The public’s understanding of tuna often begins and ends with the familiar canned product found on supermarket shelves. This widely consumed and commercially important fish is not a single species, but rather a complex group of highly migratory, predatory oceanic fishes. The common label of “tuna” covers several distinct species that vary significantly in size, habitat, and market value. Many fish labeled as tuna are only distant relatives of the species scientists consider to be the “true” members of the group, leading to confusion among consumers.
The Definition of True Tuna
The scientific definition of a true tuna is confined exclusively to the eight species belonging to the genus Thunnus. This genus is part of the Scombridae family, which also includes mackerels and bonitos. All tuna species, including the true tunas and their close relatives, fall under the tribe Thunnini.
The eight species of Thunnus are distinguished by unique physiological traits. They exhibit endothermy, a rare characteristic among bony fish, which is the ability to maintain a body temperature higher than the surrounding water. Specialized blood vessel structures called the rete mirabile help them conserve metabolic heat. This adaptation allows them to hunt effectively in colder, deeper waters and achieve high swimming speeds. True tunas are large, fast-swimming pelagic predators with sleek, streamlined bodies.
The Eight Species of the Thunnus Genus
The eight species of true tuna are organized into two subgenera, reflecting their evolutionary relationships. The first subgenus is Thunnus, often called the Bluefin group, and contains the three largest and most highly valued species: Atlantic Bluefin (T. thynnus), Pacific Bluefin (T. orientalis), and Southern Bluefin (T. maccoyii). Atlantic Bluefin tuna are the largest, capable of reaching lengths of over ten feet and weights exceeding 1,500 pounds.
Pacific and Southern Bluefin tuna share the massive size and dark, fatty red meat that makes them prized for high-end sushi and sashimi. All three Bluefin species are characterized by relatively short pectoral fins and make vast transoceanic migrations. Their immense size and high commercial demand have made them susceptible to overfishing, leading to significant conservation concerns.
The second subgenus, Neothunnus, contains the remaining five species, commonly known as the Yellowfin and Albacore group.
Yellowfin and Albacore Group
Yellowfin tuna (T. albacares) are named for their distinctive bright yellow finlets and the elongated, sickle-shaped second dorsal and anal fins on mature adults. Found in tropical and subtropical waters worldwide, this species is a primary source for both canned products and fresh steaks. Albacore (T. alalunga) are unique for their exceptionally long pectoral fins, which extend past the second dorsal fin. Their pale, firm meat is often marketed as “white meat tuna” in the canning industry.
Bigeye tuna (T. obesus) are named for their disproportionately large eyes, an adaptation that assists them in hunting in the deeper, darker layers of the ocean. Their flesh is fattier than Yellowfin, making it highly desirable for sushi. The Blackfin tuna (T. atlanticus) is the smallest Thunnus species and is restricted to the western Atlantic Ocean, where it is often targeted by recreational fishermen. The Longtail tuna (T. tonggol) is predominantly found in the coastal waters of the Indo-Pacific region and is distinguished by a slender body shape and a short tail fin.
Other Fish Commonly Marketed as Tuna
Not all fish sold or referred to as tuna are members of the Thunnus genus, which creates confusion for consumers. The most commercially important non-Thunnus species is the Skipjack tuna (Katsuwonus pelamis). It accounts for the vast majority of the world’s canned tuna, typically labeled as “chunk light.” Skipjack belongs to a separate genus and is significantly smaller than the true tunas, rarely exceeding 75 pounds.
The Skipjack’s abundance and rapid reproductive cycle make it a more sustainable and economically efficient target for large-scale commercial fishing operations. Biologically, it is distinguishable by the four to seven dark, longitudinal stripes running along its belly.
Another group of fish frequently mistaken for tuna are the Bonitos, which belong to the genera Sarda and Euthynnus. Bonitos are smaller, coastal species that share the streamlined body and dark meat color of the true tunas. They are classified separately because of differences in their scale patterns and body structures, such as the arrangement of teeth and the number of finlets. While Bonito and Skipjack are both part of the broader Scombridae family, their different biological classifications mean they lack some of the advanced physiological adaptations, such as the highly developed endothermy, found in the eight true Thunnus species.