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Blue Crab

Description

blue crabBlue crabs are members of the most abundant group of animals on Earth, the arthropods. Their Latin name, Callinectes sapidus, translates to “beautiful, savory swimmer.” Like all true crabs, they have five pairs of legs: three pairs of walking legs, one pair of paddle-like swimming legs, and one pair of claws. Their bodies are dorsally compressed, and their carapace ends in two sharp points on either side. Male crabs may be differentiated from females by their size and the shape of their abdomen. An adult male is typically larger than the female, with an abdomen that looks like an upside-down “T” embedded in its underside. Immature females have a triangular abdomen, while that of mature females is rounded and can be easily pulled away from the body. Coloration also varies between the sexes. Males usually have bright blue claws, while those of the female have orange tips.

Habitat and Biology

Blue crabs rely on estuarine habitats during most of their life cycle, although the larval stages develop in the ocean. The species ranges from Nova Scotia to Uruguay; however, it is most common in estuarine and nearshore habitats from Massachusetts to Texas. Shallow intertidal marsh habitats, such as those that line the South Carolina coast, are important nursery areas for young crabs, whereas larger adults prefer deeper waters.

In South Carolina, the mating season extends from February to November. Spawning peaks occur from March to July and again in October and November. Mating, which takes place in waters of low to moderate salinity, is a once-in-a-lifetime event for a female blue crab. After an elaborate courtship ritual a few days before mating, a male pairs with a female which is about to shed her shell for the last time (pubertal molt). He cradles her and carries her until she molts. Mating takes place while the female is still soft. Sperm that is transferred from the male is stored in receptacles on the female’s body and used to fertilize her eggs throughout the following year or two. After mating, the male continues to carry and protect the female until her shell hardens. Spawning occurs during late spring and summer near inlets and in nearshore ocean waters. As the eggs are extruded, they are fertilized and attached to pleopods, located on the female’s abdomen. The female crab carries the eggs under her abdomen until they hatch. Following mating, females move seaward, carrying their eggs in an external mass called a “sponge.” Ovigerous females have been collected in Charleston Harbor from April through October, with a peak in July. Hatching occurs in nearshore waters where larvae undergo a series of seven developmental stages as zoea and transform to the postlarval or megalopal stage to begin the cycle again. Megalopae enter inshore waters during the summer and fall. Young crabs typically move up estuaries to mid and low salinity water and grow quickly. Growth is rapid, and blue crabs reach maturity and the five inch legal harvest size in one to two years. Blue crab larvae feed primarily on coastal zooplankton, whereas adults are opportunistic, feeding on fish, other crustaceans (including blue crabs), mollusks and decaying plant and animal matter.

In addition to movements related to the crab’s life cycle, weather conditions are known to affect the distribution of crabs. If water conditions become too warm or too cold, crabs move to more favorable areas. Typically, cold water forces crabs, especially mature females, to deep water where they bury themselves in the sediment and become inactive. Crabs have also been known to leave shallow waters because of extreme heat on both a daily and a seasonal basis.

Species Significance

crab traps

Blue crabs are harvested commercially and recreationally in South Carolina, with crab traps, or pots, as the primary method used in their harvest. The trap is cube-shaped and made of wire mesh. A partition bearing two holes separates the trap into an upper and lower chamber. The lower chamber is equipped with two to four entrance funnels and a bait well. Crabs are attracted into the lower chamber and, once confined, try to escape by swimming upwards. This response causes them to move into the upper chamber, decreasing their chances of escape. Commercially, blue crabs are also harvested by means of trawling and as incidental catch in the shrimp trawl fishery. Recreational fishermen can use several harvest methods besides crab pots, such as dip nets, drop nets, and collapsible baskets.

A small portion of the blue crab fishery targets softshell crabs. Softshell crabs are those which have recently molted and whose shell is still soft. These crabs are sought after by seafood lovers and typically earn a fisherman a substantially higher price than hard crabs. The shell of the blue crab is only soft for short periods. The outer hard shell of crabs cannot grow larger at the crab grows. Periodically, crabs go through a “softshell” stage by shedding their outer hard shell. Prior to molting, a new shell develops underneath the old shell. It remains soft until the animal crawls out of its old shell once a new shell has developed underneath. Over a period of a few hours, the premolt or peeler crab molts, and the underlying softshell hardens to become the familiar hard shell. Some commercial crabbers have learned to predict the time of the next molt based on changes in color of the crab's exoskeleton in the region of the dactyl of the fifth pereopod. Blue crabs are not attracted to bait during the premolt or peeler softshell stage; thus methods other than bait odor must be employed to attract them.

The population of blue crabs in South Carolina is stable, and there are no indications that this resource has been or is currently being overexploited.

References

Low, R., R. Rhodes, E. R. Hens, D. Theiling, E. Wenner, and D. Whitaker. 1987. A profile of the blue crab and its fishery in South Carolina. South Carolina Marine Resources Center. Technical Report No. 66.

Sandifer P. A., J. V. Miglarese, D. R. Calder, J. J. Manzi, and L. A. Barclay. 1980. Ecological characterization of the Sea Island coastal region of South Carolina and Georgia, Vol. III: Biological features of the characterization area. United States Fish and Wildlife Service, Office of Biological Services, Washington, D.C., FWS/OBS-79/42.

Whitaker, D. J. 1995. Blue Crabs. Sea Science Educational Series. South Carolina Department of Natural Resources, Charleston, SC.



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