Not all fish are cold-blooded. In 2015, researchers with the NOAA Southwest Fisheries Science Center revealed the opah, or moonfish, as the first fully warm-blooded fish. Although not as warm as mammals and birds, the opah circulates heated blood throughout its body, giving it a competitive advantage in the cold ocean depths from 150 to 1,300 feet below the surface.
Its body temperature isn’t the only thing that makes this fish stand out from the rest in its environment. Most fish living in the dark and chilly depths rely on ambush to catch their prey, but the agile opah is fast and efficient, flapping its bright red pectoral fins to race through the water. The constant flapping of its fins heats the opah’s body, speeding its metabolism, movement, and reaction times.
About the same size as a large automobile tire, the opah is equipped with specialized blood vessels that carry warm blood to its gills to rewarm the blood that cools as the fish breathes and absorbs oxygen from the water. These heat-exchanging blood vessels minimize the loss of body heat to the opah’s cold environment, ensuring a warm core body temperature, increasing muscle output and swimming capacity, and boosting eye and brain function. The opah is also able to stay in deep water longer without risking reduced function to its heart and other organs because the fatty tissue surrounding its gills, heart, and muscle tissue acts as insulation against icy waters.
The opah is one of the most colorful of the commercial fish species, and is particularly popular in Hawaii. It is overall red with white spots and turns a silvery-grey when it dies. Its fins are crimson, and its large eyes are encircled with gold. The fish’s large, round profile is thought to be the origin of its “moonfish” nickname. These combined characteristics certainly make this “warm-blooded fish” unique among the many wondrous creatures of the ocean.
A few other fish such as tuna and some sharks warm certain parts of their bodies, boosting their swimming performance. But internal organs including their hearts cool off quickly and begin to slow down when they dive into cold depths, forcing them to return to shallower depths to warm up.