First warm-blooded fish, the Opah, discovered by NOAA scientists

Until recently, everybody believed the only fully warm-blooded animals on Earth were birds and mammals. A new study has found that the Opah (Lampris guttatus), also known as Moonfish, is fully warm-blooded, making it the first ever fish with warm blood known to science.

Scientists at NOAA (National Oceanic and Atmospheric Administration) Fisheries say this unique feature gives the Opah a significant competitive advantage in the cold ocean depths.

The Opah, a silvery fish that can reach a total length of 2 metres (6.6 feet) and weigh up to 270 kg (600 lbs), exists hundreds of feet beneath the ocean’s surface in very cold, dimly lit waters.

With its large, red pectoral wing-like fins, it is a very fast swimmer.

Most very-deep sea-dwelling fish tend to be slow and sluggish, and conserve their energy by ambushing prey rather than chasing it.

Lampris guttatus behaves differently, the researchers reported in the journal Science (citation below). It constantly flaps its fins, which keeps its body warm, its metabolism fast, giving it super-fast reaction times.

Lead author, fisheries biologist Nicholas Wegner, of NOAA Fisheries’ Southwest Fisheries Science Center in La Jolla, California, explained that the warm-blooded advantage turns the Opah into a fearsome predator that swims more rapidly, reacts faster and possesses sharper vision, compared to other fish deep down in the ocean.

Wegner said:

“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.”

Unusual gills

The researchers realised the fish was unusual when co-author, biologist Owyn Snodgrass gathered samples of its gill tissue. Wegner noticed an unusual design – it had blood vessels that carry warm blood into its gills that wind around those carrying cold blood back to the body core after absorbing the water’s oxygen.

In engineering, the design is known as ‘counter-current heat exchange’. In Lampris guttatus it means that warm blood leaving the body core helps warm up the cold blood returning from the respiratory surface of the gills where oxygen is absorbed.

A bit like a car radiator, it is a natural adaptation that conserves heat. Because of the unique location of the heat exchange within the gills, nearly the whole of the fish’s body maintains an elevated temperature (endothermy), even in the deep cold water.

Wegner said:

“There has never been anything like this seen in a fish’s gills before. 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 scientists gathered and analyzed temperature data from Opah caught during surveys off the US West Coast, and found that their body temperatures were repeatedly warmer than the water around them.

They attached temperature monitors to the fish as they tracked prey on dives to several hundred feet. Despite diving into very cold, deep waters, their body temperatures remained steady.

While swimming between 150 and 1,000 feet below the surface, the fish’s average muscle temperature was about 5°C warmer than the surrounding water, the authors found.

Some species of fish such as tuna and a few types of sharks, warm certain parts of their bodies (muscles), thus boosting their swimming performance. However, their internal organs including their hearts soon cool off and start to slow down when they dive into cold depths, forcing them to come back up to raise their temperature.

Warmth gives the opah an edge

According to satellite tracking, opahs spend most of their time between depths of 150 and 1,300 feet, without regularly surfacing.

Wegner said:

“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. Fatty tissue surrounds the gills, heart and muscle tissue where the opah generates much of its internal heat, insulating them from the frigid water.”

While other fish have regional endothermy – limited warm-bloodedness – which helps them stay longer and go deeper into cold water, the opah can stay deep down as long as it wants. This gives it a consistent edge over prey and other competitors.

According to recent research, there are some distinctive differences among opah from different parts of the world. The science team is interested in comparing their warm-blooded features.

Wegner said:

“Nature has a way of surprising us with clever strategies where you least expect them. It’s hard to stay warm when you’re surrounded by cold water but the opah has figured it out.”

Over recent years, NOAA research has caught more opah off California. Biologists are not sure what the reason is. Either their population is growing, or current conditions are favouring the fish, they say.

Californian fishermen do not generally target opah, but local recreational anglers do sometimes catch the species. Some seafood markets say the opah’s rich meat has become increasingly popular.

Francisco Werner, director of the Southwest Fisheries Science Center, said:

“Discoveries like this help us understand the role species play in the marine ecosystem, and why we find them where we do. 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.”

Citation: “Whole-body endothermy in a mesopelagic fish, the opah, Lampris guttatus,” Nicholas C. Wegner, Owyn E. Snodgrass, Heidi Dewar and John R. Hyde. Science. Published 15 May, 2015. DOI: 10.1126/science.aaa8902.

Video – Opah, the first known warm-blooded fish