Copper is necessary in all higher plants and animals. Copper is carried mostly in the bloodstream on a plasma protein called ceruloplasmin. When copper is first absorbed in the gut it is transported to the liver bound to albumin. Copper is found in a variety of enzymes, including the copper centers of cytochrome c oxidase and the enzyme superoxide dismutase (containing copper and zinc), and is the central metal in the oxygen-carrying pigment hemocyanin. The blood of the horseshoe crab, limulus Polyphemus, uses copper rather than iron for oxygen transport.