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. Crabs communicate with one another by several means, including claw waving and drumming, as in the fiddler crab, Uca. Stone crabs, Menippe, Chinese land crabs, Helice, and others stridulate much like crickets by bending and rubbing the first walking leg against itself or against ridges on the body. Crabs belong to the order Decapoda, class Crustacea, phylum Arthropoda.