There are 8 species of tuna: Albacore tuna, Atlantic bluefin tuna, Bigeye tuna, Blackfin tuna, Longtail tuna, Pacific bluefin tuna, Longtail tuna, Yellowfin tuna.
Tunas 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.
The lifespan of tunas varies by species, but ranges from about 15 years to over 40 years.
Tunas are elongated, robust, and streamlined fishes; they have a rounded body that tapers to a slender tail base and a forked or crescent-shaped tail. In color, tunas are generally dark above and silvery below, often with an iridescent shine. They have a conspicuous keel on either side of the tail base, a row of small finlets behind dorsal and anal fins, and a corselet of enlarged scales in the shoulder region.
Tunas vary considerably, both within and among species.
The blackfin tuna (Thunnus atlanticus) is the smallest tuna species, generally growing to a maximum of 100 cm (39 in) in length and weighing 21 kg (46 lbs).
The Atlantic bluefin tuna (Thunnus thynnus) is the largest tuna species. Fully mature adult specimens average 2–2.5 m (6.6–8.2 ft) long and weigh around 225–250 kg (496–551 lb). The largest recorded specimen taken under International Game Fish Association rules was caught off Nova Scotia, an area renowned for huge Atlantic bluefin, and weighed 679 kg (1,497 lb) and 3.7 m (12 ft) long.
Tunas are highly specialized migrating species. They swim continuously to counterbalance their negative buoyancy, travelling hundreds of kilometers / miles. This strategy has a high energy cost, forcing them to move in search of food and has resulted in morphological and physiological adaptations for thermoregulation and high oxygen extraction efficiency. Therefore, temperature and dissolved oxygen concentration affect tuna behavior significantly.
Tunas have a well-developed network of blood vessels below the skin that acts as a temperature-regulating device associated with long-term, slow swimming. Because of this vascular system, they are able to maintain the temperature of their bodies above that of the surrounding water, often between 5 and 12 °C (9 and 21.7 °F) above ambient water temperature.
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 75 km/h (47 mph).
The tuna will swim near the surface of the ocean to look for food, however they can also dive to the depth of 900 meters (3000 feet) to search for their next meal.
At a young age, tunas eat tiny zooplankton, and their prey increases in size as they do. As adults, they eat different types of fish and invertebrates.
Dolphins swim beside several tuna species. Tuna schools are believed to associate themselves with dolphins for protection against sharks, which are tuna predators.
Tunas are among the most commercially valuable fish on the planet.
As a result of overfishing, stocks of some tuna species, such as the southern bluefin tuna, are close to extinction.
Also unlike most fish, which have white flesh, the muscle tissue of tuna ranges from pink to dark red.
The fresh or frozen flesh of tuna is widely regarded as a delicacy in most areas where it is shipped, being prepared in a variety of ways for the sake of achieving specific flavors or textures.
Tuna has many nutritious elements. It is an excellent source of selenium, vitamin B3 (niacin), vitamin B12, vitamin B6, and protein. Tuna is a very good source of phosphorus as well as a good source of vitamin B1 (thiamin), vitamin B2 (riboflavin), choline, vitamin D, and the minerals potassium, iodine, and magnesium. Some of the selenium is found in the form of selenoneine, which may be especially valuable in terms of its antioxidant properties. In addition, tuna provides valuable amounts of omega-3 fatty acids.
The health benefits of tuna fish include its ability to reduce cardiovascular conditions, stimulate growth and development, lower blood pressure and cholesterol levels, and help in weight loss. Tuna also has the ability to boost the immune system, increase energy, maintain the health of the skin, increase red blood cell count, prevent cancer, protect against various kidney diseases, reduce general inflammation, and inhibit cell membrane damage.
Canned tuna was first produced in Australia in 1903, quickly becoming popular.
In the United States, 52% of canned tuna is used for sandwiches; 22% for salads; and 15% for casseroles and dried and pre-packaged meal kits such as General Mills’s Tuna Helper line.
In 2013, a 222-kilogram (490-pound) bluefin tuna was sold at Tokyo’s Tsukiji market for an all-time high of 155.4 million yen, or 1.8 million dollars, at the annual new year auction.
Besides for edible purposes, many species of tuna are caught frequently as a game fish, often for recreation or for contests in which money is awarded depending on how heavy the fish weighs in at. Larger specimens are notorious for putting up a fight while hooked, and have been known to injure people who try to catch them, as well as damage their equipment.