A Blog by Ed Yong

Meet the Comical Opah, the Only Truly Warm-Blooded Fish

There’s nothing about the opah that says “fast-moving predator”. Tuna, sharks, and swordfish are fast-moving predators and accordingly, their bodies look like streamlined torpedoes. By contrast, the opah looks like a big startled frisbee, with thin red fins stuck on as an afterthought.

It’s pretty (silver body and red fins) and big (up to two metres long), but fast? Nicholas Wegner from the National Oceanic and Atmospheric Administration certainly didn’t think so when he first started studying it. Since then, he has discovered that the opah is an active predator, which has a trait that no other fish possesses.

It is warm-blooded.

Most fish have body temperatures that match the surrounding water. A small number of them can warm specific parts of their bodies. Swordfish, marlins, and sailfish, can temporarily heat their eyes and brains, sharpening their vision when pursuing prey. Tuna and some sharks, including the mako and great white, can do the same with their swimming muscles, going into turbo mode when they need to. But none of these animals can heat their entire bodies. Their hearts and other vital organs stay at ambient temperature, so while they can hunt in deep, cold waters, they must regularly return to the surface to warm their innards.

The opah has no such problem. It can consistently keep its entire body around 5 degrees Celsius warmer than its environment. It doesn’t burn as hot as a bird or mammal, but it certainly outperforms its other relatives.

Nick-Wegner-OpahWegner discovered its ability by accident. His team just happened to catch more opah during one of their research trips, and they used the opportunity to learn more about this little-known species. As they dissected the animals, Wegner immediately noticed that its gills contain a beautiful and intricate tangle of red and blue blood vessels. “That was when we realised what it was capable of,” he says. Wegner had seen blood vessels like those before. They’re called retia mirabilia—Latin for “wonderful nets”—and they’re the secret behind the heating systems of tuna and sharks.

All animal muscles produce heat when they contract, but in most fish, that heat is almost immediately lost to the environment through the skin or the gills. The gills are especially problematic. No matter how much insulation a fish has, the blood that runs through the gills has to make close contact with the seawater. A tuna can produce as much heat as it likes in its swimming muscles, but as soon as the blood from those muscles reaches the gills, as it must do to be reloaded with oxygen, it ought to quickly cool. But it doesn’t, because of the wonderful nets.

In those nets, the veins that carry warm blood away from the hot muscles are interwoven with the arteries that carry cold blood in from the gills. They run so close that the veins offload their heat onto the arteries, before it can reach the gills and disappear. Through these “countercurrent exchangers”, the tuna can retain whatever heat it generates. But since its retia mirabilia are located in its swimming muscles, those are the only body parts that stay warm. That’s why its heart still runs cold.

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.

Wegner’s team confirmed this by catching opah, implanting them with small thermometers, and then releasing them. The instruments inside the fish recorded consistently higher temperatures than those dropped into the surrounding water. The opah’s brain is warm. Its muscles are warm. And perhaps most importantly, its heart is warm—a first for a fish. Not even a 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.”

So, it’s fast, then? Despite the somewhat comical physique? “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.”

Reference: Wegner, Snodgrass, Dewar & Hyde. 2015. Whole-body endothermy in a mesopelagic fish, the opah, Lampris guttatus. Science http://dx.doi.org/10.1126/science.aaa8902

45 thoughts on “Meet the Comical Opah, the Only Truly Warm-Blooded Fish

  1. If it’s warm blooded, then isn’t it, by definition, not a fish?

    [No. The definition of a fish has nothing to do with being cold-blooded. – Ed]

  2. had one of these in my freezer for long time it was caught off queen charlotte islands way to north for a tropical fish my son thought. he was a commercial troller at the time .

  3. It’s an endotherm. ‘Warm blooded’ is a false title, as is ‘cold blooded’. Nice discovery, though.

  4. One reason fish are cold blooded is because there is not enough dissolved oxygen in ocean water to support the higher metabolism of a warm blooded animal. How does this fish overcome this limitation?

  5. Hi Ed, you do good work, but I don’t think this statement is correct: “It can consistently keep its entire body around 5 degrees Celsius warmer than its environment.” There is considerable thermal variation throughout the body, as Figure 1A in the Science paper shows. I think the suggestion that opah exhibit “whole body endothermy” is debatable, and I think the press in general has been misrepresenting it a bit by saying it’s “fully warm-blooded” “truly warm-blooded” etc, which I think implies to the general reading public that it’s thermoregulating like a bird or mammal–which is clearly not the case.

  6. you catch these on long line boats ,they taste good ,used to sell to Japanese markets,we used to call them moon fish ,and I have a photo of me holding 1

  7. Dear Ed,
    fish 1 |fiSH|
    noun (pl.same or fishes)
    a limbless cold-blooded vertebrate animal with gills and fins and living wholly in water: the sea is thick with fish.

  8. Dear Bill,

    “Cold-blooded” only pertains to the metabolic heat production vs waste heat produced through physical muscle contraction. Tens of thousands of animals in the “cold blooded” animal kingdom produce their own “warmbloodedness” through vibrations and muscular activity, but are fully considered ectothermic due to no active metabolic heat production.

  9. “the Only Truly Warm-Blooded Fish”. What a truly shortsighted statement. If you only just found the first, how many more potentially exist?

  10. The evolutionary, systematic, definition of fish has to do with ancestry, not any one trait. The Opah’s ancestors and cousins and such are fish, so it’s a fish.

    Of course, this means that we are fish also in evolutionary terms, though very odd fish. (Our distant, distant ancestors were fish.) The Opah is a ray-finned fish, and not ray-finned fish was an ancestor of ours.

    That definition of fish that says they are cold-blooded is defining by description. It can, and should, be changed (slightly).

  11. Good to see the editor not posting my comment. You are clearly not a fish biologist. I am. Your article describing the opah as warm-blooded is incorrect. My last comment, which you did not post, pointed that out. It’s sad to see that National Geographic has become a place where actual scientific fact is not important and sensational headlines are. Warm blooded animals maintain a constant body temperature that is controlled by metabolic heat production. Cold-blooded animals eg. fish do not. The opah is very good at conserving heat generated within muscles but does not generate heat metabolically in order to maintain a constant internal body temperature regardless of external water temperature. Your article is wrong and sensationalizing incorrect information and you should be ashamed to call yourself a science writer. I realize this post will just be deleted by you, way to control the message and crush dissent.

  12. Suzie, do you mean troller or trawler? (I’ve come across trollers – normally called trolls – before, but not commercial ones, and certainly not someone who THINKS he’s a commercial troller)

  13. Whatever the finer points the surrounding definition, this is a fascinating discovery which I enjoyed reading about – thanks for a good article.

  14. This is way cool, read about in the paper today!! I was gonna go online and read about this amazing discovery!!! Thanks for sharing!!! Love national geographic!!!!

  15. Dan Kratville,

    The common phrases “warm-blooded” and “cold-blooded” are simply shorthand for three separate conditions. Can the animal maintain a constant temperature (homeothermic) or not (poikilothermic)? Does it have a high metabolism (tachymetabolic) or not (bradymetabolic)? Does it generate its internal body heat primarily by its own internal processes (endothermic) or not (ectothermic)?

    The tone of your comment was far too intemperate. Ed Yong is an excellent science writer who was simply using a bit of shorthand, as were you.

  16. I have been a reader of National Geographic since i was a kid and they unearthed Pompeii and i have also been fishing since i was 2 years old, and i find this story great and wonderful. It was told so very well and ppl are over looking the basic fact that there is a WARM blooded fish in the ocean pure amazement! someone above said “can u see the universe in his eyes” he gets it! And to answer ur question yes i can see it, so ppl stop bashing the writer and the column and enjoy the fact in this day n age we are finding these wonders on our beautiful planet!

  17. @ Dan Kratville

    Your opening statement informs me with high probability that you are neither a fish biologist nor someone versed in biological science. Merely someone who poses as such using the “I’m a so-and-so and you should listen to me since I say so”. Otherwise I am forced to assume you are incompetent at your profession.

    For starters as Jerrald Alphern pointed out, your criticism of the semantics regarding “warm-blooded” is ill placed and likely not thought through, you mean to say the ‘fish biologists’ at NOAA are incompetent since they are the source of that statement (not Ed)?

    But the more serious problem with your comment regards your concept of metabolism, the statement of which you make borders on outright untruth with intent to mislead.

    “…but do not generate heat metabolically…”

    Pray tell me then, what is the definition of ‘metabolism’ and what do you call the conversion of energy via cellular means (muscle contractions; which btw is the source of said thermal energy that the fish ‘conserves rather than generates’ as you so succinctly put.)

    Your last statement is something that a true scientist would never say, only an attention seeking conspiracy theorist. I’ve nothing against egotists, but spreading untruths and ill measured comments that only serve to mislead people who don’t know better is what sets me off.


    1. @Spencer

      Endothermy is not energy that is simply conserved from natural muscle movements, but an increase in metabolic rate specifically for the purpose of generating heat. One is a passive, the other is active. Cold-blooded and warm-blooded are incomplete, out-dated, and terms that gets you mocked after you ruin someone’s previous day’s dinner with said day’s idle chat, but if you’re eating at red lobster, what do you expect?

  18. The fish is still technically ectothermic, not “warm blooded” per se. The countercurrent exchanges that allow the fish to maintain its higher body temperature do not make it endothermic, but this is still an awesome find! The same adaptation can be found in the feet of arctic and antarctic birds that spend long periods of time standing on ice.

  19. Even more interesting is the Opah’s unusual behavior of approaching schools of fish while saying “YOU get a car!, and YOU get a car!, and YOU get a car!!…”

  20. While everyone expresses their jealousy for not making the find, while simultaneously being reminded that you can’t put a round world in a square box, I will simply enjoy the discovery for what it is… awesome!

  21. Maintaining constant body temperature associated to changes in insulation in birds and mammals is usually restricted to a thermoneutral range of temperatures and some heterothermy as regards to limbs and external areas usually occurs. From the picture I gather the opah is quite round thus reducing surface volume relationship (fish skin is normally a poor insulator although if subcutaneous fat is thick that would be enhancing it) and reducing thermal exchange. The fact that the countercurrent exchange is in the gills area is certainly very promising as regards to thermoregulation abilities but ¿are they restricted to keeping active exercise?.

  22. This is just a plain ordinary comment…..that fish looks intelligent…..and kind….and just a beautiful little guy to go floating by on a nice day…..he is very interesting looking….and after the studies are completed…..I do hope they let them live in peace down under…

  23. Jerrold Alpern; Many thanks for your clarification. Like Sarah, I also enjoyed the article. It has always been fun to learn new things. This has piqued my interest and will lead to further investigation and discovery.

  24. There is a serious logical error in comparing this fish’s heat generation with that of “warm-blooded” creatures. The observation is that the fish keeps its body temperature five degrees above the water temperature.

    However, “warm-blooded” organisms such as mammals or humans
    do not maintain a certain temperature above that of their surroundings; they very closely maintain a certain ABSOLUTE temperature, regardless of the surrounding temperature.

    All animals with muscles are as “warm-blooded” as the opah, but just a little more locally.

  25. WOW! I can’t beleve this is possible! I wonder if since this fish is warm blooded, if it evolved from a mammal? This fish has all kinds of possibilitys in its fins!

  26. Sammie,

    Mammals, like all tetrapods, evolved from fish, not the other way around. Lobe-finned fish, specifically, not ray-finned such as the Opah.

  27. i ate sometimes this fish, specially the cheeks. ( Since i saw the whole fish i’m ashamed)..i regret i will never do it again it s too magic animal..
    By the way, why men are always so happy and proud when they’ve killed a big or beautiful fish ?

  28. This is a very cool (but warmer than expected) discovery, thanks for passing it on!
    Sorry about all the dictionary-wielding trolls, though.

  29. Thanks for the findings, so begs question how adaptation to external temperature moderated vascular architectural evolution fish to man … to cetacean, or back?

  30. Camblam,

    Humans and whales are both mammals. Neither evolved from the other. They share a common ancestor whose own ancestors arose among the lobe-fin fishes. The process cannot be reversed or re-run.

  31. I don’t like the cold vs. warm blooded statement. Shades of grey, there are different degrees and mechanisms for biological functions and we should give them all their due.

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