The Barney Family  

 

Mating occurs primarily in relatively low-salinity waters in the upper areas of estuaries and lower portions of rivers. Blue crabs mate from May through October. Extended periods of low temperatures will usually significantly shorten the spawning season.

The male may mate during its third or fourth intermolt phase after it matures. Females mate only once in their lives, following the pubertal molt (when she changes from "sally" to "sook".) When approaching this pubertal molt, females release a pheromone in their urine which attracts males. Male crabs vie for females and will protect them until molting occurs. The male will clasp the immature female with his walking legs and carry her underneath him until she is ready to molt. This is called a precopulatory embrace, or more commonly known as a "doubler" or "buck and rider."

When the female begins her molt, the male releases her and will stand nearby to guard and protect her. Immediately following her molt she is now sexually mature ("sook") and her abdomen will change from an inverted "V" shape into an inverted "U" shape. The male will once again clasp the female with his walking legs, only now she is held upside-down (abdomens facing). This is called a copulatory embrace since this is the time that mating takes place, her shell still soft. Pleopods of the male and swimmerets (the paired abdominal appendages under the apron of the female crab on which the eggs are carried until they hatch) of the female are intromittent organs which aid in copulation. Mating may last as long as 5 to 12 hours. During mating, the female captures and stores the male's sperm in seminal receptacles for use at a later time.

After mating has taken place, the female with turn right-side up and the male will continue to carry her for several more days, called a postcopulatory embrace, until her shell completely hardens and she can once again defend herself

Although the female will mate only once in her life, the stored sperm may be used as often as the female spawns, generally two or more times during a 1- or 2-year period.

After mating, females migrate to high-salinity waters in lower estuaries, sounds, and near-shore spawning areas. 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. They over-winter before spawning by burrowing in the mud. 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. 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 setae on the appendages of the abdomen until the larvae emerge. The sponge which may contain 700,000 to 2 million eggs, is formed in about two hours.

The eggs are bright orange when first deposited, but become yellow, brown, and then dark brown before hatching. This incubation period generally requires one to two weeks. The color change is caused by absorption of the yellow yolk and development of dark pigment in the eyes and on the body. Eggs are about 0.25 mm in diameter. Hatching of blue crab eggs only occurs at salinities of 23-33 ppt and temperatures of 66-84 ºF Mortality of eggs has been attributed to fungal infection, predation, suffocation in stagnant water, and exposure to extreme temperatures. On the average, one out of every million eggs survives to become a mature adult.

Growth 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.
 
 

First stage larvae, called zoeae, measure approximately 0.25 mm wide at hatching. The larvae bear little morphological resemblance to adults, are filter feeders, and planktonic.

Evidence suggests that blue crab zoeae hatch in the 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.

Zoeae are phytoplanktivorous and readily consume algae, phytoplankton and zooplankton.The zoeae and all subsequent life stages can increase body size only by molting. Zoeal development depends on salinity and temperature, but development time has been shown to be variable even in a single salinity-temperature regime. Larvae molt seven to eight times before entering the next stage of development.
 
 

The final molt of the zoea is characterized by a conspicuous change to the second larval stage (megalops) at about 2.5 mm carapace width; development to this stage requires 31 to 49 days. The megalopa larva is more crablike in appearance than the zoea; its carapace is broader in relation to its length, and it has pinching claws and pointed joints at the ends of the legs. It swims freely, but generally stays near the bottom in nearshore or lower-estuarine, high-salinity areas. The megalopal stage lasts 6 to 20 days, after which the larva molts into the "first crab" stage, or juvenile stage, characterized by adult proportions and appearance.

There are usually seven zoeal stages and one postlarval, or megalopal, stage. On occasion, an eighth zoeal stage is observed. Larval release is often timed to occur at the peak of high tide, thus assuring that larval abundance is greatest when the tide begins to ebb. Blue crab larvae are advected offshore, and complete development in coastal shelf waters. Typical time for development through the seven zoeal stages is between 30 to 50 days before metamorphosis to the megalopal stage. The megalopa then persists between 6 to 58 days. It is widely believed that it is the megalopal stage that subsequently return to estuaries for settlement, and eventual recruitment to adult populations.

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.
 
 

The juvenile "first crab" is typically 2.5 mm wide (from tip to tip of the lateral spines of the carapace). Juveniles gradually migrate into shallower, less-saline waters in upper estuaries and rivers, where they grow and mature. It was previously reported that many juveniles had completed this migration by fall. New evidence, however, suggests the bulk may not reach the upper parts of tributaries and Deleware Bay until the following summer. Males, which prefer low-salinity waters, generally migrate farther upstream than do females, which tend to stay in the lower rivers and estuaries.

Juvenile  blue crabs feed mostly on benthic macroinvertebrates, small fish, dead organisms, aquatic vegetation and associated fauna.

Growth and maturation proceed during a series of molts and intermolt phases; each of these crab stages is identified by the number of molts that have occurred since the megalopal stage. It has been estimated that juveniles reached the 9th or 10th crab stage by October in the Deleware Bay. Molting and growth stop during winter, but resume as waters warm. The crabs generally reach maturity during the spring or summer of the year following the year of hatching.
 

Adult 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.

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.

Blue crab 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.

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.

Predators of blue crabs include fish as well as other blue crabs. The major fish predators of blue crabs include the Black Drum, Red Drum, the American Eel, and the American Croaker.