Is a blue crab really blue? Well actually, all adult blue crabs are dark green on top and white underneath. So what’s the story with the blue? Blue crabs are called blue because of the striking deep blue coloring that the male crabs have on the top of their largest claw. Females don’t have this blue, but instead show red tips at the end of their claws.
The scientific name of the blue crab, Callinectes sapidus, is actually a more accurate description for both males and females. Callinectes, means beautiful swimmer, and the blue crab is indeed a good swimmer. Blue crabs belong to the family Portunidae and one thing that separates crabs in this family from other crabs is that the portunids all have a special pair of swimming legs.
Sapidus, finally, means tasty or savory. So, the scientific name of the blue crab, Callinectes sapidus, means a “beautiful swimmer that tastes good.” Not a bad name for this tasty swimmer.
- Kingdom: Animalia
- Phylum: Arthropoda
- Class: Crustacea
- Subclass: Malacostraca
- Order: Decapoda
- Suborder: Pleocyemata
- Infraorder: Brachyura
- Superfamily: Portunoidea
- Family: Portunidae
- Subfamily: Portuninae
- Genus: Callinectes
- Species: sapidus
Blue crabs exhibit obvious sexual dimorphism. Male blue crabs have a T-shaped abdomen readily distinguishable from both female stages. Immature females have triangular shaped abdomens with fused segments; however, at the terminal or pubertal molt, the final ecdysis and onset of sexual maturity, the mature female abdomen becomes broad and rounded.
Morphological variants are so different from each other that they could easily be interpreted as distinct species, but there is no point of demarcation — morphological, geographic, or bathymetric — between the “typical” rather blunt-spined form predominating along the east coast of the United States and the “acute-spined” form named C. sapidus acutidens predominating from Florida southward.
One way to learn more about blue crabs is to take a closer look at their anatomy.
Crabs have compound eyes consisting of several thousand optical units. The eyes are on stalks which can be lowered for protection into sockets on the carapace. Crabs appear to see very well with some species detecting movement 20 or 30 metres away. Crabs can also hear and produce a variety of sounds. In courtship some species attract the females attention by banging their cheliped on the ground or vibrating their walking legs. Each species has its own unique sound that can attract a female or intimidate a competing male.
Crabs have bristles and hairs which act as touch receptors. The bristles occur all over the body but are most frequently found in clumps on the walking legs. These bristles signal contact with a hard surface simply by bending, while other shorter hairs are sensitive to water currents. Crabs can find food using chemical stimuli. The antennae have “smell detectors” which detect chemicals that stimulate a search for food. When similar detectors on the legs contact food the cheliped quickly grasps the object and passes it into the mouth. Crab mouthparts have further receptors which are sensitive to particular chemicals. There are two prominent triangular teeth with nine anterior lateral teeth, the hindmost tooth being the longest and directed outward. Crabs rely on a combination of these sense organs to find food and mates and flee predators.
The outside parts of a blue crab are hard. This is called their exoskeleton. Its back legs look like paddles and are called swimming legs. Crabs also have six walking legs and two claws. Using their swimming legs or walking legs, crabs can move pretty quickly to catch prey or to escape from predators. And those claws? If you’ve ever been pinched by these crustaceans, you would know what they use them for. When crabs are threatened, they extend their claws in a defensive posture.
You can also see two eyestalks protruding from the front part of the shell. These eyestalks can stick out so the crab can see in the front, back, and sides if need be. There are two sets of antennae. These are used by the crab to sense vibrations in the water and collect chemical clues for detecting predators and prey. Click on the image to see the crab move one of the sets of antennae.
Visible on the underside of a crab are the mouthparts and the abdomen. The gills through which the animal obtains oxygen cannot be seen. They are soft structures under the side of the carapace. The eyes which protrude from the front of the carapace are on the ends of short stalks. The mouthparts are a series of pairs of short legs, specialised to manipulate and chew food.
Because the shell of a blue crab, the exoskeleton, is hard and doesn’t grow, crabs must shed their shells in order to grow. This process is known as molting. The molting page contains a comprehensive breakdown of the many stages with easy to follow pictures!
Inside the crab are the soft parts. Crabs, like all crustaceans, have gills to take up oxygen in the water. Crabs also have a stomach and a heart that pumps a liquid through the body to supply the tissues with oxygen. The many nooks and crannies in the crab contain the muscles that move the claws, walking legs, and swimming legs.
Male or Female?
An easy way to tell the difference between an adult male and female crab is look at a crab’s underside and examine the shape of its abdomen. By looking at the abdomen you can see the difference between adult males, adult females, and juvenile females.
We have a guide written up to show you exactly how to tell the difference between male and female blue crabs with lots of pictures.
Diet & Food Web
Blue crabs are an important link in the food web of the Chesapeake Bay. They are predators as well as prey for other animals, including humans. Because crabs mainly stay on the bottom when looking for food, their preferred diet is clams, which they even dig up in the muddy or sandy bottom of the Bay.
Under healthy environmental conditions, blue crabs also eat range of other animals, such as fish, worms, and crustaceans like shrimp, barnacles, mud crabs, and other blue crabs. Some think blue crabs are also scavengers and when food is scarce blue crabs are found to be willing and able to eat anything: plants and animals, alive or dead.
Blue crabs don’t usually eat oysters because the shell of the oyster is too hard to open. They do like to crawl around an oyster reef and look for other fish and invertebrates.
Blue crabs are also prey for many other animals, such as fish, birds, and even other blue crabs. When blue crabs molt, their shell is soft, which makes them vulnerable to other predators. A small hard shell blue crab can easily capture and eat a larger soft shelled crab.
The blue crab also spends its first month or so in the plankton community, during which time it is a very easy prey for many of the filter feeders in the Bay, such clams, oysters, menhaden, bay anchovies, and barnacles.
Blue crabs mate in early summer and fall. The crabs can only mate when the female crab has just molted and her outer shell is still soft.
Female blue crabs, like the males, molt several times in order to grow, but a particularly important molt when an immature female becomes mature. Immature female crabs that are ready to mature on the next molt are called pre-pubescent females and release chemicals to attract mature male crabs in the area.
The male crabs may have to compete with each other for the female. During a distinct mating ritual the male crab stands on the tips of his walking legs and uses his swimming paddles to rock from side to side.
If the pre-pubescent female accepts the male, the two “double-up” and the male carries her around–usually for one or two days–underneath him until she is ready to molt.
When the female molts, they separate for a little while, but immediately after she has molted and shed her old outer shell, the crabs double-up again and begin to mate.
Depending on the conditions, the crabs will mate for a few hours. The females will keep the sperm in a sac until they use it to fertilize the eggs later that year, usually during her fall migration to the mouth of the Bay.
After they mate, the crabs stay together and the male continues to cradle the female crab until her shell hardens again. According to the researchers at SERC, the male crab may be doing this to protect his “investment” by making sure that the female doesn’t mate again. Researchers have noticed that some females, if released before their new shells had hardened, may mate a second time.
There’s even more information on the reproductive cycle of the blue crab on our mating & reproductive page.
Development and hatching of eggs takes place under a relatively wide range of salinity and water temperature, but this range narrows in successive zoeal instars. The optimum salinity and temperature ranges for larval development and survival, as indicated by laboratory experiments, correspond closely with ranges for these factors in the lower estuary and marine waters. Juvenile or adult blue crabs can tolerate a wide range of salinities and water temperatures in contrast to the zoeae and megalopae. Juveniles and adults are exposed to a salinity range of 0 to 35 ppt.
A complex behavioral repertoire has been demonstrated for blue crabs. Brachyuran crabs are highly aggressive animals, having agonistic interactions consisting of visual threat displays and actual physical combat, which may be formal and ritualized or wild and irregular. Most agonistic acts employ chelipeds as organs of expression as well as weapons.
Found Around the World
Callinectes sapidus, the best known species of crab found in the Chesapeake Bay, is also found in many other parts of the world. They live from Nova Scotia to Uruguay and from the Baltic Sea to France. They were found in the Black Sea in 1967 and in Japan in 1974. Other areas include:
- Israel in 1955
- Greece in 1959
- Turkey in 1961
- Egypt in 1963
- Lebanon in 1965
- Italy in 1965
- Adriatic Sea in 1972
- Cyprus in 1986
- Ionian Sea in 1999
The larvae are easily transported in the ballast of ships and have been found in the western and eastern Atlantic and Mediterranean ports. The Chesapeake Bay crabbing industry started in the 1870s, and has expanded over many years becoming an annual 30 million dollar business. Many people have seen the skipjacks plying the Bay’s waters. These few remaining members of a once proud fleet of almost 100 in the early 1950’s, are harvesting oysters. The Bay has the only commercial sailing fleet in the country.
Just how long the blue crab or any crab, such as the fossilized horseshoe crab pictured here, has been around is open to conjecture. If one uses fossil records then it can be reasonably assumed that it dates back to the Paleozoic Period, some 400 million years ago. This is well within the acceptable limits.
The blue crab is a Malacostracan, a large sub class of crustaceans having a thorax consisting of eight segments usually covered by a carapace, an exoskeleton (hard shell) including decapods and isopods.
More than 20,000 species have been found. Crustaceans are the most prevalent of the Arthropods. Many times it has been found that there are more of them than any other type of water dweller. According to the University of California at Berkley, more than 38,000 species have so far been identified. Exploration in the ocean depths continues to reveal others. Several new and as yet unnamed species have been found near volcanic vents on the ocean floor. These ‘smokers’ provide a source of warm water in the depths where sea life flourishes. Ice flows in the arctic also provide a home for different species of arthropods.
There are so many shapes of crustaceans that finding a similar feature is difficult. It has been found that all at some time had two pairs of antennae in their development.
Fossil records are few for many species. The horseshoe crab has left numerous fossil records but the swimming crab, such as the blue crab, has left far fewer. Still we know that if you were to go out and catch a blue crab today it would be the same species as it was 400 million years ago!
The crustaceans living today are quite different from their great aunts and uncles of eons ago. They were thought to have consisted of many similar segments, each with a pair of biramous appendages (having two branches) that resembled each other. All the variations in the form and structure found today in crustaceans arose from ancestors with this basic body shape.