Crab External Anatomy

Crustaceans are a highly diverse group of arthropods with respect to morphology, physiology, and life history. Due to the fact that crabs comprise several thousand species and the number keeps growing, it is absolutely not possible to mention all the differences between them in one article. Therefore, I will describe the crab anatomy in general.

All crabs are ten-footed crustaceans, or decapods (the name Decapoda from the Greek means “10 legs”). Crab’s body is generally covered with a thick exoskeleton, composed primarily of highly mineralized chitin, eight walking legs, and two grasping claws (chelae).  Unlike shrimp, lobsters, and crayfish, in crabs, the abdomen is wrapped under the body (cephalothorax) and is not visible dorsally. 

In this article, I am going to talk about the external anatomy of the crabs in detail. So, keep reading for everything there is to know about these amazing creatures. 

Interesting fact: Crab-like creatures date back to the Late Devonian Period, about 365 million years ago.          

Quick Notes about Crab External Anatomy

Scientific name	Common name	Function
Crab’s cephalothorax consists of
Cephalic	Head	Contains a few nerve cell clusters (cerebral ganglion or ganglia).
Rostrum	Beak or nose	Additional protection of eyes
Thoracic	Chest, Carapace or Upper body	Protection of inner organs.
Antennae	Long whiskers	The sensor of orientation and coordination.
Antennule	Short whiskers	The sensor of chemical information (food, gender discrimination, etc.)
Maxillae	Jawfoot	Help to eat and to draw water over the gills.
1st, 2nd, 3rd Maxillipeds	Jawfoot	Eating (rummage and bring food to the mouth).
Mandibles	Jaws	Hard, powerful cutting jaws.
Pereiopods	Walking legs	Movement
Chelipeds	Claws	Holding and picking food. Defense and/or aggression.
Eyes	Eyes	Vision
Crab’s abdomen consists of
7 Abdomen segments	Stomach	Protection of inner organs.
Telson	Tail	Movement (coordination)

Crabs Body Structure

Like most crustaceans, crabs have a hard outer covering called an exoskeleton made of chitin. This exoskeleton provides protection from predators and the rigors of the physical environment (like cold, heat dryness, etc.). It also serves as the basis for the attachment of the muscle system.

Even though technically, their body is divided into two main parts – the cephalothorax and the abdomen, in crabs, the abdomen is the flap that is flexed underneath the body.

1.     The cephalothorax

The cephalothorax (cephalic+ thoracic) consists of the cephalic (or head) region and the thoracic (or chest) region.

The hard dorsal and lateral covering of the head and thorax of the body is also called the carapace. It protects their vital organs of the crab (brain, heart, stomach, bladder, testicular, or ovarian) from above but not ventrally.

Note: Many crab species have some kind of groove, that separates the head and chest regions. However, this separation is nominal because head-chest areas are basically ‘fused’ together. It means that the crab absolutely cannot turn its head.

Head

Crabs are characterized by a joined head and chest. The head segment (cephalic) has several appendages:

• the 1st and 2nd antennae (antennule and antenna), and
• all mouthparts (mandible, 1st and 2nd maxilla, 1st to 3rd maxilliped).

It also bears the (usually stalked) compound eyes.

Chest

The chest segment (thoracic) is the most calcified upper part of the crab’s body. It is thicker than the shell elsewhere on the crab and connects to 5 pairs of appendages (peraeopods).

• In most cases, the first peraeopods have enlarged pincers (chelae) and are therefore called chelipeds.
• The next four pairs of pereiopods are mostly used for walking and sometimes for gathering food. 

Stalked Eyes

The eyes are located on the short, independently movable, and adjustable eyestalks. Crabs can fold their eyes into sockets in the shell for protection. Having eyes on stalks also means crabs have a better field of vision.

Crabs have compound eyes, which give them panoramic vision. Even though compound eyes do not focus well, they are very sensitive. It helps them to detect UV light in low light conditions and notice the slightest movements around them.

The eye-stalks of the crabs produce a special hormone that regulates the metabolism and molting process in the body.

Interesting fact: Crabs can regenerate their lost limbs (claws or legs) with every molting process. However, unlike other body parts, they cannot regenerate their eyes.

Antennae and Antennule

Crustaceans like crabs generally have two pairs of antennae (also called “feelers”):

• Two sets of short antennules (first antennae).
• One set of long antennae (second antennae).

Note: Compared to crayfish and shrimp, their antennae and antennule are pretty small. Even more, in some species, they are so small that we can barely see them at all.

They use long antennae to orientate in murky environments and gather tactile information. Antennae can easily catch water vibrations around the crab.

The short ones also help crabs to assess the suitability of food and provide chemical information (“taste” and “smell”) about what they are touching. That is why they are also called chemoreceptors.

Mouthparts (Mandible, Maxillae, and Maxilliped)

Crabs have modified feeding appendages called mandibles and maxillae.

Maxilliped (jawfoot, and foot jaw Origin: [Maxilla + L. pes, pedis, foot.]) is one of a pair of 3 sets of mouth appendages on the head of the crab modified to rummage and bring food to the mouth.

One of the functions of the 3^rd maxilliped is to protect the more delicate appendages anterior to it. The 1^st and 2^nd maxillipeds are smaller, they primarily take part in the feeding process. They also pump water over the crab’s gills, so the crab can breathe while eating.

Maxillipeds are almost in constant movement as crab finds and manipulates tiny morsels of food to its mouth. They are fused to each other and to the head. 

The mandibles are heavily calcified and equipped with powerful muscles. They are the most anterior of the mouthparts. The mandibles shred food into pieces. Opposed to a human’s jaw, the crab’s mandibles move side to side.

Pereiopods (Legs) and Chela

Crabs have 5 pairs of jointed legs attached to the thorax. However, they have a different function:

• the first pair of pereiopods have claws/pincers (chelipeds). Crabs use them for catching food, defending, and fighting between themselves.
• the next 4 pairs of pereiopods are used for walking as well as for “sensing” the environment. In some aquatic crab species, the last pair of pereiopods are flattened, it lets them push themselves through the water and move faster. This pair is often called swimming legs.

Crabs typically walk sideways. This is because their legs are located on the sides of their bodies and their joints point upwards. In addition, at some point in evolution, sideways movement is quicker and more efficient.

Interesting fact:  Typically most crabs have one crushing claw and one cutting claw.

Note: Although various other crustaceans aka false crabs (like hermit crabs, porcelain crabs, etc.) superficially resemble true crabs but have only three pairs of walking legs.

2.     The abdomen.

In crabs, the abdomen is located beneath the cephalothorax and usually includes the 7-segmented pleon (abdominal segments).

The small, triangular, terminal portion is the telson (the tail), which is not a true segment.

The segmentation of the thorax is apparent ventrally where it is not covered by the carapace. As with cephalothorax, these segments are partially or completely fused. It means that crabs sacrificed their swimming ability (compared to shrimp and crayfish) for flexibility and mobility.

How to determine the sex of the crab from the shape of the abdomen?  

In mature females, the abdomen is broad (wider and oval) with convex sides and covers most of the posterior ventral surface of the thorax. Whereas the abdomen of males is very narrow although it has a broad base.

In Conclusion

All crab species have segmented bodies (up of 20 body segments grouped into two main body parts the cephalothorax (head and chest) and the abdomen). Crabs have wide, flat bodies with no obvious tail. The head and thorax are merged together under the carapace.

Crabs are ten-footed crustaceans or decapods. They have 5 pairs of legs: four pairs of walking legs and one pair of front legs are called chelipeds, they have a pair of claws on them. At the rear of the thorax is the crab’s abdomen, which is no more than a small flap in most crab.

You do not have to be a master of anatomy and physiology or know every single piece of the body but it is certainly to your advantage to learn the main body parts.

Related articles:

• Crayfish External Anatomy
• Crayfish Internal Anatomy
• How to Tell If a Crab is Male or Female
• Dwarf Shrimp External Anatomy
• Dwarf Shrimp Internal Anatomy
• How to Set Up a Freshwater Crab Tank