Blue Crab (Callinectes sapidus): Facts, Habitat and Range

What does a blue crab look like?

The blue crab’s shell, or carapace, is much wider than it is long, about two to two and a half times as wide. Each front edge carries nine teeth, and the last one on each side is a long, sharp spine, which gives the crab its distinctive pointed outline. The shell is olive, grayish or bluish green above and white to cream below.

Like all crabs, it has ten legs. The first pair are claws, called chelipeds, which are long, ridged and bright blue. Behind them are three pairs of walking legs, and the last pair end in flat, oval paddles used for swimming. Those paddles are the mark of the swimming crab family, Portunidae.

Males and females are easy to tell apart. Under the body, the folded abdomen, called the apron, is narrow and T-shaped in males. Young females have a triangular apron, and after their final molt it becomes broad and rounded. Mature females also have red or orange tips on their claws, which crabbers call painted fingernails.

The blue color comes from a pigment-protein complex in the shell combined with red astaxanthin, the same pigment that colors lobsters. Cooking breaks the complex apart and leaves only the red, so cooked blue crabs are orange-red.

How big do blue crabs get?

Blue crab size is measured across the shell from spine tip to spine tip.

Males usually grow larger than females and have longer side spines. Growth depends heavily on water temperature, so crabs in the warm Gulf of Mexico reach adult size faster than those in Chesapeake Bay. Blue crabs are mid-sized among crabs; for the giants, see what is the biggest crab in the world?

Blue crab look-alikes

Several relatives can be mistaken for the Atlantic blue crab. The lesser blue crab (Callinectes similis) shares much of its range in the United States but lives farther offshore, stays smaller and has a smoother, more granular shell. The ornate blue crab (Callinectes ornatus), common in Florida and the Caribbean, can be told apart by the teeth between the eyes: the Atlantic blue crab has four, the ornate blue crab six.

In Europe, the native green shore crab (Carcinus maenas) is much smaller and lacks swimming paddles. In the eastern Mediterranean, the blue swimming crab Portunus segnis, which arrived from the Red Sea through the Suez Canal, also has bluish claws, and in some fish markets both species are sold simply as blue crab.

Habitat and range

Blue crabs are estuary specialists. They live in bays, lagoons, tidal creeks, salt marshes and seagrass beds, from the low-tide line down to about 36 m (120 ft). They tolerate a huge range of salinity, from almost fresh water at the heads of estuaries to full seawater offshore.

Males and females often live apart. In Chesapeake Bay, males prefer the fresher upper bay and its rivers, especially in Maryland, while females favor the saltier lower bay toward Virginia. In winter, crabs in northern waters move to deeper channels and bury themselves in mud, becoming almost inactive until spring.

The native range runs along the western Atlantic from Nova Scotia and Cape Cod south through the Gulf of Mexico and the Caribbean to Uruguay and northern Argentina. Populations are most abundant from New England to Texas, and warming water has helped blue crabs settle farther north in the Gulf of Maine.

Since the early 1900s, blue crabs have also been carried to other parts of the world, most likely as larvae in ships’ ballast water. They now live along European Atlantic coasts, around the whole Mediterranean, in the Black Sea and in Japan. In the Mediterranean they have become a major pest of shellfish lagoons, explained in why are blue crabs invading Europe?

What do blue crabs eat?

Blue crabs are omnivores and eat almost anything they can catch or find. Their main foods are thin-shelled clams, mussels and young oysters, along with snails, marine worms, small fish, shrimp and other crabs. They also eat dead animals, eelgrass and algae.

They open clams and mussels with their strong claws, often by cutting at the hinge or chipping the shell edge. Larger crabs tackle bigger and harder-shelled prey. Blue crabs are also cannibals: big crabs readily eat smaller ones and freshly molted crabs with soft shells, which is one reason young crabs hide in seagrass.

How blue crabs swim and behave

Blue crabs are strong swimmers. They scull their paddle-shaped back legs rapidly, a little like oars or a propeller, and usually swim sideways, the same direction in which they walk. That sideways movement is explained in why do crabs walk sideways?

They are active, alert and quick to defend themselves. A threatened blue crab backs away with its claws spread wide and raised, and it can also bury itself in sand or mud with only its eyes showing. Tracking studies show that individual crabs often move a few hundred meters a day, and females migrate many kilometers down estuaries to spawn.

Blue crabs have good color vision for a crab, and studies suggest males prefer females with red-tipped claws. Most courtship signals, however, are chemical, carried in urine.

Molting and growth

Like all crustaceans, a blue crab grows by molting. It forms a new soft shell under the old one, splits the old shell open at the back and backs out, then swells with water before the new shell hardens. A crab about to molt is called a peeler, and one that has just molted is a soft crab.

A blue crab molts about 18–20 times after its larval stages before reaching adult size, and roughly 25 times in all. The new shell hardens fastest in low-salinity water. Molting also lets a crab replace lost legs, as described in can crabs regrow legs?

Reproduction and life cycle

A female blue crab mates only once in her life, right after her final molt, when her shell is still soft. Before that molt, a male carries her beneath him, cradled in his walking legs, for several days, a pair known as a doubler. After mating he guards her until her new shell hardens. Males can mate many times.

The female stores the sperm for up to a year and uses it to fertilize one or more batches of eggs. She then migrates to the salty mouth of the estuary. Each batch, called a sponge, holds roughly 750,000 to 3.2 million orange eggs, according to the Chesapeake Bay Program, and hatches after about two weeks.

The tiny larvae, called zoeae, drift out to sea and molt through about seven stages over a month or more. They then become a postlarva, the megalopa, which rides tides back into estuaries and settles in seagrass or marsh edges. Young crabs mature in about 12–18 months, faster in warm southern waters.

How long do blue crabs live?

Most blue crabs live only 1–3 years. NOAA Fisheries gives a typical lifespan of 3–4 years, and the Chesapeake Bay Program notes that few live longer than three. A small number reach 5 years and, very rarely, about 8. Because crabs shed their whole shell when they molt, scientists have no simple way to read their age.

Predators and threats

Huge numbers of larvae and young crabs are eaten by fish, and adults have plenty of enemies too. Predators include red drum, striped bass, Atlantic croaker, eels, cownose rays and sharks, along with herons, gulls and sea turtles. Other blue crabs are among the most important predators of juveniles.

In Chesapeake Bay, numbers rise and fall naturally from year to year, but the population fell sharply from the 1990s, linked to heavy fishing, loss of underwater grass beds and low-oxygen dead zones. Parasites and diseases, such as bitter crab disease caused by the parasite Hematodinium, can also kill many crabs. Warming water may help blue crabs in the north of their range by shortening winters.

Conservation status and people

The IUCN Red List has not assessed the blue crab, so its status is Not Evaluated. It is abundant over most of its range and is managed as a fishery rather than as a threatened species. Maryland and Virginia run a bay-wide winter dredge survey each year to track Chesapeake numbers, and managers use the results to set catch rules.

The blue crab is the state crustacean of Maryland, and crab feasts, crab cakes and soft-shell crabs are part of the culture of the Chesapeake and the Gulf Coast. Louisiana now lands more blue crabs than any other state.

In Europe, by contrast, the same species is seen as an invader. It eats young clams and mussels, damages fishing nets and has spread rapidly through Italian and Spanish lagoons since the 2010s. It looks quite different from its long-tailed cousin, the American lobster, compared in crab vs lobster.