Fish are known for being seemingly cold-hearted creatures, swimming the cool depths of our vast oceans. But now scientists have discovered the first fully warm-blooded fish, and it's making them question how much we really know about marine life.
A team from the NOAA Fisheries describes in the journal Science the opah, or moonfish, which circulates 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 tire, is known from oceans around the world and dwells hundreds of feet beneath the surface in chilly, dimly lit waters. It is recognized by its large, red pectoral fins, which help it to swim rapidly through the water.
Most fish that inhabit such dark depths are slow and sluggish, conserving energy by ambushing prey instead of chasing it. But unlike its neighbors, opah is a fast swimmer, constantly flapping its fins which heats its body, speeds its metabolism, movement and reaction times.
By being warm-blooded, opah is a fierce deepwater fish that swims faster, reacts more quickly, and sees more sharply than others.
"Before this discovery I was under the impression this was a slow-moving fish, like most other fish in cold environments," biologist Nicholas Wegner of NOAA Fisheries, who led the study, said in a statement. "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."
This discovery was made after Wegner and his colleagues studied opah's unique design of its gills. What they saw is described as "counter-current heat exchange," in which 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. This means that warm blood leaving the body core helps heat up cold blood returning from the respiratory surface of the gills where it absorbs oxygen, thus allowing the fish to maintain an elevated temperature throughout its entire body (endothermy). (Scroll to read on...)
"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."
To better understand this mechanism, the NOAA team caught several opah off the West Coast and attached temperature monitors for tracking during deep dives. They found that fish had an average muscle temperature about 5 degrees Celsius (41 degrees F) above the surrounding water while swimming about 150 to 1,000 feet below the surface.
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.
It's true that other fish like tuna and some sharks warm certain parts of their bodies such as muscles, boosting their swimming performance. But that does not mean they are fully warm-blooded like opah. When diving into cold depths, their internal organs, including their hearts, cool off quickly and begin to slow down, forcing them to return to more shallow waters to warm up.
This makes opah especially unique, and suggests that over time it evolved its warm-blooded ways to gain a competitive edge so far beneath the surface.
"Nature has a way of surprising us with clever strategies where you least expect them," Wegner concluded. "It's hard to stay warm when you're surrounded by cold water but the opah has figured it out."
"Discoveries like this help us understand the role species play in the marine ecosystem, and why we find them where we do," added Francisco Werner, director of the NOAA Fisheres' Southwest Fisheries Science Center. "It really demonstrates how much we learn from basic research out on the water, thanks to curious scientists asking good questions about why this fish appeared to be different."
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