Page:Dungeness Crabs of Glacier Bay.pdf/3

 extrudes them under her abdomen where they are carried for 3­5 months until hatching. A large female crab can carry 2.5 million eggs. Egg incubation tends to be much longer in Alaska than further south. The female crab stores the sperm until her eggs are fully developed. In Alaska, fertilization typically occurs in September.

Many female crabs do not reproduce annually in Alaska. Larger females (greater than 141 mm in carapace width) are probably the group of females that do not reproduce annually, possibly because they don't molt as often and rely on stored sperm to fertilize their eggs. Egg­bearing females often congregate and bury themselves in areas of suitable substrate, sheltered from winter storms while they incubate their eggs.



When hatched, the new crabs are planktonic and swim freely away from the female. Larval development takes from 4 months to a year in Alaska. In the larval stage, these babies are free­floating in the water column and are transported primarily by currents. There are 6 successive life stages (5 zoeae and 1 megalop) that occur before they enter the first juvenile stage and begin to resemble a miniature crab ­ ¼ to "dime­size." Female crabs grow more slowly than the males.

With claws and legs, the megalop is more recognizable as a young crab, but it still has a shrimp­like abdomen. Megalopae are relatively strong swimmers, moving at speeds up to 22 centimeters per second, and are typically present in surface waters only at dawn, dusk and night. During the day, they usually move down in the water column to a depth of at least 20 meters. In Glacier Bay, a significantly larger number of megalops were caught at high tides than at low tides, suggesting that late­stage larvae may be transported to nearshore areas with flooding tides.

Crabs can only grow by periodic shedding of their shell in a process called molting. The carapace does not grow in the continuous, gradual manner of animals with internal skeletons ­ such as fish and mammals. Instead crabs must regularly shed their exoskeletons in order to increase in size. This process is known as molting, and individuals typically increase in size by about 15 to 25 per cent with each molt. All the hard parts of the crab are shed as one piece. During a molt, the old shell splits at the back and along both sides, and the crab backs out of its shell. The opening in a molted shell closes up after the crab has exited. Young crabs, which put all their energy reserves into body growth, molt more frequently and to a relatively larger size than adult crabs, which devote more of their energy to reproduction.