The blue crab is a decapod, which means it possesses ten appendages, of which four pairs are segmented walking legs. Its last pair of legs is paddle-shaped, permitting the crab to swim more actively than most other crab species. These legs, called the paddle fins, are flexible so that they can be rotated in a sculling action that allows the crab to propel itself through the water. The crab is capable of swimming at burst speeds of up to one meter per second.

The largest appendages are a pair of segmented arms ending in pincer-like claws, which the crab uses for defense and to grasp and crush prey. The carapace is the shell that covers and protects the crab. Eyes, mouth, and antennae are located at the front edge of the carapace.

Crabs have an outer hard shell called an exoskeleton. Unlike skin, the hard shell can't stretch to accommodate growth, so the crab must molt, or shed its shell, in order to grow. As the crab grows, the shedding process occurs again and again, as often as 25 times.

• Anatomy of a Blue Crab
• Blue Crab Health Field Identification
• Lifecycle of a Crab