A tuna is a saltwater fish that can maintain a body temperature higher than that of the surrounding water. (the other species of fish which can do that are opah and mackerel sharks). The tuna is among the fastest-swimming pelagic fish - for example, the yellowfin tuna can swim as fast as 75 km/h.
The largest tuna, the Northern (or Atlantic) Bluefin Tuna, can reach lengths of nearly 13 feet (4m) and historically, up to 2000 pounds (900 kg). The Atlantic bluefin tuna eat a variety of prey but apparently prefer pelagic fishes and invertebrates that they can swallow whole. The Atlantic bluefin tuna has also been the foundation of one of the world's most lucrative commercial fisheries. Medium-sized and large individuals are heavily targeted for the Japanese raw fish market, where all bluefin species are highly prized for sushi and sashimi.