Blue crabs mate in early summer and fall. The crabs can only mate when the female crab has just molted and her outer shell is still soft.
Female blue crabs, like the males, molt several times in order to grow, but a particularly important molt when an immature female becomes mature. Immature female crabs that are ready to mature on the next molt are called pre-pubescent females and release chemicals to attract mature male crabs in the area.
The male crabs may have to compete with each other for the female. During a distinct mating ritual the male crab stands on the tips of his walking legs and uses his swimming paddles to rock from side to side.
If the pre-pubescent female accepts the male, the two "double-up" and the male carries her around--usually for one or two days--underneath him until she is ready to molt.
When the female molts, they separate for a little while, but immediately after she has molted and shed her old outer shell, the crabs double-up again and begin to mate. The photo below shows mating crabs with the female's empty exoskeleton on the left.
After the female crab has shed her outerskeleton (on the left) the male and female crabs "double-up" to mate..
Depending on the conditions, the crabs will mate for a few hours. The females will keep the sperm in a sac until they use it to fertilize the eggs later that year, usually during her fall migration to the mouth of the Bay. The pictures below show front and rear views of the mating crabs.
Front and rear view of mating blue crabs.
After they mate, the crabs stay together and the male continues to cradle the female crab until her shell hardens again. According to the researchers at SERC, the male crab may be doing this to protect his "investment" by making sure that the female doesn't mate again. Researchers have noticed that some females, if released before their new shells had hardened, may mate a second time.