermit crabs also have pleopods, but they are much smaller and not nearly as hairy. Pleopods are also called Swimmerets or swimming legs.

Setae - sensory hairs found on the exoskeleton.[iii]

Telson – at the end of the crab’s abdomen is the telson, which contains the anus. A crab produces small feces that are deposited in its shell. By moving its abdomen about inside its shell, the crab causes the droppings to fall out. [iv]

Uropods – at the tip of the crab’s abdomen are small appendages called uropods. Although you will never see them, they do an important function – the uropods hook onto the spiral of the shell and help the crab grip its shell.

[iii] Setae definition and photos are available at Hermit Crab Patch


Exercise 1 :  Label the Body Parts

I was given a diagram of a hermit crab, and descriptions of body parts and had to label up the body of a hermit crab.

This exercise sheet can be found in The Project Documents section.

Once I’d labelled my own diagrams, I was given the answer sheet showing hermit crab diagrams which are posted in the next sections.

From this exercise I learnt  what the parts of the body are called and what they are used for.

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