The Opah is a striking fish species that can grow as large as 7-feet in diameter and weigh over 100 pounds. Also referred to as moonfish, the giants that can be found in both temperate and tropical seas reside at depths of 150 to 1,300-feet where the waters can be extremely chilly. Scientists have often wondered how the fish can withstand the temperatures given that they lack typical deep sea dweller characteristics like a large heart. Now thanks to some curious California researchers that mystery has finally been solved - The Opah has warm-blood!

The chain of events leading to the discovery was accidental. It all began in 2012 when Southwest Fisheries Science Center researchers Owyn Snodgrass and Heidi Dewar caught a few more Opah specimens than normal during a research trip off the Southern California coast. Since very little was known about the elusive fish, the scientists decided to take advantage of the unexpected catch and sent some tissue samples to their colleague Nicholas Wegner for dissection.

While the researchers had expected to learn some intriguing facts about the fish's lifestyle, they had not anticipated anything earth shattering. Even Wegner, who specializes in the respiratory adaptations of fast-swimming fish, later admitted that he had always thought the Opah was just another sluggish cold-blooded deep-water species.

So you can imagine everyone's surprise when Wegner discovered that the opah's gill tissue is webbed with red and blue blood vessels or "rete mirabile." This means the fish has one set of blood vessels to carry warm blood to the gills, and another to transport cold, oxygen-rich blood to the body core. Because the two sets of blood vessels are entwined tightly, the warm blood heats up the colder blood as it circulates. As a result, the Opah can maintain higher body temperatures than cold-blooded fish or "ectotherms" that rely on the temperature of their surroundings to regulate body heat. The Opah's body is further optimized to retain heat with insulating layers of fat that protect its heart from the gills and the pectoral muscles from the cold ocean water.

There are a few other fish like tuna and sharks that possess rete mirabile. However, their ability only extends to warming their blood to heat individual body parts for short bursts of time. This means that the fish cannot maintain their core body heat for too long in deep waters and have to surface periodically to shallower waters to warm up. The "endothermic" Opah, however, can maintain a constant body temperature, enabling it to remain in deeper, colder waters for extended periods of time.

To measure how much warmth the Opah can retain, the scientists fitted some with tracking thermometers. What they found was that the fish were consistently able to maintain a body temperature of about 5°Ç higher than the surrounding water, regardless of depth. The researchers say the fish's ability to keep its muscles, brain, and heart warm means it can swim faster and see better than both, its prey and predators, allowing it to react quicker when hunting or being hunted.

As to how this amazing fish manages to warm its blood even in the coldest environments? By constantly flapping its wing-like pectoral fins. This action not only heats its blood but also helps it propel faster.

These surprising findings have resulted in a new respect for this fish. According to Wegner, who published the results in Sciencemag on May 15th, "All indications are that this is a very fast fish and an active predator . . . Specialized for living deeper than those other predators." Hardly the lazy giant it had been made out to be! Just shows how much we still have left to learn about the amazing marine animals that reside in our oceans.