A tuna is a saltwater finfish that belongs to the tribe Thunnini, a sub-grouping of the mackerel family (Scombridae) – which together with the tunas, also includes the bonitos, mackerels, and Spanish mackerels. Thunnini comprises fifteen species across five genera, the sizes of which vary greatly, ranging from the bullet tuna (max. length: , weight: ) up to the Atlantic bluefin tuna (max. length: , weight: ). The bluefin averages , and is believed to live for up to 50 years. Their circulatory and respiratory systems are unique among fish, enabling them to 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 – the yellowfin tuna, for example, is capable of speeds of up to . Found in warm seas, it is extensively fished commercially, and is popular as a game fish. As a result of over-fishing, stocks of some tuna species such as the Southern bluefin tuna have been reduced dangerously close to the point of extinction.