The blue swimmer crab (Portunus armatus) is a crab native to the Indo-Pacific, found from East Africa to Japan and northern Australia. It has a blue-green carapace and white-tipped legs, growing to 12-20 cm across the shell, with sweet, delicate meat prized from the body (lump meat) and claws.
Blue swimmer crab meat is used in seafood salads, crab cakes, dips, and as a topping for pasta, risotto, or pizza; in Asian cooking it appears in crab fried rice, noodle soup, and steamed preparations. Its sweetness pairs well with lemon, lime, and other acidic or aromatic ingredients.
Despite the similar name, blue swimmer crab is a different species from the Atlantic blue crab (Callinectes sapidus); the two aren't interchangeable when a recipe specifies one, though both work in dishes that simply call for crab meat. In parts of Australia it's also called sand crab, a name that separately causes confusion with true sand crabs (mole crabs), an unrelated, mostly inedible family.
Live blue swimmer crabs should feel heavy for their size with intact shells and legs; frozen crab meat is a convenient substitute, though fresh crab has better flavor and texture.