The first known warm-blooded fish

May 17, 2015 • 10:00 am

Everybody knows that the only warm-blooded (“endothermic”) animals—animals that can keep their entire body temperature well above ambient temperature—are birds and mammals. But many of you probably know that some animals, like lizards and turtles, can gain the advantages of endothermy by raising their blood temperature via basking in the sun.(There are speculations that some of the dinosaurs were truly warm-blooded, but we don’t know for sure.) Others—most notably fish like tuna, some sharks, and swordfish—can raise parts of their bodies above ambient temperature by generating heat from metabolism, usually in the muscles. Insects like bees can do the same, but only when flying or buzzing their wings.

The advantages of endothermy are many. Because the body is basically a giant chemical reactor, low temperature slows down everything, including your speed and your ability to function over a range of environmental temperatures, and thus to escape predators and find mates and food. Your rate of digestion is also higher. But of course there are costs, too, which probably explains why many species haven’t evolved it and remain “ectothermic,” or cold-blooded. Sometimes there’s no need to keep your body temperature above ambient, or you can do it behaviorally rather than metabolically (lizards bask in the sun, some fish make sporadic visits to the water surface to warm up).

As I said, fish like tuna and swordfish can keep parts of their body, like the muscles, above ambient temperature, but the body core, including the crucial heart, cannot be kept warm, and that severely limits their activity and ability to function at low temperature. (After all, your heart rate determines the rate at which blood gets to the rest of your body.) But now, according to a new paper in Science by Nicholas Wegner et al. (link and free view below), there is at least one species of fish that is truly warm-blooded, keeping its entire body temperature 5 °C or so (9° F) above the water temperature. This is the opah (Lampris guttatus), the fish shown below.


The opah is distributed worldwide, and is “pelagic”, swimming in the open ocean from the surface to several hundred meters down.  It’s a big fish that can reach 2 meters long and weigh 600 pounds.

The authors discovered, by implanting thermocouples in swimming fish, that the opah keeps its body temperature well above ambient, even in cold waters. The diagram on the left shows the thermal profile of a opah swimming at 10.5 °C. The thermocouple measurements show that its entire body is above ambient temperature, and the crucial core is about 4 degrees above ambient. Note that the fish is warmest around the eyes, something also seen in billfish. It may be due to movements of the eye muscles.

On the right you see a diagram of the temperature of the fish’s pectoral muscles (a small, 39-kg specimen) as it dives to different depths up to 100 m (right scale). Although the ambient temperature of the water varies from 18°C at the surface to about 9.5 °C below 50 m, the body temperature (red line) stays between 13 and 14 degrees.


The interesting thing is how the fish does this. It requires several adaptations involving behavior, morphology, and physiology.

The main problem with a fish trying to conserve body heat is its need to oxygenate its blood by bringing blood vessels in its gills close to the water (the oxygen source). That, of course, cools down the blood severely. The oxygenated but cold blood then flows back to the fish’s core, cooling it down.

The opah has three ways to overcome this problem. First, it generates a lot of body heat by swimming forward not by undulating its body, as do most other fishes, but by vigorous “rowing” of its pectoral fins (see the video below). That generates a lot more body heat than does undulation.

Second, and most striking, the opah has evolved its circulatory system into a system of heat conservation called “countercurrent heat exchange.” If you think of the blood vessels in the gills as a series of pipes, the opah has “cold water” pipes running away from the gills, containing cold oxygenated blood, that lie side-by-side with “hot water” pipes running from the body toward the gills. Their proximity allows the cold blood flowing toward the body core to be warmed by the muscle-generated heat; all of it keeping the blood and body temperature higher than ambient.

In the right picture below (“C”), you can clearly see the proximity of the two types of vessels. The blue ones are the “afferent” vessels coming from the body to the gills (don’t be fooled by the color; the blue ones have warm blood), while the red vessels are the “efferent’ vessels containing oxygenated but cold blood coming from the gills. You can see that they’re interspersed, like like side-by-side wires. The temperature differential thus allows heat transfer from the blue vessels to the red ones. This arrangement is called the retia mirabilia (“wonderful net”), which is also seen in other fish that can keep parts of their body above ambient temperature.

Third, as you see on the left (“B”), the gill arches, where the blood gets oxygenated and chilled by the water, and which contains the retia mirabilia, is insulated from the rest of the body by a thick layer of fat (“adipose tissue” in the diagram). This further insulates the body from the cold blood, allowing it to be warmed up before it leaves the gill arch:

Screen Shot 2015-05-17 at 6.30.29 AM

Here’s a video (some of it taken from a video in the Science paper) showing the opah swimming with its pectoral fins.

Here’s a big advantage of being an endothermic fish. The diagrams below show the percentage of time that various specimens of opah and albacore tuna spend at various depths in the water (dark bars at night, light bars during the day). The figures at the top show the percentage of time that each species spends above 50 meters. You can see that at night it’s only 31.2% for the opah but 89.1% for the tuna, with a similar disparity during the day (but with both fish going lower during the day.  Most important, the opah is able to forage much deeper than the tuna, almost certainly because its higher both temperature allows it to dive deeper without losing mobility, digestive capacity, and metabolic activity.


It’s almost certain that this one species of opah is not unique, and the authors say that other species in the group should be studied. I suspect that these innovations were present in the common ancestor of the group, and so we should find other endothermic fish. Further, it’s hard to even find this adaptation in any fish, for that requires putting thermocouples into individuals and monitoring their body temperature while they’re swimming (the opah in this study was tethered to a surface float so it could be retrieved).

Finally, why don’t more fish do this? As I said, for some fish it’s undoubtedly not an evolutionary advantage to keep a high body temperature, as they probably do just fine swimming near the surface, or, if found in the depths, have adapted in other ways to cold temperature (and darkness). Alternatively, the requisite mutations for warm-bloodedness might simply not have occurred in other species, even if they would be beneficial.  We don’t know the answer to these questions, but we can at least find out how many fish have adapted a quasi-mammalian/bird-like system of keeping their temperatures high.

h/t: Heather Hastie


Wegner, N. C., O. E. Snodgrass, H. Dewar, and J. R. Hyde. 2015 Whole-body endothermy in a mesopelagic fish, the opah, Lampris guttatus. Science 348 786-789

60 thoughts on “The first known warm-blooded fish

  1. Interesting.
    Wouldn’t the fact that being surrounded by cold water make a warming process difficult and expensive?
    We can’t last more than a few minutes in really cold water.

    1. Yes, but many large vertebrates manage it. An important thing is that the seas are generally a very food-rich environment, so animals have found a way to thrive there — evolving to exploit niches food is abundant b/c ‘nature finds a way’ to quote the 1st Jurassic Park movie.

      1. I didn’t quite think it through as I was going to sleep and also wanted to be the first comment. Although I did think of insulation.

        Then as I was going to sleep I thought of those vertebrates, and that those vertebrates have large amounts of insulation.

        I don’t know enough about the biology of these things but do fish have something like fat cells that could be used?

        I suppose a fish in warmer water may have to not have to worry about real though.

  2. I was under the impression that the possible endothermy of dinosaurs, while not certain, was rather more than speculation.

    1. Especially for the small, agile ones, and especially for those covered in feathers. Having feathers (or fur) is as far as i know a clear indicator of endothermy as these things exist to hold heat in. An animal that depends more on its environment to get warm cannot afford to have these coverings, since they would prevent them from getting warm from the sun or from warm rocks.
      If you put a fur coat on a lizard it will freeze.

    2. An important point to keep in mind is that birds are dinosaurs. So yes, we know that many dinosaurs are warm-blooded.

      And if it is only about the dinosaurs from before the K/T boundary, again, because birds are part of the theropod lineage, it seems quite likely that many of the characteristics of modern birds evolved somewhere in their ancestors already as opposed to all characteristics suddenly appearing at the same time in the most recent common ancestor of birds. This might apply to warm-bloodedness as much as to feathers and brooding of eggs.

      1. Exactly. The ancestors of birds had many of the anatomical traits of birds, so of course they would have many of the physiological traits as well.

  3. Since warm-blooded animals generate heat and one would certainly call the animal “the system”, why do biologists call them endothermic? Didn’t you guys learn thermodynamics? 😀

    1. I guess you didn’t learn biology/zoology; defines endothermic as an animal capable of internally generating heat.

      1. Yes, I know. But since we’re told here that “the body is basically a giant chemical reactor” it seems that the chemical thermodynamic definition should hold sway.

        1. By the chemical definition, all animals are exothermic; they all oxidize food to produce energy. So that doesn’t seem like a very useful definition for distinguishing different types of metabolism.

    2. Termed that way so that freshmen who find this counter-intuitive can be quickly identified as being be better off as physics majors.

    3. I am with you here … exothermic would be ‘more’ logical. And I am a mere chemist by trade.

      But since most chemical reactions with a favorable free energy are exothermic, I doubt “cold blooded” critters actually have many reactions that are substantially endothermic.

      So what we have are animals that don’t have systems in place that burn carbohydrates specifically to overcome heat losses.

      In both cases warm and cold blooded are giving off energy derived from some oxidation reduction reaction.

      My Greek is non existent … but thermostatic seems a better term.

    4. The terms can be a little weird, once they are considered critically. All animals, plants, even bacteria generate some heat by their metabolism. But the ‘cold blooded’ species do not generate enough heat to raise their temperatures consistently above ambient temperature. Animals need some heat to get their muscles going for sustained exertion, and for other physiological processes like digestion. ‘Ectotherms’ are so-named b/c they get their additional heat from the environment by basking in the sun or on warm rocks. The term is meant to mean that they rely on an external source of heat.
      Animals like mammals and birds (and this fish) have a souped up metabolism, and the extra heat they generate is enough to operate their systems efficiently 24/7. They do not need to spend time warming up, which has advantages (and disadvantages). They are called ‘endotherms’ b/c they get their needed heat internally.

  4. I was hoping you would have a post on this having seen a small snippet elsewhere. However I would be interested to know if it can “cool down” if the water temperature rises or would it just have to dive deeper. Could this be part of the explanation it spends more daylight time at deeper depths?

    1. I would predict that they could cool down by seeking a cooler environment like any animal, including mammals. The counter-current system that they have would simply exchange heat in reverse, always moving heat from hot –> cold.

  5. I bet the quickest way to identify which species are endothermic is to look for ones that spend the days at a roughly equal distribution of depths as opposed to those that spend proportionally more in the shallows.


    1. I also wonder if pelagic fish might be another factor in identifying endothermic fish. Wiki says 11% of known fish are pelagic.

  6. The early studies on lactate and glyceraldehyde-3-phosphate dehydrogenases were done on the enzymes from lobster muscle. Those labs that did that work were famous for their lobster parties.

    So the question is, what does opah taste like?

  7. In some alternate universe wise fish are astounded at the adaptations mammals have to make to live in an environment without omnipresent water…

  8. So the fish can’t maintain temperature unless it swims nearly constantly, right? Is that truly endothermic? Mammals and birds can maintain temperature just sitting there.

    1. Yes indeed. A much better, more detailed posting than I’ve read elsewhere. The other news sites, even the so-called “science” sites dumb things down so much that it’s little use if one really wants to grasp the significance of the original paper. This is the main issue with the death of the science reporter, now replaced by someone in the newsroom who just reports science.

  9. The Opah is certainly in the news at the moment. Having read several stories on this amazing discovery, this post is IMO by far the most informative and fascinating.

    Thank you very much Prof CC !!

  10. That heat exchange system does more than warm the oxygenated cold blood coming from the gills. It preserves some of the heat that would otherwise be lost from the deoxygenated warm blood headed to the gills. Reduce, reuse, recycle!

    1. Incidently, humans have a similar construct from fingers to elbows and from toes to knees. It’s called venae comitantes. In our case, the additional veins on either side of main arteries help cool us in summer as well as shunting blood away from the extremities in winter. And, it’s why ballerinas wear leg warmers. Granted, it’s far less for direct artery-vein heat transfer, but its anatomy and physiology are closely related, as they deal with maintenance of core body temperature.

  11. Thanks! I must have seen the odd swim style before. And now I know why it behaves that way!

  12. Now that you’ve written about it, I understand it much better than when I read about it from other sources. Thank you Jerry.

  13. Surely part of the reason why there aren’t more endothermic fish is the high heat capacity of water compared to air. This is why we immerse boiled eggs in cold water to cool them quickly, and also why hypothermia is a greater danger in cold water than in cold air. A warm-blooded fish spends more energy keeping warm (for the same temperature differential) than a land mammal.

    Also, birds and mammals can use feathers and fur to trap an insulating layer of air near the skin. Fish (and aquatic mammals) don’t have that option due to hydrodynamic constraints.

    What’s missing from that opah/tuna comparison chart (cue David Letterman) is a measure of hours per day spent hunting, and calories consumed per kilogram of body mass. My guess is that the opah scores higher on both.

    1. I wonder whether the opah will evolve to have more fat to help it keep warm? However, that would also make it harder to dive deeply. So maybe it will also evolve denser bones?

    2. Surface-to-mass ratio is another factor in the practicality of aquatic endothermia. It may be that the particular tuna species used for comparison (T. alalunga or albacore), at about 1/4 the body mass of L. guttatus, is simply too small to make a go of it. A larger tuna species (such as yellowfin or bluefin) might have made for a fairer comparison.

      1. It’s worth noting that the opah seems much more spherical than, for example, a shark. That gives it a much smaller surface / mass ratio.

        Probably something else to look for….


  14. Thank you Prof Coyne for a wonderful science post.

    Just in awe of the wonderful diversity of life.

  15. It is quite surprising that it has taken this long to find the first warm blooded fish! I guess the technology needed in the way of implants has been less available till now. But still, their physiology predicts a warm blooded system. Are researchers now going to put implants into more fish that have large hearts?

    I wonder how many more warm blooded fish will now be found?

  16. This post is a big improvement over the article in the Washington Post, which was pretty long. I read it and thought, I’ll bet WEIT will do this much better.

  17. Ah, fish! Now you’re talking my language. I’ve fished the waters off San Diego and down into Mexico on the sportfishing boats since the late 1940s. I have seen the occasional opah caught and brought aboard but have never caught one myself. (Caught plenty of albacore and other tuna, most in the 15 – 30 lb range.) Often wondered how well an opah fights, compared to tuna. Apparently, not well if they only use their pectoral fins to swim rather than body undulations with tail. Thanks for the post.

  18. It wasn’t so unexpected. It isn’t such a big step to evolve full body endothermy from partial endothermy present in many fish. Still that endothermy is muscle powered.

  19. Leatherback sea turtles have also evolved a type of endothermy, as well as other sea turtles in a much lesser degreee. It is hypothesised that plesiosaurs and other mesozoic marine reptiles had similarly elevated body temperatures. Perhapse endothermy of large marine animals, which can afford it, is advantageous regardless of clade membership.
    The discovery of endothermy in that fish brought to my mind another question, which puzzles me for much time. Why fish, which generally operate in cooler temperatures than terrestrial vertebrates, are much more active than terrestrial ectotherms? Most fish swim energetically all day, resembling birds and mammals, but amphibians and reptiles have disproportionately more ambush predators or sedentary species. Amphibians, which operate in lower temperatures than reptiles, seem more active, while even active reptiles which operate in above mammalian temperatures take long pauses throughout the day. It seems evolution went from active fish to slow bottom-dwelling sarcopterygians, then to equally slow heavy and bony tetrapods, then to more active smaller species and most recently to the two clades of endothermic vertebrates, archosaurs and mammals.
    By the way, why is dinosaur endothermy still regarded with so much skepticism? Isn’t most evidence pointing to endothermy of dinosaurs as well as to endothermy or at least tachymetabolism of most of the archosaurs?
    ps. For a reason, I wrote the comment previously, but only the first paragraph appeared, so this is the rest.

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