Callinectes sapidus, commonly known as the blue crab, Atlantic blue crab, or, regionally, the Maryland blue crab, is a species of crab native to the waters of the western Atlantic Ocean and the Gulf of Mexico, and introduced internationally.
C. sapidus is of considerable culinary and economic importance in the United States, particularly in Louisiana, the Carolinas, the Chesapeake Bay, Delaware, and New Jersey. It is Maryland's largest commercial fishery and has been designated its state crustacean. Due to overfishing and environmental pressures some of the fisheries have seen declining yields, especially in the Chesapeake Bay fishery.
Unlike the other fisheries affected by climate change, blue crab is expected to do well; warming causes better breeding conditions, more survivable winters, and a greater range of habitable areas on the Atlantic coast. Whether this will have negative effects on the surrounding ecosystems from an increased crab population is still unclear. However, rising sea levels as a result of climate change will likely hinder blue crab reproduction as they will decrease the amount of eelgrass necessary for female crabs to complete their final shedding stage.
Etymology The genus name Callinectes comes from Ancient Greek κάλλος (kállos), meaning 'beautiful', and νήκτης (nḗktēs), meaning 'swimmer'. The specific epithet sapidus is Latin for 'savory'. A common name for crabs in Kikitowämank is , used by the Powhatan people, and in Nanticoke, .
Description
C. sapidus is a decapod crab of the swimming crab family Portunidae. The genus Callinectes is distinguished from other portunid crabs by the lack of an internal cartilaginous spine on the carpus (the middle segment of the claw), as well as by the T-shape of the male abdomen. Blue crabs may grow to a carapace width of . C. sapidus individuals exhibit sexual dimorphism. Males and females are easily distinguished by the shape of the abdomen (known as the "apron") and by color differences in the chelipeds, or claws. The abdomen is long and slender in males, but wide and rounded in mature females. A popular mnemonic is that the male's apron is shaped like the Washington Monument, while the mature female's resembles the dome of the United States Capitol. Claw color differences are more subtle than apron shape. The immovable, fixed finger of the claws in males is blue with red tips, while females have orange coloration with purple tips. A female's abdomen changes as it matures: an immature female has a triangular-shaped abdomen, whereas a mature female's is rounded.
Other species of Callinectes may be easily confused with C. sapidus because of overlapping ranges and similar morphology. One species is the lesser blue crab (C. similis). It is found further offshore than the common blue crab, and has a smoother granulated carapace. Males of the lesser blue crab also have mottled white coloration on the swimming legs, and females have areas of violet coloration on the internal surfaces of the claws. C. sapidus can be distinguished from another related species found within its range, C. ornatus, by number of frontal teeth on the carapace. C. sapidus has four, while C. ornatus has six.
The crab's blue hue stems from a number of pigments in the shell, including alpha-crustacyanin, which interacts with a red pigment, astaxanthin, to form a greenish-blue coloration. When the crab is cooked, the alpha-crustacyanin breaks down, leaving only the astaxanthin, which turns the crab to a bright orange-red color.
Organochlorides are found by Sheridan et al 1975 to be transferred to the C. sapidus hepatopancreas. AGRIS id XF2016073628. They find that among organochlorides, DDT specifically is converted both to DDE and DDD in this crab.
Distribution
C. sapidus is native to the western edge of the Atlantic Ocean from Cape Cod to Argentina and around the entire coast of the Gulf of Mexico. It has recently been reported north of Cape Cod in the Gulf of Maine, potentially representing a range expansion due to climate change. It has been introduced (via ballast water) to Japanese and European waters, and has been observed in the Baltic, North, Mediterranean, and Black Seas. The first record from European waters was made in 1901 at Rochefort, France. In some parts of its introduced range, C. sapidus has become the subject of crab fishery, including in Greece, where the local population may be decreasing as a result of overfishing. In Italy, public awareness of the detrimental impact of this species on local mollusks is rapidly growing and, especially in the Po delta area and on the Adriatic Sea coast, eradication efforts are undergoing, both by local authorities and by local fishermen. In 2023, Italy's Ministry of Agriculture appointed a "commissioner for the blue crab" to coordinate efforts to address the species' spread in Italian waters. Representatives of the agricultural and fishing industries have attributed significant losses in clam fisheries to blue crab predation, with some industry estimates putting the economic damage at up to €100 million since the infestation began.
Ecology Some of the natural predators of C. sapidus include eels, drum, striped bass, spot, trout, some sharks, humans, cownose rays, and whiptail stingrays. C. sapidus is an omnivore, eating both plants and animals. It typically consumes thin-shelled bivalves (such as clams, mussels, and oysters), crustaceans, annelids, small fish, plants (such as eelgrass), and nearly any other item it can find, including carrion, other C. sapidus individuals, and animal waste. In salt marshes, C. sapidus will eat marsh periwinkles, Littoraria irrorata during high tides. Although an aquatic predator, C. sapidus will remain in shallow pits in salt marshes at low tide and ambush intertidal prey such as fiddler crabs (e.g., Minuca pugnax) and purple marsh crabs (Sesarma reticulatum) C. sapidus may be able to control populations of the invasive green crab, Carcinus maenas; numbers of the two species are negatively correlated, and C. maenas is not found in the Chesapeake Bay, where C. sapidus is most abundant.
C. sapidus is subject to a number of diseases and parasites. These include a number of viruses, bacteria, microsporidians, ciliates, and others. The nemertean worm Carcinonemertes carcinophila commonly parasitizes C. sapidus, especially females and older crabs, although it has little adverse effect on the crab. A trematode that parasitizes C. sapidus is itself targeted by the hyperparasite Urosporidium crescens. The most harmful parasites may be the microsporidian Ameson michaelis, the amoeba Paramoeba perniciosa and the dinoflagellate Hematodinium perezi, which causes "bitter crab disease".
In 2021, scientists from the University of Maryland completed DNA sequencing of the genome of C. sapidus in Baltimore after six years of research to help better understand the species. This genetic map is expected to help scientists understand how the blue crabs will be affected by climate change and warmer water temperatures, along with which mutations cause disease, traits that influence meat production, and which females have the best reproductive ability.
Life cycle
Growth Eggs of C. sapidus hatch in high-salinity waters of inlets, coastal waters, and mouths of rivers, and are carried to the ocean by ebb tides. During seven planktonic (zoeal) stages, blue crab larvae float near the surface and feed on microorganisms they encounter. After the eighth zoeal stage, larvae molt into megalopae. This larval form has small claws called chelipeds for grasping prey items. Megalopae selectively migrate upward in the water column as tides travel landward toward estuaries. Eventually, blue crabs arrive in brackish water, where they spend the majority of their lives. Chemical cues in estuarine water prompt metamorphosis to the juvenile phase, after which blue crabs appear similar to the adult form.
A blue crab grows by shedding its exoskeleton, or molting, to expose a new, larger exoskeleton. After it hardens, the new shell fills with body tissue. Shell hardening occurs most quickly in low-salinity water where high osmotic pressure allows the shell to become rigid soon after molting. Molting reflects only incremental growth, making age estimation difficult. For blue crabs, the number of molts in a lifetime is about 25. Females typically exhibit 18 molts after the larval stages, while postlarval males molt about 20 times.
Male blue crabs tend to grow broader and have more accentuated lateral spines than females. Growth and molting are profoundly influenced by temperature and food availability. Higher temperatures and greater food resources decrease the period of time between molts, as well as the change in size during molts (molt increment). Salinity and disease also have subtle impacts on molting and growth rate. Molting occurs more rapidly in low-salinity environments. The high osmotic pressure gradient causes water to quickly diffuse into a soft, recently molted blue crab's shell, allowing it to harden more quickly. The effects of diseases and parasites on growth and molting are less well understood, but in many cases have been observed to reduce growth between molts. For example, mature female blue crabs infected with the parasitic rhizocephalan barnacle Loxothylacus texanus appear extremely stunted in growth when compared to uninfected mature females. Blue crabs may reach maturity within one year of hatching in the Gulf of Mexico, while Chesapeake Bay crabs may take up to 18 months to mature. As a result of different growth rates, commercial and recreational crabbing occur year-round in the Gulf of Mexico, while crabbing seasons are closed for colder parts of the year in northern states.
Reproduction
Mating and spawning are distinct events in blue crab reproduction. Males may mate several times and undergo no major changes in morphology during the process. Female blue crabs mate only once in their lifetimes during their pubertal, or terminal, molt. During this transition, the abdomen changes from a triangular to a semicircular shape. Mating in blue crab is a complex process that requires precise timing of mating at the time of the female's terminal molt. It generally occurs during the warmest months of the year. Prepubertal females migrate to the upper reaches of estuaries, where males typically reside as adults. To ensure that a male can mate, he actively seeks a receptive female and guards her for up to seven days until she molts, when insemination occurs. Crabs compete with other individuals before, during, and after insemination, so mate guarding is very important for reproductive success. After mating, a male must continue to guard the female until her shell has hardened. Inseminated females retain spermatophores for up to one year, which they use for multiple spawnings in high salinity water. During spawning, a female extrudes fertilized eggs onto her swimmerets and carries them in a large egg mass, or sponge, while they develop. Females migrate to the mouth of the estuary to release the larvae, the timing of which is believed to be influenced by light, tide, and lunar cycles. Blue crabs have high fecundity; females may produce up to 2 million eggs per brood.