Calories in Tuna, ahi or yellowfin

What makes tuna so powerful?

Yellowfin tuna (Thunnus albacares) possesses a unique vascular structure called a rete mirabile that allows them to retain metabolic heat. This adaptation keeps their core muscles significantly warmer than the surrounding ocean water, enabling them to hunt in colder depths. Because their muscles require a constant oxygen supply for sustained, high-speed cruising, their flesh is packed with myoglobin. This oxygen-binding protein gives ahi its characteristic deep red color, distinguishing it from the paler flesh of other fish. However, when exposed to air, the iron in myoglobin oxidizes, turning the meat brown. This is why fresh ahi steaks are sometimes treated with carbon monoxide to maintain a bright red appearance—a practice banned in some countries but permitted in the US. Unlike most fish, tuna do not possess true white flesh because they lack the typical slow-twitch muscle fibers used for leisurely swimming; their muscles are almost entirely fast-twitch, built for explosive speed and endurance.