Crabs

MatingFemale blue crabs mate only once in their lives, when they become sexually mature immediately following their pubertal molt (immediately following this molt, the female is known as a "sook.") When approaching this pubertal molt, females release a pheromone in their urine which attracts males. Male crabs vie for females and will carry and protect them, called "cradle carrying," until molting occurs. Following this molt, when the female's shell is soft, the pair will mate. During mating, the female captures and stores the male's sperm in sac-like receptacles so that she can fertilize her eggs at a later time. Once the female's shell has hardened, the male will release her and she will migrate to higher salinity waters to spawn.

Spawning

SpawningAfter mating, females migrate to high-salinity waters in lower estuaries, sounds, and near-shore spawning areas. They over-winter before spawning by burrowing in the mud. Most females spawn for the first time two to nine months after mating, usually from May through August the following season. The female extrudes fertilized eggs into a cohesive mass, or "sponge," that remains attached to her abdomen until the larvae emerge. The average sponge contains about two million eggs and is formed in about two hours.

Development - Growth Stages

Growth StagesGrowth and development of the blue crab, as in other crustaceans, consist of a series of larval, juvenile, and adult stages during which a variety of morphological, behavioral, and physiological changes occur. These changes are most dramatic when the animal molts (sheds its rigid exoskeleton) permitting growth and changes in body shape. Before molting, a new shell is formed underneath the old exoskeleton, which then loosens and is cast off. The new shell is initially soft, but it expands and hardens in a few hours. The stage between molts is termed intermolt. LarvaeStage 1 - ZoeaeFirst stage larvae, called zoeae, measure approximately 0.25 mm at hatching. They bear little morphological resemblance to adults (Hopkins 1943), are filter feeders, and live a planktonic existence in the high-salinity surface waters near the spawning grounds (Pyle and Cronin 1950; Darnell 1959). Tagatz (1968) found more zoeae near the water's surface than at the bottom. Evidence suggests that blue crab zoeae hatch in the Chesapeake Bay, Chincoteague Bay, Delaware Bay, and other estuaries and drift out to sea, where they feed and grow. These larvae may migrate vertically in the water column to reach flood and ebb tides, which transport them back into the bay area.

The zoeae and all subsequent life stages can increase body size only by molting (Hay 1905; Pyle and Cronin 1950). Zoeal development may require 31 to 49 days, depending on salinity and temperature, but development time has been shown to be variable even in a single salinity-temperature regime (Williams 1965). Zoeae molt four to seven times before entering the next stage of development. The final zoeal stage is about 1.0 mm in width (Hopkins, Rogers 1944).

Stage 2 - MegalopsThe final molt of the zoeae is characterized by a conspicuous change to the second larval stage, called a megalops (also termed megalopa [singular] or megalopae [plural]. Development to this stage requires 31 to 49 days. The megalops larva is more crablike in appearance than the zoeae, its carapace is broader in relation to its length, and has biting claws and pointed joints at the ends of the legs. It measures about 1.0 mm in width. The megalops swims freely, but generally stays near the bottom in nearshore or lower-estuarine, high-salinity areas (Tagatz, 1968). The megalops stage lasts 6 to 20 days, after which the megalops molts into the "first crab" stage, with proportions and appearance more like those of an adult.

There are usually seven zoeal stages and one postlarval, or megalopal, stage. On occasion, an eighth zoeal stage is observed.

Juveniles

JuvenilesThe juvenile "first crab" is typically 2.5 mm wide (from tip to tip of the lateral spines of the carapace). These juveniles gradually migrate into shallower, less-saline waters in upper estuaries and rivers where they grow and mature (Fischler and Walburg 1962). Van Engle (1958) and Tagatz (1968) reported that many juveniles had completed this migration by fall and early winter. New evidence, however, suggests the bulk may not reach the upper parts of tributaries and Chesapeake Bay until the following summer.

Males generally migrate farther upstream, preferring low-salinity waters, whereas females tend to stay in lower rivers and estuaries (Dudley and Judy 1971; Music 1979).

Growth and maturation occur during a series of molts and intermolt phases, each of which is termed a "crab" stage according to the number of molts that have occurred since the megalops stage. Churchill (1921) reported that juveniles reached the 9th or 10th crab stage by October in Chesapeake Bay, but growth varies considerably among years and with latitude along the Atlantic coast. Molting and grown stop during winter (Churchill, 1921; Darnell 1959); growth resumes as waters warm, and juveniles generally reach maturity during the spring or summer of the year following their hatching.

Adults

AdultsIn the Chesapeake Bay, sexual maturity is reached after 18 to 20 postlarval molts, at the age of 1 to 1½ years. Males continue to molt and grow after they reach sexual maturity. It is generally accepted that females cease to molt and grow (terminal molt) when they mature and mate. However, new research suggests that mature females (sooks) will continue to molt given the right set of circumstances. Jeffery Shields of the Virginia Institute of Marine Science says that it was once believed that blue crabs experienced a terminal molt which was considered the second molt after puberty for males and the only molt after puberty for females. While this is generally the case, most portunid crabs do continue to molt. There are some caveats that apply to blue crabs:

• Molting takes energy; energy that is better used for reproductive output. Since it takes more energy to produce eggs, the theory is that mature female crabs don't grow as large or molt as frequently because of their reproductive energetics. The converse is that sperm production is cheap, so males don't put energy into reproduction, rather they put it into somatic growth.
• Molting is energy dependent. Larger animals must store far more nutrients for molting than do smaller juveniles. Thus, a really big lobster only molts every 2 to 5 to 10 years. Similarly for blue crabs, the larger the crab, the more difficult to store energy for molting.
• Molting is risky business. Larger animals may be at more risk, hence, they are not frequent in the population.

After the females mate and migrate to spawning areas, they either remain there for the rest of their lives or move only short distances out to sea. In warmer months, males generally stay in low-salinity waters such as creeks, rivers, and upper estuaries. Research on blue crabs in the Chesapeake Bay indicated that females over-wintered at the mouth of the bay and spawned there in spring, whereas the migration of males was non-directional. Crabs bury themselves in mud in winter and emerge when temperatures rise in spring. The maximum age for most blue crabs in the Mid-Atlantic Region is three years; adults thus live an average of less than one year after reaching maturity.

Food

FoodBlue crabs are classified as general scavengers, bottom carnivores (eats other animals), detritivores (eats decaying organic matter), and omnivores (eats either other animals or plants). At various stages in the life cycle, blue crabs serve as both prey and as consumers of plankton, benthic macroinvertebrates, fish, plants, mollusks, crustaceans (including other blue crabs), and organic debris. Food is located by a combination of chemoreception (chemical sense) and taction (touch). Blue crabs may play a significant role in the control of benthic populations.

Macroinvertebrates are organisms without backbones (e.g., insect larvae, annelids (leeches), oligochaetes (worms), crustaceans (crabs, crayfish and shrimp), mollusks (clams, oysters and mussels), and gastropods (snails)) and inhabit bottom substrates (e.g., sediments, debris, logs, macrophytes, and filamentous algae.)

Adult FoodAdult blue crabs prefer mollusks such as oysters and hard clams as their primary food sources. The crab uses the tips of its front-most walking legs to probe the bottom for buried bivalves and to manipulate them after they are located. Some other common food items include dead and live fish, crabs (including other blue crabs), shrimp, benthic macroinvertebrates, organic debris, and aquatic plants and associated fauna such as roots, shoots and leaves of sea lettuce, eelgrass, ditch grass, and salt marsh grass. It will also prey on oyster spat, newly set oysters and clams, or young oysters and quahogs if other food is unavailable. Juvenile FoodJuvenile blue crabs feed mostly on benthic macroinvertebrates, small fish, dead organisms, aquatic vegetation and associated fauna. Larval FoodZoeae are phytoplanktivorous and readily consume algae, phytoplankton and zooplankton. Megalope are considered general scavengers, bottom carnivores, detritivores, and omnivores. Megalope are more omnivorous than zoeae and prey upon fish larvae, small shellfish, and aquatic plants.

Abundance & Predators

Abundance & PredatorsPredators claim large numbers of young crabs, and crab populations may vary from year to year according to the abundance of predators. Blue crabs are subject to predation throughout their life cycle and are particularly susceptible when they are soft during the molting process.

As larvae, they are vulnerable to fishes, jellyfish, shrimp, and other planktivores. Plankton feeders eat the larvae as they drift in the water; after they settle, eel, drum, striped bass, sea trout, catfish, spot, and other blue crabs* are primary predators.

The megalopae and juvenile crabs are consumed by various fishes and birds, as well as other blue crabs*.

Adults are consumed by other blue crabs*, American eels, striped bass, Atlantic croakers, cobia, red drum, black drum, oyster toadfish, sandbar sharks, bull sharks, cownose rays, speckled/spotted trout, weakfish, catfish, gars, largemouth bass, loggerhead turtles, Atlantic Ridley turtles, herons and egrets, various diving ducks and raccoons.

*The blue crab is well known for its cannibalistic habits. Cannibalized blue crabs make up as much as 13% of a crab's diet. Blue crabs in poor health, missing important appendages, heavily fouled with other organisms, and those during or immediately following molt are more likely to be cannibalized.

Autotomy & Regeneration

Autotomy & RegenerationBlue crabs have the ability to sacrifice limbs (called autotomy) in order avoid capture. Missing limbs are regrown by a process called regeneration. RangeRangeThe range of the blue crab is from Nova Scotia, down the east coast of North America (including the Chesapeake Bay), off Bermuda, throughout the Gulf of Mexico and the Caribbean Sea (including the Antilles Islands), and down the east coast of South America to northern Argentina. Although the blue crab is rarely found north of Cape Cod, it has been seen in Maine and Nova Scotia following consecutive warm years.

The blue crab has been introduced, probably via ship ballasts into Europe, north Africa, and southwest Asia. Introductions into the Mediterranean Sea and adjacent waters have produced breeding populations whereas others were probably temporary occurrences. The blue crab also has been introduced into Japan.

Range (for Chesapeake Bay)Blue crabs are found from the mouth of the Chesapeake Bay to tidal fresh areas. There are distinct differences in the ranges of males and females. During the summer months, males are found from freshwater to the polyhaline zone (waters with salinities of 18-30 ppt), although they occur in the greatest numbers in salinities of 3-15 ppt. Maximum numbers of females occur down Bay at salinities of 10 ppt to ocean salinities.

Environment

EnvironmentWater TemperatureWater temperature requirements vary and are considered important, but no optimal range is reported.

When air temperatures drop below 50°F (10°C), adult crabs leave shallow, inshore waters and seek deeper areas where they bury themselves and remain in a state of torpor throughout the winter. Blue crab growth is regulated by water temperature. Growth occurs when water temperatures are above 59°F (15°C). Water temperature above 91°F (33°C) is lethal. Blue crabs are susceptible to sudden drops in temperature.

Water SalinitySalinity is important, but requirements vary by life stage. Generally optimum is 3-15 parts per thousand (ppt). Water pHTolerance range is pH 6-8. Less than 6 is lethal.