First fully warm-blooded fish: The opah or moonfish

New research by NOAA Fisheries has revealed the opah, or moonfish, as the first fully warm-blooded fish that 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 automobile tire, is known from oceans around the world and dwells 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.

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 of NOAA Fisheries’ Southwest Fisheries Science Center in La Jolla, Calif., lead author of the new paper.

Wegner realized the opah was unusual when a coauthor of the study, biologist Owyn Snodgrass, collected a sample of its gill tissue. Wegner recognized 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.

The design is known in engineering as “counter-current heat exchange.” In opah it 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. 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.

“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.”

(read more) Science Daily || photo: NOAA || [paper]


Of all the fish in the world, only a few have the ability to keep warm in certain parts of their bodies. But new research reveals that opah (Lampris guttatus) have the ability, unprecedented, of circulate warm-blood throughout the body, so it is the only known fish that has warm-blood.

The ability to generate heat internally is what distinguishes birds from mammals and fish. So far, only some predators, such as tuna, have demonstrated the ability to heat certain muscles and organs while chasing their prey, but they have to return to the surface to increase the temperature of the rest of its body.

According to researchers Lampris guttatus mainly increases its body temperature through constant flapping of its pectoral fins

Werner, who leads the study adds that opah has specialized blood vessels in their gills, forming countercurrent heat exchangers to minimize this loss of heat to the water when the fish breathe .

An advantage of constant heating of the fish is seen in improved performance. “A hot cranial region allows opah to increase reaction rates and visual resolution. This is important when these fish look for food in the cold, deep waters, where they spend most of their time, "remarks Wegner.


Only Known, Truly Fully Warm-Blooded Fish Swims Like a Bird

by Tim de Chant

Scientists have discovered a fish that can keep its entire body several degrees warmer than the waters it lives in, suggesting that endothermy evolved on yet another occasion.

The fish, known as the opah or Lampris guttatus, is a large species with a conspicuously round profile. It can weigh almost 600 pounds and swims not by flipping its tail but by flapping its pectoral fins. The opah lives in the mesopelagic zone, which is about 660 to 3300 feet deep where the water is particularly cold.

Ichthyologists had long thought that such deep-water predators hunted more like alligators—they conserved their energy, preferring to ambush their prey rather than actively hunt it. Since the discovery by Nicholas Wegner and his colleagues from the National Oceanic and Atmospheric Administration, they’ve revised their view of opah, at least. The fish is actually a swift and agile predator, similar to the tuna…

(read more: Nova Next)

photograph by NOAA

13. endotherm

An organism that produces its own heat.

This is James.

James shared dragonfruit with us in the kitchen.

He brought it from his own garden, and it was really very good.

He seems like quite the sweetheart, though I only met him shortly.

Being a mammal, specifically, a human, James produces heat internally by metabolizing fats and sugars very fast.

The processes of generating and conserving heat in an endothermic body are really quite complex and amazing.


Sacred Savagery

I want to build a fire in the grass, in the dying blue of twilight,
a steady fire with an unreal heart that glows and undulates,
translucence of oak and with the rest of the world emptied out
around us, tear away our clothes, dance and howl for the virtues:
red-blooded existence, for spirits of endothermy, you and I spinning
and colliding at last: I want to be ridden, ridden by your sweet body
in the night’s cooling grass in the smoke of a boundless fire,
in the shuddering nexus of every sense to witness your form
and the forms it takes taking pleasure from me,
translucence of body, phoenix wood of our sex.

A fish that is warm through and through

Fishes are typically considered ‘ectotherms’ – their body temperature is at the mercy of their surroundings. This contrasts with mammals and birds, the ‘endotherms’ that can generate heat internally and thus regulate body temperature. However, it has also been known for decades that some fishes blur these lines- certain sharks and tuna, for example, have warm swimming muscles, whilst others, such as billfishes, maintain warm brains to speed up neural activity. These fishes are considered ‘regional endotherms’ as they can only maintain high body temperatures in specific bodily locations. It has recently been shown that regional (red muscle) endothermy has striking advantages, including increasing swimming speed and the scope for migration (see the previous post!). However, these fishes remain constrained because the heart, and other vital organs, remain cold- they receive blood directly from the gills, which are designed to efficiently exchange gases (oxygen and carbon dioxide) with external water, but this also makes them excellent heat dissipaters. 

Reported today in the prestigious journal Science, Nicholas Wegner and his colleagues report the first known case of a fish that can maintain an elevated temperature throughout- the opah (Lampris guttatus). The team caught these large fish and quickly inserted thermometers into various regions- including the heart- and showed that temperature consistently averaged 13 degrees C- which was at least 3 degrees above the ambient water temperature. To confirm this in vivo, temperature loggers were inserted into the pectoral muscle of four fish, which were released and recaptured after a few hours. During deep dives, in which external temperature could fall below 10 degrees C, the opah could maintain body temperatures above 14 degrees. Wegner and colleagues believe that the body heat is mainly generated by the unusual, rapid ‘flapping’ motion of pectoral fins.

As alluded to earlier, the large surface area and thin barrier of gills are normally primed to release heat. Not surprisingly, the thermostated opah shows key anatomical adaptations in the gills to conserve heat. An elegant arrangement of blood vessels in the opah gill forms a ‘counter-current’ heat exchanger- warm blood arriving from the body transfers its heat to the cooler blood that has just taken up oxygen- thus heat is retained in the fish. Further, the gills, heart and muscle are insulated by a thick layer of fat. 

The team additionally compared opah distribution with Albacore tuna (a classic ‘regional endotherm’) and it appears that whole-body endothermy may endow opah to reach greater depths (with lower temperatures) and conquer new niches. 

The key difference between regionally endothermic fishes and opah (at least in the eyes of a comparative cardiovascular physiologist…) is the ability of the latter to supply the heart with warm, oxygenated blood. This will likely allow more blood to be circulated during deep dives into the cold. Indeed, it is a captivating possibility that the heart itself has adaptations geared towards endothermy. 


Wegner, Snodgrass, Dewar and Hyde. 2015. Whole-body endothermy in a mesopelagic fish, the opah, Lampris guttatus. Science. https://www.sciencemag.org/content/348/6236/786.full.pdf

Opah (Lampris guttatus): First Known Warm-Blooded Fish Species

A team of researchers led by Dr Nicholas Wegner of NOAA Fisheries’ Southwest Fisheries Science Center has discovered a whole-body form of endothermy in a deep-water fish, the opah (Lampris guttatus), that produces heat through the flapping of wing-like fins and minimizes heat loss through a series of counter-current heat exchangers within its gills. Unlike […]

Opah (Lampris guttatus): First Known Warm-Blooded Fish Species

Forget Everything You Knew: Some Fish Are Warm-Blooded

Another day, another way nature forces us to change something fundamental about how we see the world.

Scientists have long assumed that all fish are basically cold-blooded. Sure, there are some limited exceptions: tuna, great white sharks and a few others can warm up parts of their body a bit when they vigorously swim. Generally, though, fish that live in the deep cold water of the sea were thought to be exclusively sluggish and slow-moving.

Naturally, then, biologist Nick Wegner of the National Oceanic and Atmospheric Administration’s Southwest Fisheries Science Center in La Jolla, Calif., assumed that a large silvery fish called opah, which hangs out in the depths, was cold and lethargic.

This turned out to be wrong, and researchers have discovered that the opah can actually continually keep its whole body, including its heart and brain, warm. This ability, known as endothermy, was thought to be unique to mammals and birds. A warm body temperature allows these animals to have a higher metabolism, and a better-performing brain and more active muscles, according to the study.

“Because the opah 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,” Wegner said in a statement.

In a study published today in the journal Science, Wegner and colleagues found that opahs produce heat by continually flapping their wing-like pectoral fins. This movement generates warmth in the fish’s enormous pectoral muscles, which take up nearly one sixth of its body mass.

Opahs also have an ingenious way of conserving and circulating this heat, Wegner says. The veins leaving the muscles with warm blood are wrapped around those leaving the gills, carrying cold blood. This heats up the colder blood, and limits loss of heat. This actually looks a lot like a design humans use to warm or cool things, called a “counter-current heat exchange,” found for example in car radiators. It’s amazing that this setup was “invented in fish long before we thought of it,” Wegner says.

“Nature has a way of surprising us with clever strategies where you least expect them,” he adds.