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Is the Opah a warm blooded fish?

Is the Opah a warm blooded fish?



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In 2015 many sources, including USA TODAY, Discover Magazine, and National Geographic, ran stories about the Opah fish (Lampris guttatus) being warm blooded.

Image Source: http://discovermagazine.com/2016/janfeb/58-meet-the-first-warmblooded-fish

Headline from USA TODAY 5/14/2015: First warm-blooded fish identified

In a discovery that defies conventional biology, a big fish that lives deep in the Pacific Ocean has been found to be warm blooded, like humans, other mammals and birds.

Article from National Geographic 5/14/2015: Meet the Comical Opah, the Only Truly Warm-Blooded Fish

The opah's wonderful nets are in its gills, and that makes all the difference. The blood vessels carrying warm blood from heart to gills flows next to those carrying cold blood from the gills to the rest of the body, warming them up. So, while a tuna or shark might isolate its warm muscles from the rest of its cold body, the opah flips this arrangement. It isolates the cold bits-the gills-from everything else.

This allows its huge pectoral muscles, which generate most of its heat, to continuously warm the rest of its body. It also keeps that heat with the help of thick layers of fat, which insulate the heart from the gills, and the pectoral muscles (which produce most of the animal's heat) from the surrounding water.

According to the Merriam-Webster Dictionary warm blooded is defined as:

having warm blood; specifically : having a relatively high and constant internally regulated body temperature relatively independent of the surroundings

This definition seems to agree with the stories about the Opah fish being warm-blooded. However, it seems different from humans, mammals, and birds which generate and maintain body heat when muscles are at rest too. Are there different definitions of warm-bloodedness that would fit the Opah fish that is different from humans, mammals, and birds?


Did you say warm-blooded?

Despite the definition you cite from Merriam-Webstre Dictionnary, the termwarm-bloodis very unclear. The correct terms are endo-, exo-, poikilo- and homeo- therm. In short…

Source of heat

  • endo = inside
  • exo = outside

Variation in inside temperature

  • Poikilo = varies
  • homeo = does not vary

Please note, that these terms refer to extreme cases. There are organism which temperature are extremely constant (homeotherm), organisms which temperature are extremely variable (poikilotherm) and anything in between.

Any combination of these two axes exist. For example: If the temperature in the environment never varies you can be homeotherm without needing to be endotherm. Also, some organisms manage to keep a quasi constant body temperature via behavioural actions (moving to the sun when cold, going into the water when it's hot). A fun example (although debated to what I've heard) are the large dinosaurs that are thought to be homeotherms because their metabolism produce some heat and they are so large that they remain warm thanks to this heat source. However, they were probably not able to regulate actively their temperature. Therefore, I would tend to qualify dinosaurs as homeo-exo-therm but I wouldn't be surprised if someone prefers to call large dinosaur homeo-endo-therm individuals.

Opah fish

From what your second quote, it sounds like the Opah fish is endotherm. It is unclear from the quotes whether Opah fish are homeothermic.

Wegner et al. (2015) is a great read on the subject. Below is their abstract but I recommend reading the whole paper, it is easy to read and is very short (3 pages).

Endothermy (the metabolic production and retention of heat to warm body temperature above ambient) enhances physiological function, and whole-body endothermy generally sets mammals and birds apart from other animals. Here, we describe a whole-body form of endothermy in a fish, the opah (Lampris guttatus), that produces heat through the constant “flapping” of wing-like pectoral fins and minimizes heat loss through a series of counter-current heat exchangers within its gills. Unlike other fish, opah distribute warmed blood throughout the body, including to the heart, enhancing physiological performance and buffering internal organ function while foraging in the cold, nutrient-rich waters below the ocean thermocline.

In the whole, paper, I could not find a mention of how much variation in body temperature there is. They highlight however that the point of such adaptation is for foraging in cold waters suggesting that they would just make a boost of temperature in extreme cases but not necessarily actively maintained a constant temperature otherwise. I would tend to think that Opah fish are much more variable than birds or mammals. To reinforce my believes, in the wikipedia > Opah fish it is written (without reference unfortunately) "The opah is not homeothermic like birds and mammals".


FYI, this answer was written by partially recycling this other answer of mine.


Meet the first fully warm-blooded fish: the opah

Though it’s a deep ocean fish, the slender opah is actually fully warm blooded – the first of its kind discovered so far. This remarkable insight was made by accident after researchers at the National Oceanic and Atmospheric Administration dissected the fish and noticed its blue and red blood vessels were located inside the gills, rather than in the fish’s swimming muscles. Tuna or sharks, which both have the same vessels but not arranged in the same way, cool their blood once it reaches the gills for oxygen reloading. The opah’s vessels are interwoven inside the gill like a net, which means the the veins that carry warm blood away from the hot muscles are interwoven with the arteries that carry cold blood in from the gills. This makes all the difference. Running so close to each other, the warm blood from the heart heats the cold blood from the gills. This way the Opah is 5 degrees Celsius warmer than its surroundings waters!

Researchers Nick Wegner holding a opah. Image: NOAA FISHERIES WEST COAST

Nicholas Wegner from the National Oceanic and Atmospheric Administration and colleagues were on research trip when they happened to catch some opah fish. While they were at it, they decided to study the fish closely – why not? They eventually found much more than they bargained for.

“That was when we realised what it was capable of,” Wegner said.

For instance, its frisbee-shape isn’t quite an indicator that this is a fast, ferocious predator. But its appearance is deceiving, as the researchers later learned after they tagged the fish with instruments.

“That’s what’s really blew my mind about this discovery,” says Wegner. “Just from looking at it, I really thought it was a slow, sluggish, deep-water fish that doesn’t do very much. But all indications are that this is a very fast fish and an active predator. We’ve put some tags on them to show that they migrate thousands of kilometres.”

The opah is as close to a full-body warm-blooded fish as science has yet discovered. Image: NOAA FISHERIES WEST COAST

Besides the net-woven blood vessels, the fish also retains heat with the help of an extra layer of fat which insulates the heart from the gills. The same applies to the pectoral muscles , where most of the heat is generated by its fins, from the surrounding water. This way, the opah’s heart, brain and muscles are all warmer than the surrounding waters, the researchers report in Science. Not even the great white shark has a warm heart.

“That’s why opah can stay at depth,” says Wegner. “These guys are specialised for living deeper than those other predators.”


Discovery and Gill Study:

The Opah has been hunted by fishermen off the coast of California, as well as other locations, for years. In 2012 though, Owyn Snodgrass, a fisheries biologist from the National Oceanic and Atmospheric Administration in San Diego, California, considered the Opah’s gills worth studying. The discovery was somewhat of an accident he and his team caught a few more specimens than intended and decided to take advantage of them. He gave a colleague, Nicholas Wegner, a fish physiologist (AKA “The Gill Guy”) the gills in a preservative.

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Wegner was not in a rush to study them. After all, this fish was not a new discovery. When he finally did, he immediately knew there was something different about the Opah’s gills.

Most fish have a few large blood vessels that carry blood to and from the gills. However, the endothermic Opah has a dense network of tiny blood vessels cozied right up to arteries and veins. Because they’re so closely entwined, the warm blood heats up the cooler blood during circulation, allowing the fish to maintain a constant body temperature.

(Here’s an image of Rete Mirabile, or “wonderful net,” the name for this arrangement of paired arteries and veins. Don’t click if you have a weak stomach. Just imagine a tightly-packed lump of angel hair pasta, but kind of grey and gross.)

Many animals use this “wonderful net” as a countercurrent heat exchanger. Some examples are aquatic birds and whales (their nets are in their tongues). The Queensland Groper uses its net to regulate buoyancy. The Tuna, Billfish, and certain Sharks mentioned earlier also use this wonderful net to keep their muscles warm, but only in short bursts.

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The difference between them and the Opah is that the Opah is the first fish discovered with this arterial organization around its gills. The heat exchanger is wrapped in a centimeter-thick layer of fat used, presumably, for insulation.


Warm-Blooded Fish

Bob Grant
May 15, 2015

Biologist Nick Wegner, coauthor of the Science paper, holding a recently captured opah WIKIMEDIA, USA NOAA FISHERIES SOUTHWEST FISHERIES SCIENCE CENTER Fish are generally a cold-blooded set. No species had been documented to stray very far from ecothermy, which makes the body temperatures of most fish&mdashalong with reptiles, amphibians, and insects&mdashtrack with environmental conditions. But researchers have determined that one species of fish, the opah, or moonfish (Lampris guttatus), is warm-blooded and can regulate its body temperature even at the frigid depths of its deep-sea habitat.

The opah was long considered a slow-moving fish, like many of the other species that inhabit the deep sea. But scientists with the National Oceanic and Atmospheric Administration&rsquos Southwest Fisheries Science Center (SFSC) reported in Science today (May 15) that the species can keep its body temperature at about 5° C warmer than the surrounding water by pumping its slender pectoral fins.

“Before this discovery I was under the impression this was a slow-moving fish, like most other fish in cold environments,” coauthor and SFSC biologist Nicholas Wegner told BBC News. “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.”

Wegner first suspected the opah’s endothermy after dissecting several species his colleagues had caught on a collecting trip off the coast of California. Noticing the intricate structure of the fish’s gills, Wegner and his colleagues decided to sample more opahs, measuring body temperatures of fish they caught at sea and inserting thermometers into some specimens before releasing them still attached to fishing gear. The team recorded consistently higher temperatures in the opah’s brain, muscles, and hearts.

Although tunas and some shark species have been shown to be “regional endotherms” because they can temporarily warm their swimming muscles for bursts of predatory speed, they are not able to maintain their hearts at temperatures higher than the surrounding seawater. Likewise, swordfish, marlins, and sailfish can temporarily warm their eyes and brains, but not their hearts. “That’s why opah can stay at depth,” Wegner told National Geographic’s Not Exactly Rocket Science. “These guys are specialized for living deeper than those other predators.”

“It’s a remarkable adaptation for a fish,” Diego Bernal, a University of Massachusetts, Dartmouth, fish physiologist who was not involved in the study, told Science.


The Opah /Moonfish. the world’s first warm-blooded fish? / May 15, 2015 by Francisco Blaha

Anyone that spent time in a longliner in the pacific, knows Opah (moonfish) Lampris guttatus. As it normally comes as by catch. Is a quite a good looking fish, but not particularly fast or smart. but it looks like it keep a good secret up its fin!

NOAA Fisheries West Coast

One of the most basic biology facts we’re taught in school growing up: Birds and mammals are warm-blooded, while reptiles, amphibians and fish are cold-blooded.

In a recent paper published published in Science, Wegner et all, explain that

The secret lies in a specially designed set of blood vessels in the fish’s gills, which allows the fish to circulate warm blood throughout its entire body.

Scientists already suspected the opah was special. Most fish who live where the opah does — that is, hundreds of feet deep, in some of the ocean’s darkest and coldest places — are sluggish, thanks to the low temperatures. At these depths, even predatory fish tend to be slow-moving, waiting patiently for prey to come by rather than actively chasing it down. But the opah, which spends all its time in these deep places, has many features usually associated with a quick-moving, active predator, such as a large heart, lots of muscle and big eyes. These characteristics made the opah “a curiosity”.

The opah’s secret first started to come out when NOAA researcher and lead author of the paper looked at a gill sample and noticed something intriguing.

All fish have two kinds of blood vessels in their gills: vessels carrying blood in from the body to pick up oxygen, and other vessels carrying oxygenated blood back out again. In the opah, the incoming blood is warm after circulating through the fish’s body. This is because the opah swims by quickly flapping its pectoral fins, rather than undulating its body like many other fish do, to propel itself through the water — a process that generates high heat. But outgoing blood, which has just been in contact with water in the gills, is cold. Wegner noticed that in the opah’s gills, the two sets of vessels are tightly bundled against each other, so that the incoming blood vessels can warm up the outgoing blood before it goes anywhere else. This set-up, known as “counter-current heat exchange,” allows warm blood to be delivered throughout the body.

Some other types of fish, such as tuna, have similarly designed blood vessels in certain parts of their bodies, allowing for “regional endothermy” — warm-bloodedness that’s limited to certain organs or muscles, such as the eyes, liver or swimming muscles. But the opah is the only fish scientists know of that has this design in its gills, where most fish lose the majority of their body heat to the surrounding cold water. By warming up the blood in the gills before it goes anywhere else, the opah achieves not just regional endothermy, but whole-body endothermy, according to the paper’s authors. Testing showed that the opah is able to maintain a core body temperature about 5 degrees Celsius warmer than the surrounding water.

While only one species of opah is currently recognized — Lampris guttatus — scientists are starting to believe that they should actually divide the opah into several different species based on genetic variations in different populations around the world. The opah in this study were found off the West Coast of North America, so the next step will be to start sampling opah in other parts of the world to see if they all have the same specialized gills.

Down the road, future studies could also examine other related types of fish to try and figure out how and when those special gills evolved. And since a variety of different fish already exhibit regional endothermy, including tuna and certain types of sharks, it may be possible that this kind of physiological adaptation, has evolved numerous times in different lineages of fishes.

It’s possible that other deep-water species have similar adaptations as the Opah, although it’s unlikely scientists will ever discover a fish that’s truly warm-blooded, in the way whales or other marine mammals are warm-blooded.


Middle School Mind Blown: Warm-Blooded Fish Exist

I don't remember a lot about biology, but there are a few unalienable truths that I took with me: There are creepy bugs that live in your eyelashes, DNA sequences are. a thing, and fish — like reptiles and amphibians — are cold-blooded. But now scientists have discovered the opah, the world's first warm-blooded fish, essentially breaking all of biology. So there's that.

The opah, sometimes called the moonfish, generates heat by constantly flapping its fins, then conserving it in specially designed blood vessels in its gills. Scientists at the National Oceanic and Atmospheric Association, which published a paper detailing its findings on Thursday, compared its specially developed system to a car radiator.

And here's the thing: Scientists didn't just discover this absurd-looking, biology-defying fish. People actually eat it! It's regularly caught by fishermen in the U.S. and New Zealand. Here's a recipe for seared opah with vine-ripe tomato garlic butter! Should we really be eating biological anomalies?

Researchers at the NOAA knew that the opah was a particularly special snowflake. Most fish that live hundreds of feet deep tend to be slow moving. But the deep-water predatory opah has a large heart, a lot of muscles, and big eyes, which are typically associated with active predators and desirable Tinder matches.

"Before this discovery I was under the impression this was a slow-moving fish, like most other fish in cold environments," Southwest Fisheries Science Center's Nick Wegner told USA Today. "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."

Can you imagine that giant frisbee chasing down a squid? The opah weighs about 100 pounds and is approximately the size of a car tire.

Researcher Heidi Dewar told The Washington Post:

What Dewar means is that it's really exciting to shake the foundations of understanding. Burn your textbooks, kids. They're all full of lies about science that will one day be debunked by even more science. Biology, like everything else, is a circle as flat as the opah. Give up now. The world is futile.


The Opah Is The First Warm-Blooded Fish Ever Found

A new study published in the journal Science has identified the world's first fully warm-blooded fish: the car tire-sized opah, or moonfish.

Unlike birds and mammals, which warm their bodies above the ambient temperature, fish, amphibians and reptiles are generally classified as cold-blooded, meaning they use external means to control body heat. Some fish can generate bursts of warmth with swim muscles to help them during times of activity, but the opah is the first fish found to use heat to warm its heart and brain.

The opah lives almost exclusively in the deep ocean, where fish are usually sluggish due to the low temperatures. "At these depths, even predatory fish tend to be slow-moving, waiting patiently for prey to come by rather than actively chasing it down," The Washington Post notes.

Because the opah has a specialized set of wing-like fins that can produce heat, it's able to have a competitive advantage and live as a quick, active predator, the National Oceanic and Atmospheric Administration reported.

“Before this discovery I was under the impression this was a slow-moving fish, like most other fish in cold environments,” Nicholas Wegner, a fisheries biologist with NOAA, said in a press release. “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.”

Other fish, like tuna -- agile predators near the top of the food chain -- can warm their muscles through similar processes, but their bodies cool when they dive to deep depths. They must return to the surface to warm back up, NOAA said. But the opah uses a unique set of gills that resemble a car radiator, which allow the fish to stay in cold depths full time.

“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.”


Gills Show Unusual Design

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. “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 researchers collected temperature data from opah caught during surveys off the West Coast, finding that their body temperatures were regularly warmer than the surrounding water. They also attached temperature monitors to opah as they tracked the fish on dives to several hundred feet and found that their body temperatures remained steady even as the water temperature dropped sharply. The fish had an average muscle temperature about 5 degrees C above the surrounding water while swimming about 150 to 1,000 feet below the surface, the researchers found.

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.


Meet The Opah, The First Known Warm-Blooded Fish

Researchers say they've discovered the first known fully warm-blooded fish.

It's called the Opah, or moonfish, and it lives in cold environments deep below the ocean's surface. Scientists say the Opah generates heat by constantly flapping its pectoral fins. (Video via Science)

It then uses fatty tissue to help trap the heat and special blood vessels to spread the warmth throughout its large, disc-like body.

Fish, of course, are generally thought to be cold-blooded animals, so researchers say this gives the Opah a "competitive edge" over some of its aquatic cousins in more frigid waters.

And, sure, the orange-and-silver fish might look pretty innocent, but researcher Nicholas Wegner of the the National Oceanic and Atmospheric Administration says the opah is actually quite the aggressor.

"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," Wegner said.

Other fish, like tuna and some sharks, can also generate heat, but their bodies can only handle short periods in the cold before they have to retreat to warmer waters. (Video via National Geographic)

The Opah can keep its body warm for long periods of time, so it doesn't have to worry about that as much.

But despite what seems like a pretty cool discovery, at least one scientist was skeptical. At least at how it's labeled, anyway.

The study refers to the Opah as a fish with "whole-body endothermy" or, basically, entirely warm-blooded.

But an associate professor of biology at the University of Massachusetts Dartmouth is quoted in The Washington Post saying that's a bit misleading.

He says the opah is warm for the most part, but, "It gets cold as you go back into the body or back into the tail or as you go up or down," so calling it a "whole-body" warm-blooded fish might be a stretch.


Is the Opah a warm blooded fish? - Biology

Researchers say they've discovered the first known fully warm-blooded fish.

It's called the opah, or moonfish, and it lives in cold environments deep below the ocean's surface. Scientists say the opah generates heat by constantly flapping its pectoral fins.

It then uses fatty tissue to help trap the heat and special blood vessels to spread the warmth throughout its large, disc-like body.

Fish, of course, are generally thought to be cold-blooded animals, so researchers say this gives the opah a "competitive edge" over some of its aquatic cousins in more frigid waters.

And, sure, the orange-and-silver fish might look pretty innocent, but researcher Nicholas Wegner of the the National Oceanic and Atmospheric Administration says the opah is actually quite the aggressor.

"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," Wegner said.

Other fish, like tuna and some sharks, can also generate heat, but their bodies can only handle short periods in the cold before they have to retreat to warmer waters.

The opah can keep its body warm for long periods of time, so it doesn't have to worry about that as much.

But despite what seems like a pretty cool discovery, at least one scientist was skeptical. At least at how it's labeled, anyway.

The study refers to the opah as a fish with "whole-body endothermy" or, basically, entirely warm-blooded.

But an associate professor of biology at the University of Massachusetts Dartmouth is quoted in The Washington Post saying that's a bit misleading.

He says the opah is warm for the most part, but, "It gets cold as you go back into the body or back into the tail or as you go up or down," so calling it a "whole-body" warm-blooded fish might be a stretch.


The first evidence of a warm blooded fish

The paper: Wegner, N. C., Snodgrass, O. E., Dewar, H., & Hyde, J. R. (2015). Whole-body endothermy in a mesopelagic fish, the opah, Lampris guttatus, 348(6236), 786–790. 10.1126/science.aaa8902.

Humans are endothermic: we produce our own body heat so that our bodies can stay warmer than the surrounding environment. So when you go swimming in 80° water you are still able to keep your body temperature warmer than the water, allowing your body to function normally. All birds and mammals are endothermic. Other animals like reptiles and fishes, however, are ectothermic- or cold blooded. This means that the temperature of their external environment regulates their body temperatures. That is why you commonly see snakes and lizards laying on sunny rocks in the morning: they need to warm up before their bodies are able to function at peak levels.

Fishes’ body temperatures are strongly regulated by the water temperature. Fishes use their gills to absorb oxygen dissolved in the water. This allows them to stay underwater their entire lives, but also comes at a cost. Because the blood vessels in the gills are in such close contact with the water, body temperatures are very susceptible to cooling down to the temperature of the surrounding water. As a result, fishes living in cold water have a body temperature very close to that of the surrounding water, and therefore, have decreased performance, a slower metabolism, swim slower, and overall are not as active as fishes living in warmer water. (For comparison sake: our lungs are located at the center of our body- far away from surfaces which come in contact with the external environment. When we breath cold air it warms up after entering our nose to a temperature that does not cause our blood to cool significantly when the oxygen is absorbed into our bloodstream.)

Figure 1: The strange but beautiful opah, Lampris guttatus. Image from swfsc.noaa.gov

Some fishes, like tunas and some sharks, are capable of warming certain parts of their bodies (i.e. brains, eyes, or muscles used for short bursts of speeds) while in colder water. These fishes are termed “regional endotherms” and have higher performance levels while hunting, especially while diving to deeper water (water below about 100 m can approach near freezing temperatures!). For example, tunas and mackerels can warm their eyes and brains, increasing vision and nervous response while hunting in colder waters and mako sharks can heat muscles allowing them to swim faster for short time periods. This alone is pretty impressive in the fishy world. Scientists have discovered something even more exciting!

Ladies and gentlemen, please put your hands together for the opah!!

The opah, or moonfish (Lampris guttatus) is an extremely bizarre looking fish (Fig. 1). This disc-shaped, silvery spotted fish with bright red fins looks like something out of a children’s book. And this fish gets big (Fig 2)- reaching lengths over 6 feet long and weights over 150 lbs! While the opah can be found in all temperate and tropical oceans, not much is known about this strange fish. It is apparently solitary and spends most of its time hanging out in deep water (50-500 m).

Figure 2: Three happy men with their large, freshly caught opahs! Image from masterok.livejournal.com

Until recently, scientists thought that this fish was lethargic, like most deep-dwelling fishes. That is until some scientists from NOAA took a closer look at a specimen they had just caught. When they looked at the gills, scientists were very surprised to see a unique network of blood vessels that appeared to be capable of warming up the tissue. This system, termed the rete mirabile (latin for “wonderful nets”), uses countercurrent heat exchange to warm the opah’s body. Essentially, the blood vessels which are cooled by the water are interwoven with blood vessels carrying warm blood from the body. The warm blood vessels, warm up the cool blood vessels, keeping the body warm even in cold water(Fig. 3)!

A Closer Look:

After the initial discovery, the NOAA scientists decided to dive a little deeper. They measured temperatures throughout the body of recently caught fish and found that, unlike regional endotherms, the entire body of the opah was warmer than the surrounding water (5° C warmer on average Fig. 4)! In situ measurements confirmed that the pectoral fin muscle generates the bulk of this heat. Layers of blubber surrounding the gills, heart, and pectoral muscles help to insulate the fish and maintain the higher temperature.

Figure 4: A) The internal temperature of the opah is higher than the surrounding environment (ambient temperature of 10.5°C). Temperatures are warmest around the brain (red area), gills, and pectoral fin (directly behind the red area- yellow to light blue in color) B) In situ measurements of body temperature at depth. Note the very stable internal temperature of the opah, even though there is a significant drop in water temperature.

Figure 3: Close up of the gills of an opah. A) shows a single gill arch. The close up in B) shows the thick blubber (adipose tissue) surrounding the gills. C) shows the rete mirabile with interwoven cold blood vessels (red – oxygenated) and warm blood vessels (blue – deoxygenated)

Using satellite tags, the scientists found that opahs spend the majority of their time in cooler water (between 50 – 500m) without spending much time in shallow water warming up. The opah is able to spend more time in deeper water and dive deeper than regional endotherms (such as tuna Fig 5) because of this whole-body endothermy which increases performance and keeps the fish from having heart failure at such cold temperatures.

Figure 5: Comparison of the percentage of time spent at depth for the opah and the albacore tuna. The opah spends more time at depth and makes deeper dive than the tuna.

Significance:

With the increased body temperature, the opah is expected to have a higher metabolism and increased performance of muscles, eyes, brain, and heart- allowing for sustained swimming speeds, higher sensory capabilities, and lower risk of having cardiac failure while at depth. All of this contradicts the initial beliefs that the opah is some slow, goofy fish that just hangs out at depth waiting for its next meal to swim by.

Besides the super cool physiological significance, this recent discovery highlights another important point in science: there are a lot of really incredible things in nature that we have yet to unveil! We know astonishingly little about our world and sometimes, the biggest discoveries come from unexpected place and funny looking fishes!

I received my Master’s degree from the University of Rhode Island where I studied the sensory biology of deep-sea fishes. I am fascinated by the amazing animals living in our oceans and love exploring their habitats in any way I can, whether it is by SCUBA diving in coral reefs or using a Remotely Operated Vehicle to see the deepest parts of our oceans.


Watch the video: Meet The Opah, The First Known Warm-Blooded Fish (August 2022).