Also known as the Chesapeake blue crab or the Atlantic blue crab, these crabs are strong swimmers—largely due to their fifth pair of legs, which are shaped like paddles. They are striking to spot with their often bright-blue claws and olive-colored carapace (shell). The claws on the adult female blue crab are tipped with red. Males can be seven to eight inches (18 to 20 centimeters) across, while females are a bit smaller in size.