Callinectes sapidus M. J. Rathbun, 1896
Blue crab (English), Bluepoint (English)
The Blue Crab (Callinectes sapidus) is found from Cape Cod to Uruguay, occurring especially in estuaries. It is a beautifully colored crustacean with bright blue claws--the mature female's claws are tipped in red--and an olive to bluish green carapace. The Blue Crab is a commercially important species and is popular with recreational crabbers as well. (Gosner 1978; Lippson and Lippson 1997). The Greek and Latin roots of its scientific name translate to "savory beautiful swimmer".
Except when they have recently molted and have still-soft shells, Blue Crabs are very aggressive when threatened, although they will also burrow into sand to hide (Gosner 1978; Pollock 1998).
Baldwin and Johnsen (2009) investigated mate choice in Blue Crabs, carrying out mate choice experiments using males and manipulated photographs of females. Their results indicated that courtship and mate choice behavior in Blue Crabs can be stimulated by visual cues alone. Males showed a preference for females with red claw dactyls ("pincers"). In binary choice experiments, males displayed more often to photographs of females with red claws than to those with white claws or black claws, strongly suggesting that these male crabs made their choices based on the hue of the red claws and, more broadly, that Blue Crabs are capable of color vision and use color in mate choice.
Lippson and Lippson (1997) describe the life cycle of the Blue Crab in Chesapeake Bay, Maryland, U.S.A., which is famous for its Blue Crabs. Blue Crabs spawn near the mouth of the Chesapeake from May to October. The sponge, or egg mass, which may contain up to 2 million eggs, adheres to the undersurface of the crab. The color of the egg mass is golden orange at first, but changes to black as hatching approaches. After a few weeks, small semi-transparent zoae larvae are released. Many of these larvae are swept out into the ocean, where they mix with Blue Crab larvae from other regions of the coast and, eventually, are blown into regional estuaries such as the Delaware and Chesapeake Bays. After additional molts, a second larval form, the megalops, is produced. The megalops, which resembles a tiny lobster, moves along the bottom and up into the Bay system, where it molts into a tiny but recognizable Blue Crab. By 12 to 16 months, the crabs have molted several times and reached sexual maturity at an average size of about 13 cm.
Place et al. (2005) analyzed the complete mitochondrial genome of the Blue Crab.
Several other Callinectes species overlap in range with the Blue Crab (C. sapidus). Callinectes similis is quite similar in appearance, but has 6 teeth along the front of the carapace between the eyes (often more technically described as "4 teeth, not counting the inner orbital teeth"), while C. sapidus has just 4 (or 2, not counting the inner orbitals)(Gosner 1978; Pollock 1998). It also reaches a size of only about 125 mm across (vesus 225 mm for C. sapidus). Callinectes similis is a common estuarine crab south of Cape Hatteras, North Carolina (U.S.A.), ranging into fresh water; from Cape May, New Jersey, south to Cape Hatteras it is usually recorded from depths of at least 9 meters or more and salinities of about 15 ppt (Gosner 1978; Pollock 1998). Other Callinectes species in the Atlantic/Caribbean/Gulf of Mexico region also have 6 frontal teeth (4, not counting inner orbitals), although the inner pair may be very reduced.
The last pair of the Blue Crab's legs are paddle-shaped for swimming. The shell is more than twice as wide as long with 9 marginal teeth (the 9th is a strong spine). The shell is usually olive or bluish-green above and the claws are bright blue below (young are paler). The male's abdomen is abruptly tapered, while the female's is more broadly rounded. Adults may reach a width of 225 mm between the tips of the longest spines. (Gosner 1978)
The Blue Crab (Callinectes sapidus) is especially common in estuaries, where it ranges into fresh water, but may be found offshore to at least 36 meters (Gosner 1978). Females remain in higher salinity portions of an estuary system, especially for egg laying (Lippson and Lippson 1997). Blue Crabs migrate to deeper water in winter (Gosner 1978).
The Blue Crab (Callinectes sapidus) is found from Cape Cod to Uruguay, sometimes north at least to Massachusetts Bay (Gosner 1978). Blue Crabs have also been introduced accidentally or intentionally in Hawaii, Europe, Japan, and Africa.
Blue Crabs feed on plants, shellfish, recently dead fish, and a wide variety of other prey they can kill or scavenge, including recently molted soft-shelled individuals of their own species (Lippson and Lippson 1997).
Barnacles often attach themselves to Blue Crabs, especially in southern regions. The Striped Barnacle (Balanus amphitrite) and Turtle Barnacle (Chelonibia testudinaria) are found externally; a small goose barnacle, Octolasmus lowei, occurs in the Blue Crab's gill chamber, and the bean-shaped sacculinid barnacle Loxothylacus texanus attaches itself under the abdomen. A parasitic nemertean worm, Carcinonemertes carcinophila, is found on the gills of female crabs; on virgin crabs, it is pinkish, while on breeders it is red. (Gosner 1978)
Feeding experiments carried out by Harding (2003) suggest that the Blue Crab may be an effective predator of the Rapa Whelk (Rapana venosa), a large predatory gastropod from Asia that was discovered in Chesapeake Bay in 1998. Although mature Rapa Whelks are probably too large and well protected to be attacked by Blue Crabs, available data suggest that predation by Blue Crabs on juvenile Rapa Whelks might control whelk populations in Chesapeake Bay and other estuarine habitats along the North American Atlantic coast.
Blue Crabs are the basis for a commercially valuable fishery in many regions, but as with so many other fisheries, overexploitation has been a major problem. The Chesapeake Bay has traditionally been one of North America’s most productive fishing grounds, supporting the world’s largest Blue Crab fishery. However, sustained fishing mortality and environmental deterioration led to an ∼70% decrease in Blue Crab abundance in Chesapeake Bay during the last decade of the 20th century and first few years of the 21st, from an estimated 900 million crabs down to ∼300 million, with 45–55% of those crabs harvested annually. Even more alarming, studied have found that spawning stock abundance and biomass in Chesapeake Bay declined by 81% and 84%, respectively, around this same period. Consequently, the Blue Crab fishery, which in the early 1990s was a 52,000-ton, $72-million industry, declined to a 28,000-ton, $61-million harvest in 2004. A multidisciplinary, multi-institutional program was developed to study the basic biology and life cycle of the Blue Crab, develop hatchery and nursery technologies for mass production of blue crab juveniles, and assess the potential of using cultured juveniles to enhance Blue Crab breeding stocks and, in turn, bay-wide abundance and harvests. Basic biology and culture studies yielded methods to mass produce larvae and juvenile Blue Crabs in captivity. Juvenile crabs have been produced year-round, with excellent survival. During 2002–2006, over 290,000 cultured crabs were tagged and experimentally released into the bay’s nursery habitats. Cultured crabs survived as well as their wild counterparts, increased local populations at release sites by 50–250%, grew quickly to sexual maturity, mated, and migrated from the release sites to spawning grounds, contributing to the breeding stock as soon as 5 to 6 months post-release. (Zohar et al. 2008 and references therein) Despite their enthusiasm and optimism regarding stock replenishment efforts, Zohar et al. (2008) emphasize their view that in addition to mass rearing and releasing of Blue Crab juveniles, successfully restoring Blue Crab populations will require the integration of adequate management strategies to protect the wild and released animals until sexual maturity and spawning, with fishery and seafood industry, policymakers, environmental activists, and scientists all working together.
Paolisso (2007) explored the evolving role of the Blue Crab in the human culture around the Chesapeake Bay watershed.