And it’s a doozy.
This is an opah, and it’s the first fish known to be fully warm-blooded.
As the image from the National Oceanic and Atmospheric Administration (NOAA) above shows, it’s a whopper, weighing an average of 100 pounds. It’s also known as a moonfish.
NOAA / Reuters
The body temperature of a cold-blooded critter like a fish, though, relies on its surroundings. Some, like swordfish and marlins, are capable of selectively warming their eyes or brains for a vision boost while hunting, according to Ed Yong at National Geographic.
Tuna and a few shark species similarly heat up their swim muscles to amp up their speed, though they eventually cool off enough to have to backtrack closer to the surface to thaw.
But unlike its brethren, the opah is the only fish that distributes warmth throughout its entire body, including its brain and heart, giving it a metabolic advantage — and competitive edge — over colder-blooded species.
The secret is the opah’s gills, which contain an elaborate net of blood vessels that help maintain its heat.