The opah is a silvery fish that lives in dark, deep ocean water, growing up to about 6.6 feet (2 meters) in length—about the size of a large car tire—and 100+ pounds (45+ kg), according to Earth Sky. It uses its large red pectoral fins to swim swiftly through the deep. That's unusual because most deepwater fish are slow-moving and sluggish—but the opah is a quick and active predator, researchers say.

So how does the opah pull this off? Unlike most fish, which are exotherms that get heat from their environment, the opah is an endotherm that generates heat from within, Scientific American explains. They use something called counter-current heat exchange to achieve this: as cool, oxygenated blood moves from the gills into the body, it comes into contact with the warm, deoxygenated blood moving from the body to the gills, so that outgoing blood warms the incoming blood. That heat is generated in the first place via the quick flapping of the opah's fins.

In 2015, researchers announced this discovery in a study in the journal Science. They showed that the opah can maintain a body temperature about 5º C warmer than the surrounding water. This gives the opah a major competitive advantage.

"If your heart's cold, there's only so much you can do," Heidi Dewar, researcher at NOAA and one of the authors of the study, told the Washington Post. "It doesn't matter how much your muscles will perform, if your heart can't deliver the oxygen and nutrients, your muscles can only do so much." Unlike cold-blooded fish like tuna, the opah does not need to continually make trips back to the surface to warm up, she explained. All of this means faster swimming, better vision, and quicker response times.

So what's next for researchers? They hope to investigate the Lampris immaculatus, a relative of the opah that lives in even colder waters, to see whether nature has any more surprises in store.