Crabs are aquatic animals that belong to the group of decapod crustaceans (“ten-legged” crustaceans). Like other invertebrates with an exoskeleton, their body is made up of external parts (shell, claws, eyes, legs, antennae…) and internal organs (gills, digestive system, reproductive organs, nervous system…).
If you’ve ever wondered “What are the parts of a crab called and what do they do?”, this guide walks you through the main body parts of a crab and their functions, explained in clear, simple language.
Carapace (Shell)
The carapace is the hard, protective shell that covers most of the crab’s body, especially the cephalothorax (the fused head and thorax).
It is made of chitin and minerals, forming a rigid exoskeleton.
Works like a shield against predators, abrasion from rocks and sand, and impacts.
Provides attachment points for strong muscles used to move the legs and claws.
Because this shell is rigid, it does not grow with the crab. For the crab to increase in size, it must periodically undergo molting, shedding the old shell and forming a new, larger one.
Abdomen (Pleon)
The abdomen, also called the pleon, is located on the underside and rear of the crab.
In most “true crabs” (Brachyura), the abdomen is folded tightly under the body, forming a kind of flap.
On the inner side of this abdomen are small appendages called pleopods (swimmerets), which can be involved in reproduction or swimming depending on the species.
The shape and width of the abdomen often help distinguish males from females:
Females usually have a wider abdomen to hold and protect the eggs.
Males typically have a narrower, more triangular flap.
Claws (Chelipeds)
The claws, or chelipeds, are the first pair of legs (the first pair of pereiopods) and are highly specialized.
Located at the front of the body, they end in strong pincers with a movable “finger”.
Main functions:
Grabbing and tearing food
Defending against predators
Fighting rivals of the same species
Used in courtship displays and communication in some species
In many decapods, the two claws are not identical. One may be larger and adapted for crushing, while the other is more slender and precise, better for manipulating food or capturing prey. This is a form of functional or sexual dimorphism.
Walking Legs (Pereiopods)
Behind the claws, crabs have four pairs of walking legs, also called pereiopods.
These legs are jointed, allowing a wide range of movement.
They are attached to the body by segments with muscles anchored to the exoskeleton.
Their lateral position on the body is what allows the crab’s characteristic sideways walk.
In some species, the last pair of legs is modified for special functions, such as swimming or digging, but in most crabs they mainly serve for walking over the bottom substrate (sand, rocks, mud).
Eyes
Crab eyes are mounted on short stalks or peduncles that protrude from the carapace.
These stalks can move and extend, giving the crab a wide field of view without moving its whole body.
Crab eyes are compound eyes, made up of many tiny units called ommatidia.
They are very good at detecting movement and changes in light, which is essential for spotting predators and prey, even if the image is not as sharp as human vision.
The ability to retract or “hide” the eyes into grooves on the carapace also helps protect them from damage.
Antennae and Antennules
Crabs have two pairs of sensory appendages on the head:
Antennules – shorter pair
Antennae – longer pair
These structures act as sensors for the surrounding environment:
Detect chemical signals (smells/tastes) in water and on the seabed
Perceive vibrations, water movements and touch
Help locate food, mates and potential danger
Through their antennae, crabs can pick up chemical cues released by other individuals, which is important in mating, territorial behavior and social interactions.
Digestive Organs
Crabs have a relatively complex digestive system adapted to a varied, often omnivorous diet (small animals, algae, detritus and organic matter).
The main parts include:
Mouth and esophagus – where food enters and is passed to the stomach.
Stomach – often divided and equipped with a “gastric mill”: a set of hard internal plates that grind the food, acting like internal teeth.
Hepatopancreas (midgut gland) – the main digestive gland; it produces enzymes and absorbs nutrients, similar to a combination of liver and pancreas in vertebrates.
Intestine – carries out further digestion and absorption and leads to the anus where waste is expelled.
This system allows crabs to efficiently process different food types and play a key role as scavengers and recyclers in aquatic ecosystems.
Gills
Crabs breathe through gills, which are located in gill chambers on the sides of the cephalothorax, under the carapace.
Water flows through these chambers and passes over the gill filaments.
Here, oxygen diffuses into the hemolymph (the crab’s blood-like fluid) and carbon dioxide diffuses out into the water.
The gills are protected by the carapace but still in contact with the surrounding water, allowing effective gas exchange.
Thanks to this design, crabs can live in marine, brackish and freshwater habitats. Some species that spend time on land keep their gills moist to continue breathing outside the water.
Reproductive Organs
The reproductive organs of crabs are located in or under the cephalothorax, and their external structure often shows clear differences between males and females.
Males usually possess specialized appendages (modified pleopods) used to transfer sperm to the female.
After mating, females carry their fertilized eggs attached under the abdomen, forming a visible “egg mass” until the larvae hatch.
This arrangement allows reproduction to occur even in moving water or confined spaces, helping ensure that the eggs stay protected until they are ready to be released.
Nervous System
The crab’s nervous system is composed of:
A small brain (supraesophageal ganglion) located at the front of the body, above the esophagus
A chain of ventral ganglia (clusters of nerve cells) along the underside of the body
From this system, nerves extend to:
The eyes and antennae, for processing visual and chemical information
The legs and claws, to control movement
Internal organs, to regulate essential functions
Because many responses are controlled at the level of local ganglia, crabs can produce very fast reflexes, which is vital for escaping predators or grabbing prey in time.
Summary: Main Parts of a Crab and What They Do
Carapace – hard shell that protects the body and anchors muscles
Abdomen (pleon) – folded underside that protects reproductive structures
Claws (chelipeds) – used for feeding, defense and communication
Walking legs (pereiopods) – used for locomotion, typically sideways walking
Eyes – stalked compound eyes that detect light and movement
Antennae/antennules – sensory organs for smell, touch and water currents
Digestive organs – process food and absorb nutrients
Gills – carry out respiration in water
Reproductive organs – produce gametes and, in females, carry eggs
Nervous system – coordinates senses, movement and reflexes
Understanding the anatomy of a crab not only helps you identify its body parts, but also shows how each structure is perfectly adapted to its lifestyle as a bottom-dwelling, armored, and highly versatile invertebrate.
