The bigeye tuna, Thunnus obesus is a member of the true tunas of the genus Thunnus, belonging to the wider mackerel family Scombridae. Bigeye tuna are found in the open waters of all tropical and temperate oceans and seas, with the exception of the Mediterranean. Its length is between 60 and 250 cm (23 and 93 inches). Maximum weight probably exceeds 400 lb with the all-tackle angling record standing at 392 lbs. A longer-lived fish than the closely related yellowfin tuna, the bigeye tuna is thought to have a lifespan of up to 12 years, with individuals achieving sexual maturity at the age of four. Spawning has been observed in June and July in the northwestern tropical Atlantic, and in January and February in the Gulf of Guinea. The latter area  is  also the only known nursery area for Atlantic bigeye tuna. Bigeye tuna are large deep-bodied streamlined fish with large heads and eyes. The pectoral fins are very long, reaching back as far as the second dorsal fin. There are 13 or 14 dorsal spines.

Feed items include both epipelagic and mesopelagic species, with deep diving behaviour during the day thought to be related to the seeking of prey. Satellite tagging has shown that bigeye tuna often spend prolonged periods cruising deep below the surface during the daytime, sometimes making dives as deep as 500 metres. These movements are thought to be in response to the vertical migrations of prey organisms in the deep scattering layer. Physiological adaptations to foraging in cold waters (bigeye tuna have been tracked entering water as cold as 5 degrees Celsius), and oxygen-poor subsurface layers, include blood that is highly efficient in extracting oxygen from the water, and vision that is highly adapted for functioning in low light conditions. The heart of bigeye tuna also has the unusual ability to function effectively at the low ambient temperatures encountered while foraging in cold subsurface waters. Nonetheless, bigeye tuna must make return trips to warmer surface waters to warm themselves up.