"Callinectes sapidus" is a decapod crab of the swimming crab family Portunidae. The genus "Callinectes" is distinguished from other portunid crabs by the lack of an internal spine on the carpus (the middle segment of the claw), as well as by the T-shape of the male abdomen. Blue crabs may grow to a carapace width of . "C. sapidus" individuals exhibit sexual dimorphism. Males and females are easily distinguished by the shape of the abdomen (known as the "apron") and by color differences in the chelipeds, or claws. The abdomen is long and slender in males, but wide and rounded in mature females. A popular mnemonic is that the male's apron is shaped like the Washington Monument, while the mature female's resembles the dome of the United States Capitol. Claw color differences are more subtle than apron shape. The immovable, fixed finger of the claws in males is blue with red tips, while females have orange coloration with purple tips. A female's abdomen changes as it matures: an immature female has a triangular-shaped abdomen, whereas a mature female's is rounded.