Researchers have discovered a first fully warm-blooded fish that circulates heated blood throughout its body much like mammals and birds.
The silvery fish, roughly the size of a large automobile tire, is known from oceans around the world and dwells hundreds of feet beneath the surface in chilly, dimly lit waters.
The warm-blooded opah or moonfish swims by rapidly flapping its large, red pectoral fins like wings through the water, giving it a competitive advantage in the cold ocean depths, reported the team from National Oceanic and Atmospheric Administration's National Marine Fisheries (NOAA Fisheries).
"That warm-blooded advantage turns the opah into a high-performance predator that swims faster, reacts more quickly and sees more sharply," said fisheries biologist Nicholas Wegner, lead author of the paper.
"It turns out to be a very active predator that chases down agile prey like squid and can migrate long distances," he added.
While looking at opah, Wegner recognised an unusual design: Blood vessels that carry warm blood into the fish's gills wind around those carrying cold blood back to the body core after absorbing oxygen from water.
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The design is known in engineering as "counter-current heat exchange."
Resembling a car radiator, it's a natural adaptation that conserves heat.
The unique location of the heat exchange within the gills allows nearly the fish's entire body to maintain an elevated temperature even in the chilly depths.
"There has never been anything like this seen in a fish's gills before," Wegner said.
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," the authors said.
Discoveries like this will help scientists understand the role species play in the marine ecosystem.
The paper appeared in the journal Science.