While tuna and a few sharks, like the Mako and great white, have the ability to warm swimming muscles when needed to move fast, and other fish, such as swordfish, marlins, and sailfish are able to heat their brains and eyes to sharpen their vision when pursuing prey, they cannot heat their entire body.
The Opah, also called a moonfish, has the ability to heat its entire body through constant flapping of wing-like pectoral fins, having an average muscle temperature of about 7 to 9 degrees Fahrenheit higher than the surrounding water. While some fish searching for prey in deeper waters must return to shallower levels to protect their internal organs, the Opah has no problem with the chillier depths.
Opahs can get up to 200 pounds and be as big as an oversized car tire with an oval body shape. The fish is a rusty- red color with white spots and bright red fins. The opah is found in oceans around the globe, spending most of its time at depths of 165-1,300 feet (50-400 meters), hunting fish and squid.
So what does the Opah have that keeps it from losing internal body heat to the surrounding environment? Nicholas Wegner, from the National Oceanic and Atmospheric Administration (NOAA), discovered this phenomenon quite by chance while on a research trip.
The research team was catching more Opahs than was usual, and Wegner decided to learn more about the little-known species. As the team was dissecting the animals, Wegner immediately noticed the gills contained a beautifully intricate mass of red and blue blood vessels.
The team discovers the Opahs “Wonderful Net”
“That was when we realized what it was capable of,” he says. Wegner had seen these blood vessels before. They’re called retia mirabilia, Latin for “wonderful nets.” They’re the vessels that allow tuna and sharks to heat their muscles when needed.
He points out that all animal muscles produce heat when they contract, but in fish, the heat is lost to the outside environment through the gills and skin. Regardless of how much insulation a fish may have, the blood vessels running through the gills has to come into contact with the seawater, and this cools the blood.
If a tuna heats up its muscles to move quickly, once the blood in the swimming muscles has moved back through the gills to reload with oxygen, the blood is cooled again. Wegner says the “wonderful nets” keep that blood from cooling. The process involves “countercurrent exchangers,” and this allows the tuna to keep it muscles warm. But with tuna, the retia mirabilia, or “wonderful nets” are located in the swimming muscles, so they are the only part of the fish that gets heated.
But Wegner had discovered the retia mirabilia were in the gills of the Opah, and this makes all the difference. The blood vessels carrying warm blood from the heart to the gills flows right next to the cold blood coming from the gills to the rest of the body. The cold blood is warmed as it passes next to the warm blood coming from the heart. Now that is amazing!
So with its ungainly size, looking somewhat like a “comical frisbee,” said Wegner, the Opah has warm pectoral muscles, a thick, fatty insulation around its internal organs, and most impressive, a warm heart. “That’s why opah can stay at depth,” says Wegner. “These guys are specialized for living deeper than those other predators.”
As for being a fast fish, Wegner says, “That’s what really blew my mind about this discovery. Just from looking at it, I really thought it was a slow, sluggish, deep-water fish that doesn’t do very much. But all indications are that this is a very fast fish and an active predator. We’ve put some tags on them to show that they migrate thousands of kilometers.”
This study were published in the Journal Science on May 15, 2015, entitled: Whole-body endothermy in a mesopelagic fish, the opah, Lampris guttatus.