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Hermit Crab vs. Crab (The Key Differences)

Hermit Crab vs. Crab (The Key Differences)

The purpose of this blog is to share general information and is written to the author's best knowledge. It is not intended to be used in place of veterinary advice. For health concerns, please seek proper veterinary care.

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When walking along the beach, you will often find little creatures with their cute sideways gait running around at the water’s edge. But are these Hermit Crabs or Crabs, and what is the difference?

Hermit Crabs have hard and soft exoskeletons. Crabs only have hard exoskeletons. A Hermit Crab relies on a shell to house its soft exoskeleton. A Crab’s exoskeleton does not need shelter. Hermit crabs walk on 4 legs, whereas Crabs use 8 legs. A Hermit Crab walks forward, and a crab walks sideways.

There are some similarities and differences between a Hermit Crab and a Crab. Read on to find out more interesting facts about these two crustacean species.

Hermit Crab vs. Crab, An Introduction

Hermit crabs and crabs are relatives, but although they look like crabs, hermit crabs are not true crabs. Instead, they are more closely related to some types of lobsters rather than a crab.

Hermit crabs are anomuran (permanently flexed abdomen) decapod crustaceans belonging to the Paguroidea family. There are about 800 species worldwide, of which almost all dwell in the sea. However, a few species of land hermit crabs are kept as pets.

Crabs are decapod (10 footed) crustaceans of the infraorder Brachyura family. More than 6 000 species of crab are found around the world. They live in the ocean, on land, and about 850 species dwell in freshwater.

Characteristics of the Hermit Crab vs. Crab

Below are some similarities and differences in the characteristics of the hermit crab and crab:

Body of the Hermit Crab vs. Crab

The front half of a hermit crab is covered with a hard exoskeleton. Its asymmetrical abdomen, which curves naturally to the right, is a softer exoskeleton.

This softer exoskeleton can adapt to occupy the shells of other organisms such as mollusks.

On the other hand, crabs have small tails and abdomens hidden behind the thorax under their body. They have a tough exoskeleton, so they are well protected from predators.

Male crabs have a narrow and triangular abdomen, whereas the female’s abdomen is more rounded.

Legs of a Hermit Crab vs. Crab

Hermit crabs have 5 pairs of legs. The first pair of legs are modified to form the pincers; the right side is larger than the other.

These pincers are shaped to act as a protective mechanism and close the shell entrance when the crab is inside.

The second and third pairs of legs are used for walking. The hermit crab uses the shorter fourth and fifth pair to grip the central column of its snail shell.

True crabs also have 5 pairs of legs. But they use four pairs for walking, and the strong front pair, are used to defend themselves.

These single pairs of claws (chelae) or pincers are the crab’s most important weapon. They use them for catching prey, opening hard shells to obtain food, and males use them for dominant fighting.

The Eyes of a Hermit Crab vs. Crab

Hermit crabs have two large compound eyes which can view at 360 degrees. It helps them to detect motion and gives them good night vision.

Crabs have a pair of primary compound eyes consisting of several thousand optical units on independently moving eye stalks to track movement. Crabs also have additional primitive eyes on top of their shell.

These eyes detect UV light from the sun and reflected light from the moon. Crabs follow the lunar cycle when spawning.

Antennae of the Hermit Crab vs. Crab

A hermit crab has 2 pairs of antennae. The longer outer pair are their sensory organs to touch and feel what’s in their path.

An inner bent pair of antennae enables the hermit crab to taste and smell.

Crabs also have antennae to smell and sense chemicals in the water and feel current and movement.

Mouthparts of a Hermit Crab vs. Crab

The maxillipeds or mouthparts of a hermit crab consist of 3 pairs of appendages. These maxillipeds are used to move food into the mouth and for grooming.

Crabs also use their mouth to ingest their food as the hermit crab. Their mouthparts are soft and feathery, but sometime of a harder substance in some crabs.

Crabs have teeth in the stomach to break up the food for digesting.

Breathing of the Hermit Crab vs. Crab

Hermit crabs and crabs breathe with gills like fish. These gills extract the oxygen that dissolves in water as the water flows over the crab.

Crabs that live on land can still absorb oxygen through their gills if the gills are kept moist. Hermit crabs on land store water in their bodies to keep their gills moist.

How Long Do Hermit Crabs vs. Crabs Live?

Hermit crabs live for more than 30 years in their natural habitat. The estimated lifespan for crabs is between 3 and 12 years.

Reproduction of Hermit Crabs vs. Crabs

Hermit crabs mate in seawater and only twice a year. The male and female will emerge partially from their shells and place their stomachs together to mate.

The female carries between 800 to 50,000 eggs under her abdomen, and then when they are ready to hatch, she releases her young into the sea. Most larvae will not reach puberty as they will be consumed by predators.

Crabs are male and female, and mating only occurs when the female has molted her shell and while the new shell is still soft. The female can store the male sperm for a long time until she thinks it is an opportune time to fertilize it.

Female crabs can fertilize up to 200,000 eggs which she carries with her for several weeks until they hatch. The larvae can swim away without any help from their mother. Still, these larvae are vulnerable to predators, and many will not survive.

The Molting Process of Hermit Crab vs. Crab

Hermit crabs molt a few times as they grow into adulthood. They shed their exoskeleton, and for a short while, the hermit crab cannot move until the new exoskeleton shell hardens.

Before molting, the hermit crab will bury itself entirely in the sand for protection. The surrounding darkness under the sand is necessary to begin the molting process (ecdysis).

When they reach adulthood, they will continue to molt every 18 months. This can differ in the size and species of hermit crabs. As they grow, the shell that they are housed in becomes too small, and they will look for a larger shell to live in.

The molting cycle in crabs is coordinated by hormones, and a young crab will molt many times as it grows into an adult. During molting, the old shell becomes soft and will partly erode.

The crab will have to extract its body parts from this old shell. This is a slow process and can take several hours.

The crab’s new shell is soft at first, making it very vulnerable, so it will hide until its shell has hardened.

Facts of Hermit Crabs vs. Crabs

Hermit Crabs use their legs to walk, bending them backward and forwards while holding up their abdomen. If they have large and heavy shells, they will drag their bellies across the sand as they walk.

Crabs mostly walk sideways because of the articulation of their legs, although some crabs do walk forwards and backward.

Other interesting facts about the Hermit Crab vs. Crab are:

Where Do Hermit Crabs vs. Crabs Live?

Some hermit crabs live entirely in saltwater, while land hermit crabs live in tropical areas near the shoreline.

Land hermit crabs rely on seawater to wet their gills and the interiors of their shell and for reproduction. The early phases of their lives are spent in water.

Depending on the species, Crabs live in estuaries, coral reefs, and some live within the high and low tides of the sea.

Some crab species can be seen running on land, and some even climb palms. When on land, they will dig deep under the sand to find water.

What Do Hermit Crabs vs. Crabs Eat?

Hermit crabs are omnivores and will eat whatever they find. However, they enjoy feasting on microscopic clams and mussels, bits of dead animals, and macroalgae.

Crabs are also omnivores and eat almost anything. Their primary diet is algae, mollusks, other crustaceans, worms, fungi, and bacteria. They use their claws to grab the food and bring it to their mouth.

Final Thoughts

Hermit crabs are different from true crabs, although they have some similarities. Hermit crabs have a hard and a soft exoskeleton.

The soft exoskeleton is housed in discarded shells of mollusks and other sea animals. On the other hand, crabs have a hard exoskeleton that covers the entire body protecting its inner organs.

Both crab species have 5 pairs of legs, but the hermit crab only uses 4 legs for walking while the crab uses 8 legs. Hermit crabs walk in a forward movement, whereas crabs walk sideways.

The hermit crab and the crab display a molting process during their growth cycle. At this stage, both crabs will outgrow their old exoskeleton. A new softshell will emerge, which will harden after a few days.