The reason I ask is because I want to create a terrestrial species that hunt fish underwater by holding their breath for hours without having a large lung capacity. In someway they are buffed Bajau sea nomads.


5 Answers 5


hunt fish underwater by holding their breath for hours without having a large lung capacity

So, it is a common misconception that diving mammals (humans included!) store much oxygen in the lungs. Indeed, for deep diving mammals, having air spaces in the body is a liability, because water pressure will crush them. Seals and so on have the ability to fully empty and collapse their lungs.

The wikipedia article on physiology of underwater diving has much more details and is well worth a read.

Of particular interest to you, though... myoglobin stores oxygen in muscles. Diving mammals have a lot more myoglobin in their muscles than their non-diving counterparts, which acts as a large reservoir of oxygen. Similarly, having more hemoglobin in the blood means you can store more oxygen in the blood too, which combines well with having blood storage organs like the spleen.

Having extra red blood cells in humans is a well established way to cheat at endurance sports (see also, erythropoietin doping), but you can also have better hemoglobin too... crocodile hemoglobin gets better at giving up its bound oxygen when CO2 levels in the body rise. Naked mole rats can survive just fine at very low atmospheric oxygen concentrations, even in the presence of very high levels of CO2 thanks to superior hemoglobin and fancy metabolism.

Furthermore, the naked mole rat is also has a superior anaerobic metabolism, seemingly much more resistant to the effects of acidosis than we are. There's plenty of interesting things to learn there, some of which are active research topics, and some of which are just plain mysteries (and therefore give you some scope for handwaving in similar adaptations of your own without too much fear of being implausible or incorrect).

Do take a moment to think about why your merpeeps need to be able to dive for so long, though. Seals and dolphins and so on are already excellent fish hunters and don't need to spend hours down there... they need to be able to hold their breath long enough but beyond that the additional complexities and tradeoffs start looking a bit undesirable. What's the point of having super-blood that means you're at greater risk of thromboembolism and your cancerous tumors get fed much better than everyone else's? Just come up for a breath after an hour instead of after three. The things that hold their breath longest are the deepest divers who can't just pop up to the surface for a quick breath of air. What are your creations hunting down there? What's wrong with the stuff in shallower water?

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    $\begingroup$ Not only is air in the lungs a liability due to crushing - it's a liability due to buoyancy. All the energy required to push two balloons underwater is wasted. $\endgroup$
    – JBH
    Nov 7, 2022 at 16:54
  • $\begingroup$ @JBH I believe this is solvable. The main problem is that pressure changes buoyancy. But you could imagine a sea beast that spits rocks the deeper it goes. Then you finally throw the last one out and shoot back to the surface. $\endgroup$ Nov 8, 2022 at 15:00
  • $\begingroup$ @ZizyArcher human freedivers who generally don't empty their lungs cope OK. They can easily overpower the excess bouyancy in shallow water, and at depth the reduced lung volume means they start "freefalling". The OP implies the possibilty of intelligent tool users, so the option of weight belts and dropped stones is always there. $\endgroup$ Nov 8, 2022 at 15:08


Frogs and salamanders do oxygen exchange through their skin while underwater. Some salamanders and newts have gills too. Like this cutie.

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Some deep-diving turtles breath through their ass. Or cloaca, the single opening through which the waste goes out and the sex goes in.

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    $\begingroup$ That fact about turtles compromises a lot of Little Mermaid songs. It's hard to have fun when you know the turtle is thinking "it smells like an ass in here" all the time $\endgroup$
    – urquiza
    Nov 7, 2022 at 13:51
  • $\begingroup$ @urquiza for me that makes it more fun. $\endgroup$
    – Willk
    Nov 8, 2022 at 1:56

CO2 is produced because our bodies use oxygen to oxidize carbon based substances to get energy, and that leads to the production of CO2. There is no way around it.

Putting it into some other substance which doesn't need to be breathed out would need energy, and making so take away energy from the vital functions.

Alternatively, you could avoid using oxygen as an oxidizer like anaerobic organisms do, but you would still need to get rid of the waste products, for which the consideration on the energy expenditure still holds.

Last but not least, if you run out of oxidizer, the body has nothing to feed the burner to get energy. You can't have long apnea without large availability of gas. The need of breathing out comes from the fact that an excess CO2 in the lungs, even though oxygen is still available, shifts the transport equation toward capturing CO2 rather than oxygen.

  • $\begingroup$ Thanks for the reply. Yea I didn't thought of the fact that O2 was consumed. We also can't really create an chemical equation that transform CO2 into O2 without having a net-zero effect. $\endgroup$
    – Arzack1112
    Nov 7, 2022 at 11:17
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    $\begingroup$ H2CO3 would be a closely-related chemical that might be secreted instead of exhaling CO2. It's only marginally less efficient, so that's not a hard problem. Running out of O2 remains a limit. $\endgroup$
    – MSalters
    Nov 7, 2022 at 22:26
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    $\begingroup$ Oxygen is not used during glycolysis (i.e. the oxidation of organic molecules for energy). That process requires no oxygen whatsoever. Oxygen is used to redox hydrogen and the reducing power generated during glycolysis in the electron transport chain. The chain is where aerobic (i.e. oxygen-requiring) organisms get most of their energy. $\endgroup$
    – Seggan
    Nov 8, 2022 at 16:15

Anaerobic metabolism.

Recycling my idea from here Could a deep ocean creature use some kind of bacteria in its body as a way to generate oxygen? !

But here is another idea for your creature as regards oxygen: it minimizes its use of oxygen by using anaerobic metabolism. When humans do this it is https://en.wikipedia.org/wiki/Anaerobic_glycolysis Energy can be derived from glucose without oxygen in short bursts. Lactate is the end product and builds up - this is the burn you feel when you feel the burn. Ultimately the liver has to oxidize the lactate when there is oxygen around again. It is an inefficient use of sugar compared to aerobic metabolism but we can do it.

But what about a creature that did anaerobic glycolysis and then ignored the end product? Yeast do exactly this - the end product is ethanol and it just builds up until they cant stand it any more. Or it evaporates away. Tapeworms do this and release the lactate into the fecal stream. A sea monster with lots of food and little oxygen could just let the lactate waste product ooze out of its pores into the water. It would need a lot more food (I think three times as much?) than a comparably sized creature using oxidative glycolysis, which extracts more energy from the sugar. But it could get by with much less oxygen.

Your sea creature does not need to breathe and it requires a lot of food.

  • $\begingroup$ Yea that could work thanks $\endgroup$
    – Arzack1112
    Nov 8, 2022 at 17:55

Have you considered gills?

There are no earth mammals that have gills, but if your species has them, then your problem is solved.


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