The Thunnini tribe is a monophyletic clade comprising 15 species in five genera:

family Scombridae tribe Thunnini: tunas genus Allothunnus: slender tunas genus Auxis: frigate tunas genus Euthynnus: little tunas genus Katsuwonus: skipjack tunas genus Thunnus: albacores and true tunas subgenus Thunnus (Thunnus): bluefin group subgenus Thunnus (Neothunnus): yellowfin group

The cladogram is a tool for 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. For example, the cladogram illustrates that the skipjack tunas are more closely related to the true tunas than are the slender tunas (the most primitive of the tunas), and that the next nearest relatives of the tunas are the bonitos of the tribe Sardini.
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. In 1999, Collette established that based on both molecular and morphological considerations, they are in fact distinct species.

The genus Thunnus is further classified into two subgenera: Thunnus (Thunnus) (the bluefin group), and Thunnus (Neothunnus) (the yellowfin group).

Thunnus, the true tunas Image Common name Scientific name Maximum length Common length Maximum weight Maximum age Trophic level Source IUCN status Thunnus (Thunnus) – the bluefin group Albacore tuna T. alalunga (Bonnaterre, 1788) 1.4 m (4.6 ft) 1.0 m (3.3 ft) 60.3 kg (133 lb) 9–13 yrs 4.31 Least Concern Southern bluefin tuna T. maccoyii (Castelnau, 1872) 2.45 m (8.0 ft) 1.6 m (5.2 ft) 260 kg (570 lb) 20–40 yrs 3.93 Endangered Bigeye tuna T. obesus (Lowe, 1839) 2.5 m (8.2 ft) 1.8 m (5.9 ft) 210 kg (460 lb) 5–16 yrs 4.49 Vulnerable Pacific bluefin tuna T. orientalis (Temminck & Schlegel, 1844) 3.0 m (9.8 ft) 2.0 m (6.6 ft) 450 kg (990 lb) 15–26 yrs 4.21 Near Threatened Atlantic bluefin tuna T. thynnus (Linnaeus, 1758) 4.6 m (15 ft) 2.0 m (6.6 ft) 684 kg (1,508 lb) 35–50 yrs 4.43 Least Concern Thunnus (Neothunnus) – the yellowfin group Blackfin tuna T. atlanticus (Lesson, 1831) 1.1 m (3.6 ft) 0.7 m (2.3 ft) 22.4 kg (49 lb) 4.13 Least concern Longtail tuna, northern bluefin tuna, tongol tuna T. tonggol (Bleeker, 1851) 1.45 m (4.8 ft) 0.7 m (2.3 ft) 35.9 kg (79 lb) 18 years 4.50 Data deficient Yellowfin tuna T. albacares (Bonnaterre, 1788) 2.4 m (7.9 ft) 1.5 m (4.9 ft) 200 kg (440 lb) 5–9 yrs 4.34 Least Concern

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The tuna is a sleek, elongated and streamlined fish, adapted for speed. It has two closely spaced but separated dorsal fins on its back; The first fin is "depressible" – it can be laid down, flush, in a groove that runs along its back; it is supported by spines. Seven to ten yellow finlets run from the dorsal fins to the tail, which is lunate – curved like a crescent moon – and tapered to pointy tips. A tuna's pelvic fins are located below the base of the pectoral fins. Both dorsal and pelvic fins retract when the fish is swimming fast.

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, often with an iridescent shine. The caudal peduncle, to which the tail is attached, is quite thin, with three stabilizing horizontal keels on each side.
Thunnus are widely but sparsely distributed throughout the oceans of the world, generally in tropical and temperate waters at latitudes ranging between about 45° north and south of the equator. All tunas are able to maintain the temperature of certain parts of their body above the temperature of ambient seawater. For example, bluefin can maintain a core body temperature of 25–33 °C (77–91 °F), in water as cold as 6 °C (43 °F). Unlike other endothermic creatures such as mammals and birds, tuna do not maintain temperature within a relatively narrow range.

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. The rete mirabile ("wonderful net"), the intertwining of veins and arteries in the body's periphery, allows nearly all of the metabolic heat from venous blood to be "re-claimed" and transferred to the arterial blood via a counter-current exchange system, thus mitigating the effects of surface cooling. This allows the tuna to elevate the temperatures of the highly-aerobic tissues of the skeletal muscles, eyes and brain, which supports faster swimming speeds and reduced energy expenditure, and which enables them to survive in cooler waters over a wider range of ocean environments than those of other fish.

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. Cavitation also slows tuna, but for a different reason. Unlike dolphins, these fish do not feel the bubbles, because they have bony fins without nerve endings. Nevertheless, they cannot swim faster because the cavitation bubbles create a vapor film around their fins that limits their speed. Lesions have been found on tuna that are consistent with cavitation damage.

Tuna is an important commercial fish. The International Seafood Sustainability Foundation (ISSF) compiled a detailed scientific report on the state of global tuna stocks in 2009, which includes regular updates. According to the ISSF, the most important species for commercial and recreational tuna fisheries are yellowfin (Thunnus albacares), bigeye (T. obesus), bluefin (T. thynnus, T. orientalis, and T. macoyii), albacore (T. alalunga), and skipjack (Katsuwonus pelamis).

Based on catches from 2007, the report states:

Between 1940 and the mid-1960s, the annual world catch of the five principal market species of tunas rose from about 300 thousand tons to about 1 million tons, most of it taken by hook and line. With the development of purse-seine nets, now the predominant gear, catches have risen to more than 4 million tons annually during the last few years. Of these catches, about 68 percent are from the Pacific Ocean, 22 percent from the Indian Ocean, and the remaining 10 percent from the Atlantic Ocean and the Mediterranean Sea. Skipjack makes up about 60 percent of the catch, followed by yellowfin (24 percent), bigeye (10 percent), albacore (5 percent), and bluefin the remainder. Purse-seines take about 62 percent of the world production, longline about 14 percent, pole and line about 11 percent, and a variety of other gears the remainder.

The Australian government alleged in 2006 that Japan had illegally overfished southern bluefin by taking 12,000 to 20,000 tonnes per year instead of the agreed upon 6,000 tonnes; the value of such overfishing would be as much as US$2 billion. Such overfishing has severely damaged bluefin stocks. According to the WWF, "Japan's huge appetite for tuna will take the most sought-after stocks to the brink of commercial extinction unless fisheries agree on more rigid quotas". Japan's Fisheries Research Agency counters that Australian and New Zealand tuna fishing companies under-report their total catches of southern bluefin tuna and ignore internationally mandated total allowable catch totals.

In recent years, opening day fish auctions at Tokyo's Tsukiji fish market and Toyosu Market have seen record-setting prices for bluefin tuna, reflecting market demand. In each of 2010, 2011, 2012, 2013 and 2019, new record prices have been set for a single fish – the current record is 333.6 million japanese yen (US$3.1 million) for a 278 kg (613 lb) bluefin, or a unit price of JP¥ 1,200,000/kg (US$5,057/lb). The opening auction price for 2014 plummeted to less than 5% of the previous year's price, which had drawn complaints for climbing "way out of line". A summary of record-setting auctions are shown in the following table (highlighted values indicate new world records):

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