Tuna are several species of ocean-dwelling fish in the family Scombridae, mostly in the genus Thunnus. Tuna are fast swimmers and include several species that are warm-blooded. Unlike most ocean fish species, which have white flesh, the flesh of tuna is pink. This is because the tuna's blood has a higher oxygen carrying ability than other fish species. Some of the larger tuna species such as the bluefin tuna can raise their blood temperature above the water temperature with muscular activity. This enables them to live in cooler waters and survive a wider range of circumstances.
Mercury levels are oftentimes relatively high in tuna, as they sit higher in the food chain. Because of this, the FDA in March 2004 issued guidelines recommending pregnant women, nursing mothers and children should limit their intake of tuna and certain other types of fish.
It is difficult to keep tuna in a captive environment; Monterey Bay Aquarium is one of the few aquariums in the world that successfully keep tuna in display.
Species of several other genera (all in Scombridae) are commonly named "tuna":
- Slender tuna Allothunnus fallai
- Bullet tuna Auxis rochei rochei
- Frigate tuna Auxis thazard thazard
- Kawakawa (little tuna or mackerel tuna) Euthynnus affinis
- Little tunny (little tuna) Euthynnus alletteratus
- Butterfly kingfish (Butterfly tuna) Gasterochisma melampus
- Dogtooth tuna Gymnosarda unicolor