Nicholas Wegner holds an opah caught during a research survey off the California coast. Credit:NOAA/Reuters Most fish that live where the opah does - that is, hundreds of metres deep, in some of the ocean's darkest and coldest places - are sluggish, thanks to the low temperatures. At these depths, even predatory fish tend to be slow-moving, waiting patiently for prey to come by, rather than actively chasing it down. But the opah, which spends all its time in these deep places, has many features usually associated with a quick-moving, active predator, such as a large heart, lots of muscle and big eyes. These characteristics made the opah "a curiosity", Dewar says. The opah's secret first started to come out when NOAA researcher and lead author Nicholas Wegner looked at a gill sample and noticed something intriguing.

All fish have two kinds of blood vessels in their gills: vessels carrying blood in from the body to pick up oxygen, and other vessels carrying oxygenated blood back out again. In the opah, the incoming blood is warm after circulating through the fish's body. This is because the opah swims by quickly flapping its pectoral fins, rather than undulating its body like many other fish do, to propel itself through the water - a process that generates high heat. But outgoing blood, which has just been in contact with water in the gills, is cold. Wegner noticed that, in the opah's gills, the two sets of vessels are tightly bundled against each other so that the incoming blood vessels can warm up the outgoing blood before it goes anywhere else. This set-up, known as "counter-current heat exchange", allows warm blood to be delivered throughout the body. Some other types of fish, such as tuna, have similarly designed blood vessels in certain parts of their bodies, allowing for "regional endothermy" - warm-bloodedness that's limited to certain organs or muscles, such as the eyes, liver or swimming muscles.

But the opah is the only fish scientists know of that has this design in its gills, where most fish lose most of their body heat to the surrounding cold water. By warming up the blood in the gills before it goes anywhere else, the opah achieves not just regional endothermy, but whole-body endothermy. Testing showed that the opah is able to maintain a body temperature about 5 degrees Celsius warmer than the surrounding water. Being warm-blooded gives the opah a major competitive advantage. Even fish with regional endothermy usually can't keep their hearts warm. "If your heart's cold, there's only so much you can do," Dewar says. "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." This means fish such as tuna must constantly make trips back to the surface to warm up if they don't want to slow down. The opah, on the other hand, can spend all its time in the deep waters without losing its edge. And being warm-blooded in such cold temperatures means its eyesight and muscle performance are sharper than its sluggish, cold-blooded cousins.

"The prey's at a complete disadvantage," Dewar says. While only one species of opah is now recognised - Lampris guttatus - scientists are starting to believe that they should divide the opah into several species based on genetic variations in different populations around the world, Dewar says. The opah in this study were found off the west coast of North America, so the next step will be to start sampling opah in other parts of the world to see if they all have the same specialised gills, Dewar says. Down the road, future studies could also examine other related types of fish to try to figure out how and when those special gills evolved. It's possible that other deep-water species have similar adaptations. For now, though, the opah enjoys the spotlight as the world's first - and, so far, only - warm-blooded fish.

"I think that it's really exciting that we spend so much time studying especially these larger fish to find something that's completely unique and has never been seen before in any fish," Dewar says. "We've been very excited about it." The Washington Post