Scientists Identify the World’s First Warm-Blooded Fish

By Carl Engelking | May 15, 2015 2:39 pm

This opah was caught during an NOAA Fisheries survey off the California coast. (Credit: NOAA Fisheries/Southwest Fisheries Science Center)

All fish are cold-blooded.

Just a week ago, that statement would have been true. But on Thursday, scientists announced they discovered the world’s first warm-blooded fish, the opah, forcing us to rethink some of the most basic biological concepts we learned back in elementary school.


Opah, also known as moonfish, are rotund — almost comically so — predatory fish that hunt for prey in cold, dark waters up to 1,000 feet below the ocean’s surface. Although they aren’t fished commercially, opah are often accidentally caught in fishing nets. Hawaiians consider opah a sign of good luck, and the fish are often given away as a gesture of goodwill.

But opah have long intrigued scientists because they are such nimble hunters in deep water; most predatory fish move slowly at these depths and wait for meals to come to them rather than chase them down. Things got even stranger when Nicholas Wegner, a researcher at the National Oceanic and Atmospheric Administration, examined an opah gill sample.

All fish gills have two types of vessels: One set transports stale blood from the body to be oxygenated, and another set transports oxygenated blood back into the body. But in opah, blood traveling into the gills is warm. Opah generate heat by vigorously flapping their fins to swim, which also speeds up their metabolism.


NOAA Fisheries biologist Nick Wegner holds an opah caught during a research survey off the California coast. (NOAA Fisheries/Southwest Fisheries Science Center)

Wegner noticed that vessels carrying warm blood away from the body wrapped around vessels carrying cold, oxygenated blood into the body. The unique setup raises the temperature of inbound blood before it travels to organs and muscles. It’s a process called counter-current heat exchange. Automobile radiators regulate engine temperatures the same way, but in reverse. In this way, opah can maintain a body temperature that is roughly 41 degrees Fahrenheit warmer than the water they swim in.

Wegner published his observations Thursday in the journal Science.

Big Advantage

Other top sea predators, such as tuna and sharks, have similar vascular systems to warm specific parts of their body, such as their eyes or swimming muscles, but they lose that heat when they dive deeper into cold water. As a result, they need to repeatedly reheat their bodies in warmer waters closer to the ocean surface.

Since opah have a heating system built into their gills, they don’t need to waste time seeking warmer waters. Instead, they can spend their time hunting in cold water and rely on their keen vision and speed, which is made possible by their warm blood.

Indeed, there may be millions of fish in the sea, but none — that we know of — like the opah.

CATEGORIZED UNDER: Living World, top posts
  • Tosin Otitoju

    oh wow. i nearly forgot how simply beautiful science can be.

    • azhar khan


  • Pete_EE

    41 °F! Is that a misprint?
    That converts to a temperature CHANGE of 22 °C. I notice that a temperature measurement (not change) of 41 °F converts to 5 °C. Did the original article publish in metric?
    5 °C or 9 °F sounds more reasonable.

    • AndyC

      Yep. 5 degrees C or 9 degrees F reported in other news outlets

    • surfeagleOrg

      I agree forgetting the fresh water freezing point does make it seem a lot higher then it is.

  • DaltonM

    ” ‘All fish are cold-blooded.’
    Just a week ago, that statement would have been true.”

    No, it would still have been false. A week ago, we were ignorant and wrong. Truth does not change like people’s opinions. Duh.

    I can’t believe I even had to write this…

    • OWilson

      More like, “Just a week ago, that statement would have been the 97% consensus”.

      Gotta love real science!

    • ottofromdurham

      You don’t. The actual intent of that statement was obvious.

      • elleko

        whatever the intent, obvious as it may be to some, the journalist certainly succeeded in showing egocentric ignorance

        • Otto

          So he is only concerned with himself and doesn’t understand that new information isn’t an actual change in reality? I don’t know how you gleaned that, especially the part about egocentrism.

          • elleko

            l don’t know this person and l don’t know if he is personally egocentric.
            new information may or may not be a change in reality. But facts that exist outside of knowledge are part of reality. A thing that was true before one knew it was true does not alter reality, but it does change one’s personal perspective of what is true. The opening of this article implies otherwise.

  • jim27182

    It has been documented that certain species of sharks maintain a body temperature higher than the ambient temperature. Tunas and marlins do likewise. The definition of homeothermy (warm bloodedness) is the ability of an organism to maintain a constant (with minor fluctuations) internal body temperature that is usually higher than the surrounding ambient temperature. Some sharks, tuna and marlins clearly have this ability. So, the lead in is inaccurate. Now, if they wish to focus on the physiological mechanism described here as being the first observed within bony fishes, that is another thing.

    • Brandy Hunt

      They actually mention that in the article. They stated that those fish warm the blood to certain parts of their bodies but they constantly have to seek warmer waters in order to do so. This fish is different in that it doesn’t need to go to warmer waters to warm up because it maintains that temperature throughout it’s ENTIRE body making it truly warm blooded whereas the other fish mentioned are not.

      • jim27182

        The point remains that some sharks, tunas, marlins are warm blooded. they maintain a temperature higher than ambient whether or not they have to move to waters having different temperatures, by definition they are still warm blooded. So, the first sentence of the article is misleading. And if the article is going to contend that moving to warmer areas disqualifies such organisms as not being warm blooded (a silly contention), then that would eliminate most mammals as well – notably rodents and chiropterans (these 2 groups comprise the great majority of mammalian species) who regularly have wide temperature fluctuations as well as needing to warm themselves via external means and not internal physiological processes. This is not to downplay the fascinating physiological mechanism described here, but the article is simply not accurate. By definition, an organism that maintains a body temperature higher than its ambient is deemed to be warmblooded. Let’s not place qualifiers which is what the article does here. That’s my quibble.

  • Bob

    I hope it is edible so it did not die being discovered for no good reason.

  • jerry

    Of course: We don’t know everything about our planet, that’s why we’re still doing reseach.

  • Syaif Rohmadi

    warm blooded yet cool-looking, hahaha


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