Although some fish are able to warm parts of their bodies to speed up movement or protect their organs, scientists have recently discovered the first fully warmblooded fish: the opah, or Hawaiian moonfish (Lampris guttatus).
“Most fish are the same temperature as the water. The opah is the first fish that can warm up its entire body,” says Nicholas Wegner, a fisheries research biologist at the National Oceanic and Atmospheric Administration credited with the discovery.
The opah’s endothermy has many advantages.
“Warmer temperatures increase reaction rates within the body,” says Wegner.
The opah maintains its body heat thanks to the pectoral muscles that it uses to flap its dorsal fins, which are its primary means of locomotion. A net of closely wound blood vessels near its gills acts as a heat exchanger, where warm blood from the center of the body flows near cold blood returning from the gills, raising the temperature of the cold blood before it recirculates throughout the rest of the body. This structure, called the rete mirabile (Latin: wonderful net), led to the research on the opah.
“A friend of mine brought me some gills from opah,” Wegner says. “When I took a look at the gills, I immediately saw these counter-current heat exchangers inside the gill arches, a bunch of blood vessels that weren’t supposed to be there. That was the smoking gun.”