When fully grown the spine may be more than 8 inches wide. The abdomen and lower legs are white. Crab claws are various shades of blue, but the claw tips of the female are red. Sexes can be identified by the abdominal flap or apron. In the male it is shaped like an inverted T, but in the female it is broader.
Parasites are common on crabs.
However, most of these parasites do not affect the life of the crab. As with other crustaceans, the blue crab must molt or shed its hard, outer shell as it increases in size. When the body becomes too large for its shell, a new soft shell is formed beneath the hard one. After freeing itself from the shell, the crab takes in large amounts of water and expands its soft and wrinkled body. During this soft stage the crab is easy prey and must seek shelter.