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In the popular topic, I always use the model of marine mammals, not water-breathing fish-like animals. They breathe air, that turbo-charged 20% oxygen fluid that enables the scale of metabolism that we enjoy as warm-blooded animals.

Recently, Dr. Mason debunked the Triton Gill crowd-funding scam (note: I was a major sponsor of the debunking video). Only, he did such a good job of showing how such a thing could not possibly work on a fundamental level that it also destroys any hard-SF portrayal of such a device (or gill implant).

In particular, a liter of air contains 200 ml of oxygen, and a liter of water contains (if you could remove it all perfectly) 5 to 10 ml of oxygen.

One of the earliest SF stories I dabbled with featured a gill-pack with the appropriate size and weight of a SCUBA system, but was a gill: it seems plausible that near-future technology could have a membrane that allowed disolved oxygen to be extracted, and that membrane would be folded and branched to have a high surface area, coming in contact with all the water that passes through, and long enough to process all the water before it passes out the other side.

The catch is that you would have to pump about 100 liters of water per minute through the device. In a device, that would be an enormous jet!

For a human-sized/human-metabolism animal, how could you process this much water over your gills? Note that adding to their expanse, such as a long train, would be more tissue to feed, too.

On the other hand, fish like tuna and baracuda exist, that are fast and have red muscles. I suppose they must use their high energy level in bursts only and take a long time to recharge. Exploring that idea is one avenue: but note that the brain takes a lot of power all the time (but see this question. But also, cold water has more oxygen.)

In short, how can be have a water-breathing animal that's not sluggish/torpid and has a large brain?

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  • $\begingroup$ What's this "enormous jet"? I figure a 10cm x 10cm aperture must flow at only 2cm per second (0.045mph) to work through 120 litres per minute. A quick search suggests that's about 1/10 of a relaxed pace for a diver. Did I screw up? $\endgroup$
    – sh1
    Commented Jul 13, 2016 at 18:27
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    $\begingroup$ Yes I did screw up. 1000cc/l, not 100. Should be 20cm per second (0.45mph), or equal to a relaxed pace for a diver. $\endgroup$
    – sh1
    Commented Jul 13, 2016 at 18:38
  • $\begingroup$ I see your point: not a huge jet if the aperture is reasonably large (not a garden hose). $\endgroup$
    – JDługosz
    Commented Jul 13, 2016 at 20:54
  • $\begingroup$ If the opening of a gill is 1cm, then that's one linear metre of gill (eg., five 10cm slits on each side, but failing to specify the inlet dimensions), of some unspecified depth to give it 100% efficiency, so that and/or the speed needs scaling somewhat. I don't know what stops an animal being more extensively covered in gills -- perhaps durability in a fight? I guess using the same hole to breath and to kill things is an issue. Humans have given up a lot of their durability and compensated with intelligence and tools, so that might give merfolk some wiggle room. $\endgroup$
    – sh1
    Commented Jul 14, 2016 at 4:19

3 Answers 3

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One reason could be your water has just an higher oxygen level. (depending on how flexible your world is).

Also if I understand the video correct it assumes that you breath out much of your oxygen for calculating the 100 liters of water. You could get much lower if you calculate with the value that is used by the body. Following numbers I use came from the German Wikipedia.

A adult human takes between 11 and 15 breaths per minute and has a lung volume of 0.5 l.

21% of the air we breath in are oxygen, 17% we breathe out. So we use 4% of the air we breath as oxygen. That is 220 ml to 300 ml.

If I take the oxygen level of water from the video (0.01 l oxygen per l water down to 0.005) that means if I could get all the oxygen from the water I only need 22 (best case) to 60 (worst case) l of water per minute.

Best case is higher oxygen level with lower consumption, worst case the lower level with higher demand.

In another thread for mermaid it was proposed that the long hair could be used for extracting oxygen. Using such big and wide spread organ to absorb oxygen would increase the water that can be used.

The last thing I could imagine would be a lower metabolism while in water. If they can use their lungs there energy demand will get higher, their body temperature rises and maybe even getting more intelligent.

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  • $\begingroup$ I didn't go into the detail of correcting for the actual amount of oxygen extracted from a breath, as I figured under load that percentage goes up. $\endgroup$
    – JDługosz
    Commented Jun 6, 2016 at 15:32
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How about a species that has both gills and air breathing? It may be less plausible to have both, but if the species could alter metabolic rate/brain function, it could use a higher metabolism when at the surface but reduced metabolism while staying underwater. The higher brain functions would be shut down while gill breathing, resulting in a more instinctive rather than rational reaction to occurrences underwater.

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  • $\begingroup$ Yes, it could stay hidden underwater indefinitely, which is an advantage. $\endgroup$
    – JDługosz
    Commented Jun 5, 2016 at 10:35
  • $\begingroup$ Are we talking a mix between a mammal and a frog? $\endgroup$
    – JohnWDailey
    Commented Jun 6, 2016 at 18:34
  • $\begingroup$ No, I don't see what frogs have to do with it. Or were you thinking of their larval stage (do tadpoles havr gills?) $\endgroup$
    – JDługosz
    Commented Jun 7, 2016 at 16:40
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    $\begingroup$ Some (admittedly ancient) fish have both gills and "lungs" en.wikipedia.org/wiki/Gar $\endgroup$
    – Bryan Goggin
    Commented Jun 7, 2016 at 23:39
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Great White Sharks and Mako sharks are warm-blooded (well, heterotherms, anyway). Could you use some of their biology? Give your creatures a more efficient respiratory pigment in their blood than sharks have and they can grab more oxygen for every litre of water that passes over the gills. Shark heterothermy

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