-- With its tire-sized frame and awkward fins, it would be easy to mistake the opah fish for the oaf of the ocean.
The fish, which swims hundreds of feet beneath the surface of the sea in frigid waters, is an oceanic predator thanks to a secret evolutionary trait -- its warm blood.
Many fish swimming in deep waters tend to be slow and conserve energy -- choosing to ambush their dinner instead of chasing it.
According to an article in the journal Science, the opah is a rarity: A warm-blooded fish that can swim fast by flapping its fins like water wings as it speeds through the water.
The movement speeds the opah's metabolism and heats its blood, leading to better reaction times, according to researchers.
Researchers first noticed the opah was special when they collected a sample of its gill tissue and noticed an unusual pattern of blood vessels carrying warm blood to the gills move around vessels taking cold blood back to to the fish's core where it can be heated.
"There has never been anything like this seen in a fish's gills before," Nicholas Wegner lead author of the paper and a researcher at NOAA's Southwest Fisheries Science Center said 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."