Crab Anatomy: Labeled Diagrams and an Interactive 3D Blue Crab

A crab is a decapod crustacean: an arthropod with a jointed exoskeleton and ten legs. True crabs belong to the Brachyura, a name that means "short tail". Their abdomen is greatly reduced and tucked completely under the body, and they have lost the tail fan of a lobster or crayfish (Queensland Museum). It is never used for swimming (FAO guide). The rest of the body is a cephalothorax, the fused head and thorax, covered by one shield, the carapace.^{[25, 16, 19, 18]}

This page describes the Atlantic blue crab, Callinectes sapidus, a swimming crab (family Portunidae) of muddy and sandy shores from Massachusetts to Texas (Van Engel, 1958). Its carapace can reach about 9 inches (23 cm) across (Chesapeake Bay Program).^{[1, 21, 2]} Other true crabs have the same organs in the same places; the main differences are listed under other crabs.

Directions used below: anterior is the front with the eyes, posterior the back edge of the carapace, dorsal the top of the shell and ventral the underside, where the sternum and the folded abdomen lie.

The external anatomy of a crab includes the carapace with its frontal teeth, orbits, anterolateral teeth and lateral spines; two stalked compound eyes; two pairs of antennae; the mouthparts; a pair of chelipeds with claws; three pairs of walking legs; and, in swimming crabs, a pair of paddle-shaped swimming legs.^{[2, 16, 21]}

The top of the carapace is divided into regions named after the organs beneath them: the gastric region over the stomach, the cardiac region over the heart and the two branchial regions over the gills (Stachowitsch glossary). Along the front margin, the blue crab has two broad triangular frontal teeth between the eyes, a key feature that separates it from other Callinectes species. On each side are nine anterolateral teeth: the first is the outer orbital tooth beside the eye and the ninth is the long lateral spine (Williams, 1974; Chesapeake Bay Program).^{[18, 2, 21]}

Under the front edge, the eyestalks lie in the orbits and the short antennules fold into pits called antennular fossae. In true crabs the antennae are short, and the outer maxillipeds are "operculiform": they close over the mouth like a pair of lids (Miers, 1886). The sides of the carapace curve down and in as the branchiostegites, the outer walls of the two gill chambers.^{[17, 18]}

A crab's appendages are, from front to back: antennules, antennae, mandibles, maxillules, maxillae, three pairs of maxillipeds, the chelipeds, three pairs of walking legs, the swimming legs and, under the abdomen, the pleopods. Each walking leg has six segments: coxa, a fused basis-ischium, merus, carpus, propodus and dactyl (FAO guide).^{[16, 18]}

This is the text version of the 3D model: every labeled structure of the blue crab, grouped by body system, with what it does.

When the carapace is lifted off a crab, you see the stomach in the front middle, the heart just behind it, the yellow to brown hepatopancreas filling both sides, the gonads lying on top of it, and the feathery gills in a chamber on each side. The brain and antennal glands are at the very front, and one large thoracic ganglion sits on the floor of the body.^{[5, 4, 9]}

This layout comes from dissections of the related blue swimmer crab, where mature orange ovaries can completely hide the hepatopancreas and the intestine runs back into the folded abdomen to the anus on the telson (Tahon & Shaker, 2023).^[5]

Crabs breathe with gills hidden in a gill chamber under each side of the carapace. A blue crab has eight gills per side. A paddle on each maxilla, the scaphognathite, beats to draw water in near the bases of the claws, across the gills and out of exhalant openings beside the mouth.^{[6, 18]}

Wortham & Pascual (2017) counted eight gills and three gill-cleaning epipods in each blue crab gill chamber; the related blue swimmer crab also has eight (Tahon & Shaker, 2023). Crab gills are phyllobranchs: flat, leaf-like lamellae stacked along a central axis. The back gills also regulate salts (Felgenhauer, 1992). Cooks call them "dead man's fingers"; they are not poisonous (Maryland Sea Grant).^{[6, 5, 4, 22]}

A crab's heart lies in the middle of its back, just under the cardiac region of the shell and behind the stomach. It is a flattened sac inside the pericardial sinus. Blood enters it through valved openings called ostia and leaves through seven arteries.^{[5, 18, 4, 3]}

Using resin casts, McGaw & Reiber (2001) mapped the seven arteries of the blue crab: the anterior aorta to the brain and eyes, two anterolateral arteries, two hepatic arteries, the small posterior aorta to the abdomen and the sternal artery. The sternal artery, the largest, drops straight down from the heart and feeds the ventral thoracic artery, which supplies every leg and the mouthparts. They found the arteries dividing into capillary-like vessels, and the return sinuses to be "discrete channels rather than random open spaces", so crab circulation is more organised than the textbook "open system" suggests. In crustaceans the heart is the first organ to receive oxygenated blood from the gills (Maus et al., 2020). Oxygen is carried by hemocyanin, a blue, copper-based respiratory protein that is made in the hepatopancreas (Todorovska et al., 2021).^{[3, 7, 8]}

Food torn up by the six pairs of mouthparts passes up a short oesophagus into the cardiac stomach, where the gastric mill grinds it. The pyloric stomach filters the result: the finest particles go to the hepatopancreas for digestion and absorption, and the rest travels along the midgut and intestine to the anus.^{[15, 4, 5]}

The gastric mill is a set of three teeth: a median tooth on the urocardiac ossicle and two lateral teeth on the zygocardiac ossicles (Felgenhauer, 1992). It is driven by the stomatogastric ganglion, a cluster of about 26 neurons (Otopalik et al., 2017) that generates two stomach rhythms, the fast pyloric rhythm and the slower gastric mill rhythm (Haley et al., 2018). The yellow "mustard" in a cooked crab is the hepatopancreas, not fat; Maryland Sea Grant advises against eating it because chemical contaminants concentrate there. Because the stomach lining is part of the exoskeleton, crabs stop feeding shortly before they molt (Van Engel, 1958).^{[4, 11, 12, 22, 1]}

Yes. A crab's brain, the supraoesophageal ganglion, sits at the front of the head above the oesophagus. Two connectives run around the gut to one large thoracic ganglion on the floor of the body, formed by fusion of the whole ventral nerve cord. Nerves fan out from it to every leg.^{[4, 9]}

In brachyuran crabs the suboesophageal, thoracic and abdominal ganglia are fused into one thoracic ganglion (Kotsyuba & Dyachuk, 2026). A long-tailed decapod such as a crayfish keeps a ladder-like cord with a ganglion in each abdominal segment (Felgenhauer, 1992). The main sense organs are the stalked compound eyes and the antennules, whose olfactory sensilla give the crab its sense of smell; contact chemoreceptors on all the appendages let it taste what it touches (Krieger et al., 2015).^{[9, 4, 19, 10]}

Crabs excrete with two antennal glands, also called green glands, at the front of the head. Each filters the blood in an end sac, adjusts the salts in a labyrinth and canal, stores urine in a bladder, and releases it through an excretory pore on the base of the antenna (Felgenhauer, 1992; Stachowitsch glossary).^{[4, 18]}

The quickest way to sex a blue crab is to turn it over. A male has a narrow apron shaped like an inverted T. A mature female has a broad, rounded apron, and an immature female a triangular one. Watermen call males "jimmies", immature females "sallies" and mature females "sooks" (Maryland Sea Grant; Chesapeake Bay Program).^{[21, 22, 1]}

Sources: Van Engel (1958), Williams (1974), Felgenhauer (1992) and Tahon & Shaker (2023). A female mates once, right after her final molt while her shell is soft, and the male carries her beneath him before and after. Eggs are fertilised as they pass the stored sperm and are glued to the pleopods as a "sponge" of about 700,000 to over 2 million eggs (Van Engel, 1958; Chesapeake Bay Program: 750,000 to 2 million). Animal Diversity Web gives 2 to 8 million.^{[1, 23, 21]}

A crab grows only by molting. It forms a new, soft shell under the old one, cracks the old shell open along fixed lines, backs out, swells with water and then hardens the new shell. Each molt adds about a quarter to a third to the crab's width (Van Engel, 1958).^{[24, 1]}

Sources disagree on how many molts it takes to grow up: Van Engel (1958) counted 18 or more after the larval stages, Animal Diversity Web gives 18 to 20, and LSU says a crab may shed "as often as 25 times". Small crabs molt every 3 to 5 days, crabs over 4 inches every 20 to 50 days. About 5% of the old shell's calcium is stored, mainly in the hepatopancreas; the rest of the new shell's minerals come from water and food (Van Engel, 1958). Molting is held back by molt-inhibiting hormone from the sinus gland in the eyestalks, which suppresses the Y-organs that make the molting hormone ecdysteroid (Legrand et al., 2021; Kozma et al., 2023). Larvae hatch as zoeae, with four or five zoeal stages according to Van Engel (1958) and seven according to Animal Diversity Web, then become a megalops before the first crab stage.^{[1, 23, 24, 13, 14]}

All true crabs share the blue crab's body plan, but details differ between groups, and several animals called "crabs" are not true crabs at all.^{[19, 20]}

Compared with a crayfish, a crab has the same organ systems (gills under the carapace, a gastric mill, green glands, an open circulation) but a reduced abdomen with no tail fan and a fused nervous system. See comparative anatomy for how crustaceans compare with other animals, and the spider anatomy page for the chelicerate body plan that horseshoe crabs share.^{[25, 16, 4]}