The deep ocean is a pretty chilly place. Unless you’re a warm-blooded whale or leatherback turtle, the odds are that the cold will make you slow and sluggish as you try to conserve energy. Not so for the opah, or moonfish. It might not look like a candidate for fastest fish in the sea, but these creatures, despite living up to 305 meters (1,000 ft) down, can hold their own against tuna and swordfish.

But unlike those other fish which have to return to the surface to warm their bodies before they can speed off after prey, the moonfish ( Lampris guttatus ) is able to stay at depth. Researchers from the National Oceanic and Atmospheric Administration (NOAA) have discovered how they manage this. They report that the opah is, quite incredibly, the first fully warm-blooded fish.

Almost nothing is known about this deep diving creature except that it’s a surprisingly effective predator. The researchers now think that this is because it can circulate warm blood throughout its body, and so maintain a body temperature above that of the surrounding water. This gives the fish a competitive advantage over its slower moving prey of squid and other fish.

“Before this discovery I was under the impression this was a slow-moving fish, like most other fish in cold environments,” says Nicholas Wegner, lead author of the study published in Science. “But because it can warm its body, it turns out to be a very active predator that chases down agile prey like squid and can migrate long distances.”

Biologist Owyn Snodgrass, co-author of the study, preparing to tag a opah, or moonfish. Image credit: NOAA Fisheries West Coast/flickr CC BY-NC-ND 2.0

The way it manages this is by flapping its large pectoral fins like wings, which warms the muscles and blood that circulates. This is then pumped through the rest of its body, quickening its metabolism. Not only does this boost allow the fish to maintain high swiming speeds, it also means the animal can elevate the temperature surrounding its eyes and brain, maintaining their function at low temperature and depth.

The researchers first realized something fishy was going on whilst looking at the creature's gills. They noticed that they were of an unusual design, with the blood vessels carrying warm blood from the body wrapping around those carrying cold oxygenated blood from the gills. This set up is known as a countercurrent heat exchange system, and it means that heat generated by the body can be recycled rather than lost to the water.

Whilst this system is seen in other cold adapted animals, such as in the feet of arctic foxes and penguins, “there has never been anything like this seen in a fish's gills before,” says Wenger. “This is a cool innovation by these animals that gives them a competitive edge.”

The team then decided to tag some wild fish to track and monitor their temperature and that of the surrounding water. They found the fish had an average muscle temperature 41oF (5oC) higher than that of the water. Whilst other fast swimming predators, such as sharks and tuna, are able to warm certain parts of their body such as their head and muscles, the opah is unique in that it also warms its internal organs.

Normally a solitary predator, in recent years, fishermen have been catching more moonfish than ever before. Biologists are unsure what might be causing this sudden increase in numbers. Perhaps their population is growing, or changes in the ocean conditions are favoring them. Either way, it seems that the ocean depths still have plenty of mysteries left to solve.

Top image: NOAA Fisheries West Coast / flickr CC BY-CC-ND 2.0