These regionally endothermic traits, shared with billfish and opah (cranial endothermy),
sharks of the family Lamnidae (white, porbeagle, salmon and
mako sharks), and some thresher sharks, rely on a system of countercurrent
heat exchangers, or retiamirabilia. Retia, densely bundled arteries
and veins, facilitate the transfer of heat from blood warmed by the
metabolically active tissues (e.g. muscle, viscera, brain and eyes) to
cold, well-oxygenated blood entering from the gills, allowing tunas
to maintain body temperatures up to 20 °C greater than ambient
water temperatures