Tunas are elongated, and streamlined fishes; they have a rounded body that tapers to a slender tail base and a forked or crescent-shaped tail. In color, tunas are generally dark above and silvery below, often with an iridescent shine. A row of small finlets behind dorsal and anal fins, and a corselet of enlarged scales in the shoulder region. Another notable feature is a well-developed network of blood vessels below the skin that acts as a temperature-regulating device associated with long-term, slow swimming. Because of this vascular system, tunas are unique among fishes in their ability to maintain the temperature of their bodies above that of the surrounding water, often between 5 and 12 °C (9 and 21.7 °F) above ambient water temperature. Some muscles may even be up to 21 °C (almost 39 °F) higher than the surrounding water.
