Tunas are elongated, streamlined fishes with deeply forked tails. They have a counter-shaded silvery coloration that is darker on the back than on the sides. This coloring provides camouflage that makes these fish difficult to see when viewed from above or below. Their backs are bluish to dark blue, whereas their undersides are silvery or whitish. The first dorsal fin is dark, the second is pale or bright yellow, and the third (and/or any ensuing dorsal fins) are black. The pectoral fins are short and rounded, the pelvic fins long and pointed, and all of these paired fins have white edges. The anal fin is similar to the second dorsal fin. The caudal fin is broadly forked with pointed tips, and the paired fins have white edges. Tuna range from about 20 to more than 2,700 kg (44 to 5,952 pounds) in weight, depending on the species. In most tunas, these fins are black or dusky, but in a few species they are bright yellow; for example, the yellowfin tuna ( T. albacares , or Euthynnus affinis ) has a bright yellow first dorsal fin and an anterior anal fin that is bright or dusky yellow.
Tunas are among the fastest-swimming fishes, capable of sustained bursts of speed of about 45 km/h (30 miles per hour). They are schooling fish, found most often in surface waters of tropical and temperate seas. Most tunas migrate long distances between feeding grounds and spawning sites, generally spending the daytime at greater depths sometimes more than 1,500 meters (5,000 feet), and ascending at night to feed near the surface.
Large adults are generally solitary, but younger fish often school together. Tunas feed mainly on smaller pelagic fishes and squids. They have large mouths equipped with strong teeth that are angled slightly outward to assist in capturing prey. The great white shark ( Carcharodon carcharias ) is one of the few predators capable of preying upon the larger tunas.
Tunas are viviparous fishes; that is, they give birth to live young, which develop in the uterus until ready for birth. The embryos draw nourishment from their egg yolks at first but later take it in through their intestines, as do other fish. The association of a male and female during spawning is temporary. Thousands of ova are released by each female, and they float freely until fertilized.
Tunas, particularly the larger ones such as the Atlantic bluefin ( Thunnus thynnus , or Euthynnus thynnus ) and southern bluefin tuna ( T. maccoyii ), are heavily fished commercially. The smaller species, such as the skipjack ( Katsuwonus pelamis , or Euthynnus affinis ) and bonito ( Sarda sarda ), are often canned.