Blue crabs feature a flattened, roughly hexagon-shaped carapace, or shell, that can grow to 9 inches wide. Males can be distinguished from females by the shape of the “apron”, a shell part covering their abdomen –females have a wider abdomen, shaped like the dome of the United States Capitol Building, while males have a more narrow abdomen, shaped like the Washington Monument.
The crab’s shell is mostly a drab blue-green above and white below, while its claws and legs are blue in color. This blue color can appear dull or very vibrant. Mature females have red tips on their claws.
Blue crabs have three pairs of legs for walking and a fourth pair of flatter legs that are used for paddling. Using these legs, blue crabs can paddle through the water, and their scientific name actually means “beautiful swimmer”.