A tuna is a saltwater fish that belongs to the tribe thunnini, a subgrouping of the scombridae (mackerel) family. the thunnini comprise 15 species across five genera, the sizes of which vary greatly, ranging from the bullet tuna (max length: 50 cm or 1.6 ft, weight: 1.8 kg or 4 lb) up to the atlantic bluefin tuna (max length: 4.6 m or 15 ft, weight: 684 kg or 1,508 lb), which averages 2 m (6.6 ft) and is believed to live up to 50 years. tuna, opah and mackerel sharks are the only species of fish that can maintain a body temperature higher than that of the surrounding water. an active and agile predator, the tuna has a sleek, streamlined body, and is among the fastest-swimming pelagic fish...

Source: Wikipedia