Crabs are chiefly marine, but some are terrestrial for long periods.
The abdomen of the female, wider and flatter than that of the male, forms an apronlike structure that continuously circulates water over the eggs that are carried on her underside. The free-swimming larva, which hatches in about two weeks, is easily recognized by the large spine that projects from its carapace. After several molts, the young crab settles to the bottom and begins to take on adult features. Crabs tend to move sideways, although they are capable of locomotion in all directions. It is marketed as a soft-shelled crab after it has molted and before the new shell has hardened.