Crab Anatomy

Crab anatomy is built around two innovations that no vertebrate uses: a hard outer skeleton made of chitin and protein that has to be shed and rebuilt to grow, and a body plan that fuses the head and the thorax into a single block (the cephalothorax) covered by a wide flat shield (the carapace) with the abdomen tucked underneath.

The crab (around 7,000 species in the infraorder Brachyura, “short-tailed”) carries five pairs of legs (the same decapod plan as lobsters and shrimp), with the front pair modified into pincers (chelipeds) and the four rear pairs used for walking. The eyes sit on stalks that fold into sockets in the carapace. The gills hide in a chamber under the carapace, fed by water pumped through inlet openings at the base of the legs. Sizes run from the 1 cm pea crab (Pinnotheres) that lives inside oyster shells to the Japanese spider crab (Macrocheira kaempferi) at 4 m leg span, and every species shares the same chitin-shell, fused-head-thorax, decapod-leg, gill-chamber body plan that has dominated the seafloor and the wet edge of land for around 200 million years.

True crabs sit in the infraorder Brachyura inside the order Decapoda inside the class Malacostraca inside the subphylum Crustacea. Decapoda includes lobsters, crayfish, shrimp, prawns, hermit crabs, and the squat lobsters; the hermit crabs and king crabs sit in the related infraorder Anomura, the “false crabs”. The shared decapod plan (ten legs, fused cephalothorax, paired antennae, stalked compound eyes, gills under a carapace) covers all of these. The Brachyura are split from the rest by the rear-folded abdomen: where a lobster carries a long visible muscular tail at the back of its body, a true crab’s abdomen is small, plate-like, and folded forward under the cephalothorax, hidden from sight unless the animal is flipped over. The shape of the folded abdomen is one of the easiest ways to sex a crab: a male’s abdominal flap is narrow and triangular, a female’s is wide and rounded, with a hinged door under which the eggs are carried.

The Exoskeleton: Chitin, Calcium, and the Moult

A crab’s outer skeleton is made of chitin (a polysaccharide chemically similar to plant cellulose) cross-linked with proteins and reinforced with calcium carbonate that the crab pulls from seawater and food. The result is a rigid armoured shell that protects the crab from predators and provides attachment points for the muscles. The carapace (the wide flat shield over the cephalothorax) carries the heaviest reinforcement and can take the bite of a small fish without cracking. The leg segments are tube-shaped, with thinner shell at the joints to allow movement. The abdomen, hidden under the body, carries a much thinner softer shell because it does not need to take impact.

A rigid outer skeleton cannot grow with the animal, so the crab grows by moulting (ecdysis). The crab pulls calcium back out of the old shell into the body in the days before the moult, then secretes enzymes that break down the inner layer of the old shell, then splits the carapace open along a weakness line at the rear edge. The crab pulls itself out through the opening, head first, then legs, and finally the claws (the largest body parts to extract; the crab has to compress the muscle and pull each claw through a narrow joint, which can take an hour or more for a large crab). The new shell underneath is soft, pale, and elastic; the crab pumps water into the body through the gut to inflate the new shell to the size the crab will grow into, then sits in shelter for several days while the new shell hardens. During the soft-shell stage, the crab is defenceless and is the prey form sought by humans for soft-shell crab cuisine. A small crab moults 5 to 10 times in its first year and 1 to 2 times per year as an adult; the largest species moult less.

Body Plan: Cephalothorax, Carapace, and the Hidden Abdomen

The crab body is divided into two regions: the cephalothorax in front (head and thorax fused into a single block) and the abdomen behind. The cephalothorax carries the head structures (eyes, antennae, mouthparts) and the thorax structures (the bases of all five pairs of legs and the gill chamber). The carapace is the dorsal (upper) shield that covers the cephalothorax. Its shape varies across species: round (shore crab), oval (Dungeness), wide and laterally extended (blue crab, swimming crabs), narrow and triangular (spider crabs), almost square (shore crabs of the genus Pachygrapsus). The width of the carapace is the standard measurement used to record crab size.

The abdomen is small, plate-like, and folded under the cephalothorax against the rear of the body. In a true crab the abdomen carries no swimming function, no walking function, and no defensive function; it serves only to hold the reproductive openings and (in the female) to carry eggs. The folding pattern is what gives the order Brachyura its name (Greek brachys “short” and oura “tail”). The abdomen of a true crab is the same body region as the muscular swimming tail of a lobster or a shrimp, reduced to a folded flap. The flap unfolds during reproduction so that the female can hold a clutch of fertilised eggs against her body for the weeks or months until they hatch into larvae.

Legs and Locomotion: Sideways Walking and the Decapod Count

A crab carries ten legs in total, the decapod count. The first pair is modified into chelipeds (the claws), used for defence, prey capture, food handling, and signalling. The remaining four pairs are walking legs (pereiopods). Each leg has seven segments (coxa, basis, ischium, merus, carpus, propodus, dactylus) joined by hinge joints. The sideways gait that defines the group comes from the leg geometry: the leg joints flex inward and outward in the horizontal plane only, with no fore-and-aft swing of the kind that drives a forward walk in a vertebrate. The crab leads with one set of legs and trails with the other, lifting and placing the legs in alternation. The result is a sideways scuttle that is the fastest way for a crab to move on solid ground and the only direction the limb geometry supports. Most crabs can pivot in place to change facing direction, but the actual translation across the seafloor or beach is sideways.

The fifth pair of legs is modified in the swimming crabs (family Portunidae, including the blue crab, the green shore crab of the Pacific, the Atlantic ghost crab, and the Mediterranean swimming crabs). The dactylus (the last segment) is flattened into a paddle shape, and the crab uses the paddle to swim in short bursts through open water as well as walk on the seafloor. Most other crabs cannot swim at all and walk on the bottom. A few crabs (the kelp crabs, some spider crabs) climb on stalks of seaweed or on coral surfaces using the same sideways gait scaled to a vertical surface. Land crabs (the coconut crab Birgus latro of Indo-Pacific islands, several species of Cardisoma and Gecarcinus on Caribbean and tropical Pacific coasts) walk and climb on land, return to seawater only to release larvae, and breathe with modified gill chambers that hold a moist air-breathing surface.

Chelipeds: The Two Claws

The two front limbs (the chelipeds) are the crab’s main tools. Each cheliped ends in a pincer made of a fixed lower finger (the dactylus closes against it) and a movable upper finger (the dactylus itself), powered by a heavy muscle inside the propodus. The pincer is used for defence (a snapping cheliped can sever a small fish), prey capture (gripping mussels, snails, smaller crabs), food handling (passing food to the mouth a piece at a time), and display (males waving the chelipeds to signal aggression or court mates). The mechanical advantage is high: the muscle pulls a short lever (the dactylus close to the joint) against a long lever (the closing tip), so a small muscle delivers a large force at the tip.

Crushing force scales with body size. A shore crab cheliped delivers around 5 N of pinch; a Dungeness crab around 60 N; the coconut crab, the largest land crustacean alive, delivers up to 3,300 N (around 740 lbf), the highest pinch force of any animal and enough to crack open a coconut shell or sever a small bone. The two chelipeds are unequal in size in many crab species, with the right cheliped larger in some and the left in others; the larger claw (the “crusher”) is used to break open hard-shelled prey, the smaller (the “cutter”) is used to handle and dissect soft tissue. The fiddler crabs (genus Uca) take this asymmetry to the extreme: the male carries one giant cheliped that can be a third of his body mass, used for waving displays and mate competition, while the other cheliped is small and used for feeding. Females of the same species carry two small chelipeds.

Eyes, Antennae, and the Stalk Mechanism

A crab’s two eyes sit on the ends of moveable stalks that fold into sockets in the front edge of the carapace. The stalk lifts the eye above the body so the crab can see in all directions, including up and over its own carapace, without rotating the body. Each eye is a compound eye built from up to a few thousand individual visual units (ommatidia) packed in a hemispherical array. A single ommatidium is a narrow tube with a lens, a crystalline cone, and a cluster of light-sensitive cells (rhabdom). Each ommatidium captures light from a narrow cone of space, and the brain assembles the field of view from the array. Compound eyes are poor at fine detail (a crab cannot resolve the patterns a vertebrate eye can read) but good at detecting motion, polarised light, and the shape of large objects, and the panoramic field of view is a major defensive advantage. Several deep-sea crabs and the cave crabs have lost the eyes altogether and live in permanent darkness.

The two pairs of antennae sit between and below the eye stalks. The first pair (antennules) is short and forked and carries chemoreceptors used to detect food smells dissolved in the water and to detect the chemical signals of other crabs. The second pair (antennae) is longer, thinner, and carries mechanoreceptors used to detect water currents, vibrations, and physical contact with surrounding objects. Crabs also detect water-borne sound (vibration in the surrounding water) through statocysts, paired chambers in the base of the antennules that contain a small grain of calcium carbonate (the statolith) sitting on a bed of sensory hairs. The statolith is a borrowed sand grain in some species (the crab takes one from the seafloor and inserts it during a moult) or a self-secreted calcium grain in others. Movement of the body shifts the grain across the hair bed, and the brain reads the shift as orientation and acceleration information.

Gills and Respiration: The Branchial Chamber

A crab breathes through gills hidden in a pair of branchial chambers under the lateral edges of the carapace. Each chamber holds 6 to 9 feathery gill arches that branch off the bases of the legs. The chamber is sealed against the body except for an inlet at the base of each cheliped (the inhalant channel) and an outlet at the front of the head (the exhalant channel above the mouth). A small bailer-like appendage (the scaphognathite, attached to the second maxilla) beats inside the chamber and draws water in through the inlets and out through the outlet at a steady rate. Oxygen diffuses across the gill surface into the blood (haemolymph) and carbon dioxide diffuses out. A typical pump rate is around 100 strokes per minute in a resting crab, rising to 400 in a stressed or active animal.

Land crabs and intertidal crabs have rebuilt the gill chamber for air breathing. The shore crabs of rocky beaches keep the chamber damp and use the inner wall (richly supplied with capillaries) to extract oxygen from air. The coconut crab has lost the underwater gill function altogether: the gill chamber is filled with a spongy modified lung tissue (the branchiostegal lung), the crab cannot breathe underwater after juvenile life, and an adult coconut crab placed in the sea will drown. Several other land crab species sit between these extremes, breathing both air and water depending on conditions and on the moisture state of the gill chamber. Hermit crabs occupy the same intertidal range and use moist gill chambers, with the snail shell that the hermit lives inside doubling as a humidity reservoir.

Internal Anatomy: Open Circulation, Hepatopancreas, and the Stomach Mill

A crab does not have a closed circulatory system the way a vertebrate does. The heart sits in the rear of the carapace and pumps haemolymph (the crustacean equivalent of blood) out through arteries that empty into open body cavities (haemocoels). The haemolymph bathes the organs directly, then collects back into venous channels, passes through the gill chamber to pick up oxygen, and returns to the heart. The oxygen-carrier in the haemolymph is haemocyanin, a copper-based protein that turns blue when oxygenated and clear when deoxygenated, against the iron-based haemoglobin that turns red and dark red in vertebrates. A live crab’s exposed gill chamber shows the blue colour clearly; the haemolymph itself is clear when the crab is bled.

The digestive system is built around two organs unique to crustaceans. The stomach contains a gastric mill, a set of three large internal calcified teeth (one dorsal, two lateral) that grind food after the crab has swallowed it. The crab handles food at the mouth, breaks it into bite-sized pieces with the mandibles, swallows the pieces whole, then grinds them in the stomach. The hepatopancreas (also called the digestive gland) is a large brown organ that fills much of the cephalothorax cavity and combines the functions of the liver and the pancreas in a vertebrate: it secretes digestive enzymes into the stomach, absorbs the digested nutrients from the intestine, stores fat and glycogen, and produces the moult-controlling hormones. The hepatopancreas is the part of a crab that humans eat as “tomalley” or “crab butter” in cooked crabs.

Crab vs Hermit Crab vs Lobster: Anatomy Comparison

True crabs (Brachyura), hermit crabs (Anomura), and lobsters (Astacidea, Achelata, Polychelida) all share the decapod body plan but differ in the abdomen, the legs in use, and the body symmetry.

For broader invertebrate anatomy, see comparable pages on the Jellyfish Anatomy, Octopus Anatomy, Spider Anatomy, and the broader Insects hub. Browse the full Animal Anatomy hub for comparable species.

Sources & References

• World Register of Marine Species (WoRMS): Brachyura
• IUCN Red List, Brachyura species accounts
• Smithsonian Ocean, Crabs
• Animal Diversity Web, Brachyura
• Wikipedia, Crab