What physiological adaptation primarily defines membership within the Genus *Thunnus*?

Being warm-bodied predators adapted for sustained high-speed pursuit hunting.

The Genus *Thunnus* is reserved for the larger, more robust species recognized as 'true tunas.' The defining characteristic uniting this genus is their specialization for endurance and high-speed pursuit hunting, which is physiologically supported by their endothermic capabilities—the ability to maintain elevated muscle temperatures above the ambient water temperature. This physiological edge allows these titans of the ocean to remain active and efficient in a wider range of water temperatures. While the yellow fins are specific to the species *T. albacares*, and the notochord relates to the Phylum Chordata, the warm-bodied, high-speed adaptation is what defines the elite grouping within the genus level.