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Female blue crabs mate only once in their lives, when they become sexually mature. They capture and store the males' sperm in sac-like receptacles to be used to fertilize their eggs between two to nine months after mating. Soon after mating, the female crabs migrate to high salinity waters near inlets (Figure 1).

At these more saline locations, the females fertilize and spawn their eggs into a large, cohesive mass, i.e. sponge (Figure 2). The sponge remains attached to fine hairs beneath their abdomen until the eggs hatch. The sponge can contain between 750,000 to eight million eggs, depending on the size of the female. Two million is the average. Figure 3 shows the life stages the blue crab from egg to adult.

(www.blue-crab.org/lifecycle1.htm; Etherington & Eggleston, 2000).
Figure 1. Albemarle and Pamlico Sound. Purple arrows point to three primary inlets for water exchange in this estuarine ecosystem.

 

 
Figure 2. This is a gravid female. She is holding an egg mass on her abdomen that contains millions of eggs . As the eggs mature, their color will change from bright orange to brown.
Figure 3. Life stages of blue crab. (images for diagram courtesy of Thomas Schafer, UNC Wilmington and Alicia Young-Williams, Smithsonian Environmental Research Center)
  1. The eggs take two weeks to fully develop and hatch into larvae.
  2. The released larvae are transported out to the continental shelf where they develop through 7 to 8 larval stages. This takes approximately one month, depending on water temperature and salinity.
  3. The larvae then metamorphose to the post-larval megalopa. Post-larvae are transported back into the estuary via wind or tidal driven currents. The megalopae settle on to sea grass or other hard material on the estuary floor, and then metamorphose into the first benthic juvenile stage.
  4. The juveniles migrate to the shallower, less saline waters in upper estuaries and rivers. In these shallow, protected areas, the juveniles grow and mature through each molt of their exoskeleton. The blue crab is now a benthic organism, meaning it primarily remains on the sea floor.
  5. The crabs become sexually mature adults after about 18 to 20 postlarval molts, which is about 1 to 1.5 years since they first hatched. After mating, female blue crabs migrate towards the inlet of the sound where they remain until their eggs hatch (see above).
(www.blue-crab.org/lifecycle1.htm; Etherington & Eggleston, 2000; Eggleston, personal communication 8/14/03).

In North Carolina, there are two recruitments of juveniles, one minor peak in May and a second larger peak from August to November. Recruitment refers to the life stage when the megalopae metamorphose into the first juvenile stage, and then settles on to the estuary floor. The shallower, less saline waters further up the estuary provide a safe refuge for the juvenile to grow and mature.

However, the molting and growing stops during winter, and resumes as the water warms. The juveniles usually reach maturity during the spring or summer of the year following their hatching. In addition, the blue crab's diet changes as it transitions through its various life stages.

(www.blue-crab.org/lifecycle1.htm; Etherington & Eggleston, 2000).
Table 1. Blue crab diet at different life stages.
Stage Food
egg yolk in egg sac
larva phytoplankton and zooplankton
megalopa & juvenile general scavenger, bottom carnivore, detritivore, & omnivore; includes benthic macroinvertebrates, small fish, dead organisms, aquatic vegetation
adult same as juvenile, also include mollusks and crustaceans

Blue crabs in North Carolina spend different parts of their lives in various regions of the Albemarle-Pamlico Estuarine Ecosystem (APEE).

EGGS remain attached to the abdomen of the females, who stay near the inlets, until they are hatched.

LARVAE are transported out of the estuary, and remain in water along the continental shelf.

POST-LARVAE are brought back into the estuary via tidal currents and settle on seagrass. Tropical storms can also carry post-larvae into the estuary, in which case the larvae also settle on submersed rooted vasculars.

JUVENILES migrate from the outer edges of the sound towards the upper estuary and rivers. These regions are less saline, and provide shelter from predators.

ADULTS primarily remain on the estuary floor, and move out towards deeper waters in the middle of the sound. Females remain near the inlets after they mate.

  1. How would increased rain and flooding affect the blue crabs?
  2. What ecological disturbances could give rise to increased rain and flooding?

(Eggleston, personal communication 8/14/03)

Figure 3. Three habitat types in Albemarle and Pamlico Sounds. (SG = seagrass; SDH = shallow detrital habitat; SRV = submersed rooted vasculars. Red arrows point to three primary inlets that transport blue crabs in and out of this estuarine ecosystem.

 

(Etherington & Eggleston, 2000).

Figure 4. Total pounds of blue crab landed (i.e. crabs harvested and brought to dock) in North Carolina from 1972 to 2001.

Use this graph of total blue crab landings as an indication of the blue crab population in North Carolina. There was continued increase in blue crab harvest from 1972-1998. Notice the sudden drop in population in 1999-2000.

  1. What could have caused the sudden drop?
  2. Is this decline a natural population fluctuation or an indication of overfishing?
  3. Does the steady increase in blue crab landings from 1972 to 1998 suggest that the blue crab population had been increasing during that time period? Compare this with catch per unit effort data.
(www.ncfisheries.net/comstat/index.html)

Many types of harvesting gear have been used by the commercial blue crab fishery, including trotlines, crab pots, and trawls. Currently, crab pots (Figure 5) are used in harvesting approximately 95% of total hard blue crabs in North Carolina. The huge jump from 30% crab pots used in the 1950s show that crab pots are more efficient and effective, and thus preferred method for the commercial fishery.

The pots are baited, left in the water, and then harvested at different time intervals. NC Division of Marine Fisheries estimates that over 800,000 crab pots are used annually in NC estuaries.

(legacy.eos.ncsu.edu/eos/info/mea/mea469_info/bluecrab/fishery.html)

Figure 5. Picture of a crab pot used to catch blue crabs.

  1. A crab smells the bait and circles the pot, entering through one of the throats.
  2. Once inside and unable to reach the bait, the crab feels trapped and threatened. A crab instinctively swims up towards the surface to escape, but winds up inside the parlor.
  3. It remains in the parlor until removed through a special opening along one of the top edges.
  4. Two small exit holes up high in the parlor called "cull rings" let small crabs escape.
(www.blue-crab.org)
CONTROVERSY

The primary concern regarding the use of crab pots is the potential navigation hazard they pose when the traps are set and when crab fishers abondon old pots.

(Eggleston, pers. comm. 8/18/03)
 
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This web site was created by Lynn Tran at the North Carolina State University, Department of Mathematics, Science, and Technology Education on 7/12/03. Faculty advisor Dr. David Eggleston, NCSU, Department of Marine, Earth, & Atmospheric Sciences. Last updated December 29, 2003 .