A tuna is a saltwater finfish that lies in the tribe Thunnini, a sub-grouping of the mackerel family (Scombridae) – which in common with the tunas also comprises the bonitos, mackerels, and Spanish mackerels. Thunnini encompasses 15 species across five genera, the sizes of which differ very much, ranging from the bullet tuna (max. length: 50 cm (1.6 ft), weight: 1.8 kg (4 lb)) up to the Atlantic bluefin tuna (max. length: 4.6 m (15 ft), weight: 684 kg (1,508 lb)). The bluefin averages 2 m (6.6 ft), and is deemedto live for up to 50 years.
Their circulatory and respiratory systems are sole among fish, allowing them to keep a body temperature higher than the surrounding water. An active and agile predator, the tuna has a smooth, sleek body, and is among the speediest-swimming pelagic fish – the yellowfin tuna.