Blue Crab

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Blue Crab

Although named blue crabs, the shells of these crustaceans are really more green or mottled brown, with blue only appearing prominently on their claws, with an added splash of red on females. Their scientific name is both complementary – Callinectes = beautiful swimmer- and explains their mass appeal – sapidus = savory. These crabs can be found in a large area from Nova Scotia to Uruguay but they are most famously associated with the Chesapeake Bay, thus their status as the official crustacean of Maryland. The Maryland crab fishery rivals the lobster fishery in Maine for local lore and affection, as well as for the livelihoods of thousands of fishermen and the supporting industries.


Content List

1. Scientific & Common Names

2. Characteristics

a. Breeding

b. Behavior

3. History

4. Present Status

5. References


Scientific & Common Names

Kingdom - Animalia

Phylum - Anthropoda

Subphylum - Crustacea

Class – Malacostraca

Subclass - Eumalacostraca

Superorder - Eucarida

Order – Decapoda

Suborder - Pleocyemata

Infraorder - Brachyura

Family - Portunidae

Genus - Callinectes

Species - Callinectes sapidus

Common Names – Blue Crab, Atlantic Blue Crab, Chesapeake Blue Crab, Blue Claw Crab


Characteristics

Blue crabs are opportunistic hunters and will eat anything, plant or animal, that they come across. They live around four years and can grow to a little over 9 inches across their carapace, but 5 inches is the legal minimum for fishing. They have five sets of legs, with the front converted to large claws, one slightly larger than the other, and the back into flippers for swimming. Crabs molt their outer shell in order to grow, although adult females do this only once, called the terminal or maturity molt, and this is when they mate. When they are free of the shell, they pull in as much water as they can hold, forcing their body to its largest possible size and wait for their new shell to harden. They are at their most vulnerable at this stage with no defense other than camouflage.


Breeding

Male blue crabs can be identified by the thin and pointed abdomen, while females have either a triangular (immature), or domed (mature) abdomen. Females mate only once and can release over 8 million eggs in their lifetimes, around two million per season. The female will brood the eggs before releasing them to hatch into larvae, which are at the mercy of the currents for about a month before the crab has developed enough to swim. Since blue crabs can be highly cannibalistic, the red found at the end of only female’s claws may help males identify them as potential mates instead of a potential meal.


Behavior

Blue crabs are good swimmers but they tend to live their lives on the bottom of their waterways. They are quick to use their claws to defend themselves and the term ‘crabby’ fits their temperament perfectly. Autotomy is the crab’s ability to lose or sacrifice their limbs to avoid predation and then regrow them. Males favor shallower, brackish water, while females can be more readily found in deeper, saltier waters. The tendency of females to migrate more than males has caused a problem in the Chesapeake Bay, where large ‘dead zones’ have been created where oxygen levels are dangerously low. These zones have been created from the runoff of fertilizers and animal waste, as well as some human sewage. When migrating females pass through these zones, their mortality rates rise sharply.


History

The earliest known true crabs are dated from the Jurassic period, although there is some evidence that they may have evolved earlier. Today, crabs can be found in both marine and freshwater environments as well as dry land. Although they share a name and some superficial characteristics, horseshoe crabs and hermit crabs are not true crabs.


Present status

Recent declines in their numbers, especially in the Chesapeake Bay, have begun to recover, thanks in large part to the Bi-State Blue Crab Advisory Committee (BBCAC), a cooperative between Maryland and Virginia, the two states with the most impact on the crab’s population.

Native to the eastern US, this species is now considered invasive in the Mediterranean and its impact is still being studied.


References

Beautiful Swimmers: Watermen, Crabs, and the Chesapeake by William W. Warner

The Blue Crab: Callinectes Sapidus by Victor S. Kennedy

The blue crab Callinectes sapidus in the Salento peninsula (SE Italy): Occurrence, Trophic Position, Ecological Impact of an Alien Species in the Mediterranean– November 22, 2012 by Leonardo Carrozzo Sea Grant, Maryland