The deep water dwelling Opah has become the first fish found to be fully warm-blooded, circulating heated blood throughout its body much like mammals and birds, giving it a competitive advantage in the cold ocean depths.
The silvery fish, roughly the size of a large car tyre, is known from oceans around the world and lives hundreds of feet beneath the surface in chilly, dimly lit waters. It swims by rapidly flapping its large, red pectoral fins like wings through the water.
Fish that typically inhabit such cold depths tend to be slow and sluggish, conserving energy by ambushing prey instead of chasing it. But the Opah’s constant flapping of its fins heats its body, speeding its metabolism, movement and reaction times, scientists report in the journal Science.
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 and lead author of the new paper, Nicholas Wegner, of NOAA Fisheries’ Southwest Fisheries Science Centre.
"Before this discovery I was under the impression this was a slow-moving fish, like most other fish in cold environments. But because it can warm its body, it turns out to be a very active predator that chases down agile prey like squid and can migrate long distances."
The gills of the Opah have 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.
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, known as endothermy, 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."
While mammals and birds typically maintain much warmer body temperatures, the Opah is the first fish found to keep its whole body warmer than the environment.
A few other fish such as tuna and some sharks warm certain parts of their bodies such as muscles, boosting their swimming performance. But internal organs including their hearts cool off quickly and begin to slow down when they dive into cold depths, forcing them to return to shallower depths to warm up.
Opah spend most of their time at depths of up to 1,300 ft, without regularly surfacing. Their higher body temperature should increase their muscle output and capacity, boost their eye and brain function and help them resist the effects of cold on the heart and other organs, Wegner said.
Fatty tissue surrounds the gills, heart and muscle tissue where the Opah generates much of its internal heat, insulating them from the frigid water.
"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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