Tuna are fish from the family Scombridae, mostly in the genus Thunnus. Tuna are fast swimmers—they have been clocked at 70 kilometres per hour —and include several warm-blooded species. Unlike most fish, which have white flesh, tuna flesh is pink to dark red, which could explain their odd nick-name, “rose of the sea.” The red coloring comes from tuna muscle tissue’s greater quantities of myoglobin, an oxygen-binding molecule. Some of the larger species, such as the bluefin tuna, can raise their blood temperature above water temperature through muscular activity. This ability enables them to live in cooler waters and to survive in a wide range of ocean environments.