Unique veins make car-tire-sized opah the world's first warm-blooded fish

The opah fish is able to keep its body warmer than its surroundings, which grants it some special powers over its cold-blooded compadres.

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This fish is hot blooded ... check it and see. SWFSC Large Pelagics Program

The opah, or moonfish, is kind of like the Rudolph of the fish world. It's not at all like the other reindeer -- um, I mean fish -- but its unique quality makes it incredibly cool. Well, make that warm.

Scientists at the National Oceanic and Atmospheric Administration (NOAA) Fisheries say the opah is the world's first truly warm-blooded fish -- and its unique adaptation grants it a competitive edge over other cold-blooded swimmers.

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An opah with biologist Nick Wegner, part of the team who discovered the animal's unique properties. NOAA Fisheries Service

What makes the opah different from its other fishy brethren is that in its gills, blood vessels carrying warm blood from its body are enmeshed with blood vessels carrying cold, freshly oxygenated blood back into its body. In a process called "counter-current heat exchange," the warm blood heats the cold blood and it allows the opah to keep its muscle temperature about 5 degrees Celsius (9 degrees Fahrenheit) higher than the surrounding water.

While this doesn't allow the opah the chance to light up its nose and fly, it certainly does grant it some special powers.

"There has never been anything like this seen in a fish's gills before," said biologist Nicholas Wegner of NOAA Fisheries' in La Jolla, Calif., in a statement. "This is a cool innovation by these animals that gives them a competitive edge. The concept of counter-current heat exchange was invented in fish long before we thought of it." Wegner is lead author of the new paper describing the discovery in the May 15 issue of Science.

The opah's warmer body has faster movement and reaction times at cold depths compared with other more sluggish fish. According to Wegner, the fish's warm-blooded nature should grant it other powers as well, including increased muscle output and capacity, a boost to eye and brain function and the ability to protect its heart and other organs from cold temperatures. The fact that it can keep its body warm also means the opah can stay at deeper depths for longer times than other fish -- even those that can warm only certain parts of their bodies like sharks and tuna. That's because those other fish inevitably have to swim back up to surface waters to warm their organs.

According to National Geographic, Wegner didn't set out to study the opah fish, but his team caught a lot of them during a different research trip. When they decided to dissect an opah -- because, you know, they're scientists -- they quickly saw the unique mesh of blue and red blood vessels -- known as retia mirabilia, Latin for "wonderful nets." After that the team implanted the fish with small thermometers and dropped them back into the water. That's how they discovered that the fish could keep its body temperatures significantly higher than the surrounding waters.

"Nature has a way of surprising us with clever strategies where you least expect them," Wegner said. "It's hard to stay warm when you're surrounded by cold water but the opah has figured it out."

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