FACT: All fishes are cold-blooded.
But early this year, scientists have discovered the world’s first warm-blooded fish, Opah!
Opah, also known as moonfish, are rotund (car-tire-size) predatory fish that hunt for prey in cold, dark waters up to 1,000 feet below the ocean’s surface. An opah has a keen sense of vision, swims speedily, reacts quickly, and have the stamina to chase down fast-moving prey. Although they aren’t fished commercially, opah are often accidentally caught in fishing nets. Hawaiians consider opah a sign of good luck, and the fish is often given away as a gesture of goodwill.
All fish gills have two types of vessels: One set transports stale blood from the body to be oxygenated, and another set transports oxygenated blood back into the body. But in opah, blood traveling into the gills is warm, as what Nicholas Wegner, a researcher at the National Oceanic and Atmospheric Administration, as he examined an opah gill sample.
Opah generate heat by vigorously flapping their fins to swim, which also speeds up their metabolism.
More so, Wegner noticed that the vessels carrying warm blood away from the body wrapped around vessels carrying cold, oxygenated blood into the body. The unique setup raises the temperature of inbound blood before it travels to organs and muscles. It’s a process called counter-current heat exchange. Automobile radiators regulate engine temperatures the same way, but in reverse. In this way, opah can maintain a body temperature that is roughly 41 degrees Fahrenheit warmer than the water they swim in.
Wegner published his observations in the journal Science.
One more thing, since opah have a heating system built into their gills, they don’t need to waste time seeking warmer waters. Instead, they can spend their time hunting in cold water and rely on their keen vision and speed, which is made possible by their warm blood.