For other uses, see Las Tunas. Tuna, opah and mackerel sharks are the only species of fish that can maintain a body temperature higher than that of the surrounding water. Found in warm seas, the tuna is commercially fished extensively as a food fish, and is popular as a bluewater game fish. The cladogram is a tool yellowfin tuna steak visualizing and comparing the evolutionary relationships between taxa, and is read left-to-right as if on a timeline.
The following cladogram illustrates the relationship between the tunas and other tribes of the family Scombridae. The “true” tunas are those that belong to the genus Thunnus. Until recently, it was thought that there were seven Thunnus species, and that Atlantic bluefin tuna and Pacific bluefin tuna were subspecies of a single species. The Thunnini tribe also includes seven additional species of tuna across four genera. Bigeye tuna Thunnus obesus showing finlets and keels.
The tuna is a sleek and streamlined fish, adapted for speed. The tuna’s body is countershaded to camouflage itself in deeper water when seen from above, its dorsal side is generally a metallic dark blue while the ventral or under side is silvery or whitish. Tunas achieve endothermy by conserving the heat generated through normal metabolism. In all tunas, the heart operates at ambient temperature, as it receives cooled blood, and coronary circulation is directly from the gills. Also unlike most fish, which have white flesh, the muscle tissue of tuna ranges from pink to dark red.
The red myotomal muscles derive their color from myoglobin, an oxygen-binding molecule, which tuna express in quantities far higher than most other fish. The oxygen-rich blood further enables energy delivery to their muscles. For powerful swimming animals like dolphins and tuna, cavitation may be detrimental, because it limits their maximum swimming speed. Even if they have the power to swim faster, dolphins may have to restrict their speed, because collapsing cavitation bubbles on their tail are too painful. This section needs additional citations for verification. Please help improve this article by adding citations to reliable sources. Tuna is an important commercial fish.