I’d like an alien to be warm-blooded, but have external gills. Imagine a kind of giant crab or woodlouse (pillbug) which spends part of its time on land and part of the time in the sea. However, external gills will be an enormous heat loss for a warm-blooded creature whenever it enters the water. All that surface area, shedding heat into the water like it is going out of fashion!

The normal biological solution to heat loss in an extremity is to set up a countercurrent exchange system between the outgoing and incoming blood vessels. That would work fine to save heat; chill the blood as it leaves the body for the gill, warm it up again as it comes back. Diagram of countercurrent vs normal flow

But there is one teensy, weensy problem – the countercurrent would also do the same to oxygen, and not in a good way! The outgoing blood would pinch the oxygen from the returning blood and thus no oxygen would ever get from the gills to the body. Oops.

So, can anyone suggest an alternative, biologically plausible, method of saving heat in an external gill? Otherwise my aliens will be restricted to quick dips or only going swimming in the warmest of tropical seas.

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    $\begingroup$ Have you considered lowering the baseline temperature of your creature so the difference in temperature isn't as big as you're anticipating thus reducing the amount of thermal energy lost to the water that has to flush through the gills? I'm not sure how set you are on a certain body temperature... $\endgroup$
    – Hyfnae
    Commented May 19, 2017 at 8:54
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    $\begingroup$ I'm happy for them to have a marsupial-like 31 degrees C body temperature rather than a human-like 37C or bird-like 39C. But I don't want them to be at ambient temperature, i.e. cold-blooded. I've already got some cold-blooded aliens and if possible want these guys to be more flexible in what habitats they can go set up home in - everywhere from tropics to tundra, just like humans do. $\endgroup$
    – DrBob
    Commented May 19, 2017 at 9:18
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    $\begingroup$ @DrBob, one of the reasons humans can do that is that we moved our gills inside. We also wear clothes and if the air is really cold, a scarf over our faces which has the secondary effect of heating the air we breathe. Could they wear something which has that effect on the water? or are they just beasts. $\endgroup$
    – Separatrix
    Commented May 19, 2017 at 9:21
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    $\begingroup$ External or internal does not matter for gills -- they are bathed in a current of water at the temperature of the sea. On the other hand, depending on how large the creature is this may not be a big problem; for example, large and active fish such the tuna are perfectly able to "maintain a core body temperature of 25–33 °C (77–91 °F), in water as cold as 6 °C (43 °F)" (Wikipedia). $\endgroup$
    – AlexP
    Commented May 19, 2017 at 10:18
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    $\begingroup$ Countercurrent exchange systems don't exchange oxygen. That would require fluid transfer between incoming and outgoing vessels, in order to exchange oxygenated red blood cells. Countercurrent exchange systems take the form of blood vessels in close proximity to one another exchanging heat, but not fluid. $\endgroup$
    – ckersch
    Commented May 19, 2017 at 15:00

4 Answers 4


You actuallycould use a way penguins use to protect from heat loss thorough their feet. Just make blood vessels really close to each other, to get concurrent flow.Actually, humans have that, too:

Image of concurrent flow in a human arm

That way, blood entering gills will be cold, minimal heat loss will occur, and when going back to the body, blood will collect heat from the one that is about to go to the gills.

Now note that in humans and in penguins it is essential to supply oxygen to our arms and feet, and to take away carbon dioxide. The problem you are invoking is next to nonexistent - for gas exchange to occur, we need a really thin epithelium - that's why counter-flow in gills is so effective, and that's why gills and lungs are so delicate. Blood vessels in gills and lungs are delicate, too.
But the ones that lead to and form them are not. These are thick and sturdy, and almost "airtight". If you need a proof, move your finger. If you can do it, then you know your blood has not lost oxygen going to your hand. Blood vessels have an inner layer, two elastic membranes, a muscle layer, and an outer "tunica" - not much of a border for heat, but a lot harder for oxygen to penetrate than a few layers of cells evolved to be easy for oxygen to move through.

It works well for penguins barefoot on ice. So it probably could work well enough for gills, if tweaked a bit.

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    $\begingroup$ IIRC, Mooses (Meese?) have the ability to essentially remove basically all the heat from the blood that goes to their legs, have it run through their legs cold, then reheat it using the captured heat (+ some other energy) before it continues on. $\endgroup$
    – SGR
    Commented May 19, 2017 at 14:23
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    $\begingroup$ @SGR Many much Moosen. $\endgroup$ Commented May 19, 2017 at 14:44
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    $\begingroup$ @SGR Moose is plural and singular. $\endgroup$
    – Lord Drake
    Commented May 19, 2017 at 16:28

The tough issue to solve is that you need a maximal gill surface for oxygen exchange, but this surface will allow more heat to be lost. The only way to solve this is to somehow imagine a way to coat your gills with a thermally insulative substance wich still allows oxygen transfer. So what if this substance was...

Air. Trapped in microfur (I liked the word).

Air is a good thermal insulator and we can guess that oxygen will have no problem diffusing in it. Your alien will have to coat its external lungs with a thin layer of air. How? With microscopic hairs imprisoning bubbles of air (something close to the diving bell spider web bubbles, but with hairs).

In fact, I call it hair for a lack of proper vocabulary (english is not my first language), but "organic micro tubes" would be a better description I guess.

This way, air will stick to your tubes, forming a layer of maybe half a millimeter, separating your breathing area from the cold water, insulating it quite well, while still allowing oxygen to diffuse in the air layer from the oxygen-rich water. See this paragraph about the air bubbles imprisoned in the net of the diving bell spider:


Diving bells are irregularly constructed sheets of silk and an unknown protein-based hydrogel[9] which is spun between submerged water plants then inflated with air brought down from the surface by the builder. Studies have considered gas diffusion between the diving bell and the spiders’ aquatic environment. The silk, whilst waterproof, allows gas exchange with the surrounding water; there is net diffusion of oxygen into the bell and net diffusion of carbon dioxide out. This process is driven by differences in partial pressure.

So at least the diffusion part should work.

I don't really know if the rest of my idea is realistic, biologically speaking, but well... I'd buy it!


You're describing bluefin tuna, which have a wonderful net of blood vessels which allows them to maintain core temperatures much warmer than the surrounding water - and without losing circulating oxygen!

From Wikipedia, they:

can maintain a core body temperature of 25–33 °C (77–91 °F), in water as cold as 6 °C (43 °F)


achieve endothermy by conserving the heat generated through normal metabolism. In all tunas, the heart operates at ambient temperature, as it receives cooled blood, and coronary circulation is directly from the gills.[42] The rete mirabile ("wonderful net"), the intertwining of veins and arteries in the body's periphery, allows nearly all of the metabolic heat from venous blood to be "re-claimed" and transferred to the arterial blood via a counter-current exchange system, thus mitigating the effects of surface cooling.[43] This allows the tuna to elevate the temperatures of the highly-aerobic tissues of the skeletal muscles, eyes and brain.

Just because heat is being exchanged does not mean that oxygen also would necessarily be exchanged. This: "The outgoing blood would pinch the oxygen from the returning blood and thus no oxygen would ever get from the gills to the body." does not happen.

  1. Have them live in the tropics, then the heat loss is minor enough to compensate with calories.

  2. use internal gills vs external, If the gills are enclosed and connected with a one-way-flow set of tubes you can get heat exchange going at both ends to mitigate loss, Imagine fish gills if the outlet was somewhere near the tail. You have the whole downstream exhaust tube to reclaim heat. This would be beneficial to a amphibious organisms anyway by slowing down the drying of gills in air. It could even evolve into a a dual lung/gill system in time.


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