The crab's reproductive organs are situated near and just below the heart. They open to the outside at the base of the last pair of walking legs in the male crab and at the base of the middle pair of walking legs in the female. In most species the female carries the eggs cemented to her undersides and protected by the flexed tail. Most crabs hatch at the third stage, the zoea, and then change into the fourth stage, the megalops, before assuming adult form. In some crabs, such as the freshwater Potamon, the young hatch as miniatures of the adult.
Crabs grow by shedding, or molting, their hard shells.
The blue crab, Callinectes sapidus, is believed to live about 3 years. The English crab Cancer has reached at least 12 years of age. Most crabs feed on small fish or worms or else scavenge along the shore or sea bottom. Many crabs dig or inhabit burrows, but Hypoconcha carries half of a clamshell over its body. Dromia cuts out and attaches a roof of living sponge to its back.