Blue crabs in Chesapeake Bay may take up to 18 months to reach maturity, depending on the water temperature and the length of the season.
Mating takes place in waters which are not high in salt content from June through October. It generally occurs in areas chosen by the female crab and in usually shallow marshy areas or beds of submerged vegetation.
The male carries and protects the female until molting is imminent and then releases her, but stays close and may assist her in the process. The male uses its first pair of walking legs to carry the female against his sternum in a “cradle carry” position. Mating can occur only while the female is in the soft shell state after undergoing her final molt. After insemination the male continues to carry and protect the female until her shell hardens. Unlike males, females mate only once.
Mating can take place throughout the entire Chesapeake Bay. Pairs of crabs double up when the female nears the time she will molt into a mature crab so long as she is still a soft shell crab. After mating in late summer and early fall adult female crabs migrate to the mouth of the Chesapeake Bay. Mostly during the month of October where they produce egg masses numbering 800,000 – 8,000,000. Zoea (larval stage) hatches out of the egg and becomes part of the plankton community feeding on phytoplankton. There are seven molt stages in the zoeal larval stage. The megalopa is the post larval stage and is a voracious eater in the plankton community feeding mainly on zooplankton. There is only one molt stage for the megalopa which molts into a crab. When the megalopa molts into the first crab stage, it has been swept into the mouth of the Bay by tidal currents, winds, and cool ocean water coming in from the bottom of the channel. The juvenile male and female crabs use the underwater grass beds for feeding and protection during molting.
Oval shaped bundles of spermatozoa are ejected into the female. Viable sperm is stored for later egg fertilization. Sperm may live in female seminal receptacles for a year or more and be used for repeated spawnings.
Fertilized eggs are then extruded and attached to the bottom of the outer shell to form a large, bright yellow egg mass. The sponge turns black just prior to hatching in about two weeks.
Many, if not all, females spawn twice, which may be during the same season or over two seasons.
A female crab born in May or June will generally produce eggs in two years. Female crabs usually lay their eggs shortly after copulating but can also store sperm for many months. The eggs are fertilized as they are laid by passing through the chamber holding the sperm. Eggs are brooded in a mass attached to hair on the female’s abdomen. The number of eggs carried can be very large but depends on the size of the crab. Some species may carry tens of thousands of eggs when fully grown. Once developed the egg hatches into a tiny larva called a zoea. Release of the zoea is aided by the female wafting her abdomen to and fro. The crab larvae spends its life swimming in the plankton moulting several times until it reaches a stage ready to settle on the sea floor again.
Fertilization includes copulation, which occurs only once in the life of each female, during the few hour period following the female’s terminal or puberty molt. During this doubling period, the crabs seek sheltered areas secure from predators. Immediately after the female has completed the molt, the male rotates the female so that they face abdomen to abdomen and inserts two thin appendages at the base of his abdomen into corresponding receptacles in the female. The appendages are covered with small barbs that inhibit retraction, and copulation may last several hours. Sperm is transferred to the female in packets that she stores for later use in fertilizing her eggs. Each female may fertilize as many as three broods of eggs (as many as 9 million eggs) based on this single copulation.
Hatching occurs in late July and early August. The resulting larvae (zoeae) are carried from the bay on the ensuing ebb tide. Zoeae are quite small, less than 1 mm in length, and remain in the plankton for four to six weeks. The entire period of zoeal development, including seven to eight discrete molt stages, occurs on the inner continental shelf.
After zoeal development, blue crabs enter a post-larval phase known as the megalopa, the stage that begins the “reinvasion” of the estuary. After one to two weeks, the megalopae undergo metamorphosis to the first juvenile crab stage, which is identical to the adult crab except for its tiny size. About a month later, the juvenile crab has grown to the size of a dime.
Juvenile crabs burrow in the sediment and remain inactive until April of the following year. Juvenile growth continues throughout the summer and fall as the crabs forage widely in shallow waters and marshes along the bay. The next winter, the crab burrows again in the sediment and become inactive until the following spring when most will reach sexual maturity, copulate, and start the cycle again. Most crabs die before the onset of a fourth winter, which is a life span of about three years.