Crabs grow by molting or shedding their shell. Just prior to molting, the crab is encased in both the hard, old outer shell and a soft, new one just beneath it. The formation of a new shell is evident along the margins of the swimming paddles of a crab. The earliest indication of the new skeleton is the formation of a black line along the rim of the paddles. When this line turns pink or red, the crab is referred to as a "peeler" or "shedder". Immediately after the molt, the crab's new shell is pliable and easily stretched. In this condition, the crab is called a "soft crab" or a "soft shell crab". Large amounts of water are consumed prior to and shortly after the molt, causing the soft shell to expand and increase in size. This entire process takes 2-3 hours and within 2 hours after the shed, the new shell begins to harden. The "papershell" is formed within 12 hours and an additional 2-3 days are needed before the shell fully hardens. Unlike male crabs that continue to molt and grow throughout their entire lives, females stop growing when they reach sexual maturity, usually after 21 or 22 molts. During this final molt, mating takes place.

Blue crabs mate from May to October in the brackish or slightly salty waters of Chesapeake Bay. Just prior to the final molt, an immature female crab, known as a "she-crab", is cradled by a mature male. The female is escorted by the male, commonly referred to as a "doubler", for a few days before and after her molt. During the molt, the male releases the female, but remains nearby. After molting, the female turns on her back and unfolds her abdomen. The male then transfers his sperm to the female. Although the female mates only once, she may spawn several times. The sperm received is stored and used to fertilize the eggs of all future spawnings. After mating, the two crabs resume the cradle carry until the female's shell hardens. Shortly after mating, the now mature female crab, known as a sook, migrates to the saltier waters of the Bay near the ocean. Adult males and immature females remain in the brackish waters of the Bay and its tributaries, migrating to shallow grassbeds, shallow muddy bottoms, and/or deeper waters of mid-Bay as temperatures begin to drop in the fall.

The female crab lays her eggs from 2 to 9 months after mating, depending upon when the mating took place. For example, a spring mating would result in a late summer spawn, while a fall mating would result in an early summer spawn the following year. Females develop an external egg mass on the underside of their abdomen which may contain between 750,000 and 8 million eggs, depending on the size of the crab. Young sponges are orange and gradually turn to brown and then black as the sponge matures. These crabs are called "sponge crabs" and hatching of the eggs occurs in approximately 2 weeks from June through September.

The newly-hatched larvae are called zoea and look nothing like an adult crab. These young crabs are microscopic in size and drift about in the water currents. It is believed that the majority of these developing larvae are transported into the ocean by an interaction of seasonal winds and bottom water circulation patterns, and eventually returned to settle on seagrass beds in the spawning area. After approximately 6 or 7 molts, the zoea changes into a post-larval form known as the megalops. The megalops has claws like a crab, but can swim and crawl on the bottom. Eventually the megalops settles and metamorphoses to the first crab stage which looks much like an adult crab, but is only 1/5 of an inch from point to point. As these young crabs develop their locomotion, they will migrate away from the high salinity waters near the mouth of the Bay up to more brackish regions. By winter, juvenile crabs can be found as far north as the Susquehanna Flats.

As winter approaches, most crabs will bury themselves in the mud and shallow grassbeds of the Bay. Female crabs will remain in the higher salinity waters of the lower Bay, whereas males will remain in the upper portions, migrating to deeper waters to spend the winter months. Little or no growth occurs from December to March, but when the temperature begins to rise, crabs become more active, begin feeding and searching for a mate.
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