Homo aquaticus: The Science of an Underwater Gill

Homo aquaticus: The Science of an Underwater Gill

In 1962, underwater explorer Jacques Cousteau predicted the arrival of Homo aquaticus: people surgically equipped with gills who could live and breathe in any depth for any amount of time without harm. Lately, there has been a lot of buzz about Triton, a conceptual gill (http://goo.gl/pWkd5k) that supposedly could allow humans to breathe underwater. There are many reasons why this device is still in the realm of science fiction. But first, it’s helpful to understand how some animals breathe air underwater. 

Breathe, Breathe in the Air: Like us, insects breathe oxygen from air, using a system of canals connected to the outside by breathing holes or spiracles. So how do aquatic insects survive submerged underwater, often for their entire lives? Mosquito larvae develop tiny snorkeling tubes, called siphons, that poke out of the water for regular refills. Others, like mayflies and damselflies, develop biological gills that extend into the water to extract oxygen by diffusion. The champion for ingenuity, however, is the diving beetle which carries a bubble of water tucked under its body, seen as a silvery sheath in the photograph. The air bubble is a short term supply of oxygen, that is replenished from the surrounding water based on a few simple physical principles that are fun to consider!

Love is like Oxygen: Water contains dissolved oxygen, reaching up to 5% in volume in icy-cold streams, but much less than the 20% found in the atmosphere. As oxygen is consumed by the insect, it creates a partial pressure difference inside the air bubble.  This is “corrected” by dissolved oxygen that diffuses in from the water.  There is a lot of unused nitrogen in the air bubble, 80% by volume, which is free to diffuse out , also creating a similar partial pressure deficit.  Because there is very little dissolved nitrogen present in water (it has lower water solubility than oxygen), some of the nitrogen’s partial pressure deficit is “corrected” by oxygen diffusing in, enriching the insect’s air supply.  So as long as the rate of oxygen diffusing in keeps up with the rate at which it is consumed by the insect, all is well. Unfortunately, the surrounding pressure of the water can shrink the size of the bubble over time, reducing the surface to volume ratio and hampering gas exchange. That’s why some insects make the occasional trip to the water surface, to refill their air bubbles. For those insects that don’t have this option, a plastron is the answer. 

What the Fakir?: A plastron is a special array of rigid, closely-spaced hydrophobic hairs (setae) that create a fixed “airspace” next to the body.  Air trapped within a plastron operates as a physical gill (just like air in a bubble) but this airspace cannot shrink in volume because a double layered fortress of setae prevents encroachment of surrounding water.  Think of the analogy of a fakir lying on a bed of nails: while one nail can puncture through his skin, lying on many nails effectively distributes his body weight so that the skin, like the surface of water (inset images below), is not broken. Also, the setae do a good job of repelling water using the lotus effect covered in an old post (http://goo.gl/yW7QpC). 

Triton or not Triton?: Back to the beginning, will a physical gill work for humans? Humans need a lot more oxygen than beetles, so enormous surface areas will be needed to extract oxygen from water. Too much or too little oxygen in the air we breathe can be toxic. Still, a terrier named Muggins survived a 3 hour dip in the Mississippi river using articificial gills. Check out the story (http://goo.gl/xdJeQd) and tell me if you think  Homo aquaticus  will soon be in a pool near you!  

Images: Diving beetle by Ernie Cooper (http://goo.gl/EWMwjx); Inset http://goo.gl/ci28mS


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45 Responses to Homo aquaticus: The Science of an Underwater Gill

  1. Tom Lee says:

    Fascinating ! Rajini Rao 

  2. Rajini Rao says:

    Old school physics principles to explain a champion of underwater survival! 

  3. Jesse H says:

    Awesome post Rajini.

  4. I was wondering when will you be posting again, thanks for the interesting read Rajini Rao.

    I’m sure looking forward to what Triton is to offer, hope it doesn’t stay in the realm of science fiction 😉

  5. Rajini Rao says:

    Susana M. , I need you to bug me to post more often. I’ve become an experiment in Newton’s law of inertia 🙂

  6. psher grant says:

    I learned something new today!

  7. Gaythia Weis says:

    Inertia?  I doubt it.  Someplace I heard a rumor that you run a research lab in your spare time.

  8. Rajini Rao says:

    Thank you, Gaythia Weis . More like the lab runs me 🙂

  9. Rajini Rao Of course I will. Can’t let inertia win! 😉

  10. Tom Lee says:

    Your posts are always so informative Rajini Rao. Scientific information on human and nature always make me think more of this fascinating world where we live. One great post after another. You have been here less frequent than before! I would imagine your academic work, teaching and conferencing have been taking most of your time. 🙂

  11. Rajini Rao says:

    Thanks, Tom 🙂 It’s a busy time at work, being pulled in different directions, although I would not wish it changed! 

  12. Tom Lee says:

    Sure. It looks like this year also gonna be a busy year for most. But sometimes busy is good. Hope you have a another good year and beyond ! 🙂

  13. Rajini Rao says:

    Many thanks, and much productivity to you too 🙂

  14. Amazing facts! Thanks for sharing Rajini Rao 

  15. Rajini Rao says:

    Thanks, Siromi Samarasinghe 🙂 

  16. Gaythia Weis says:

    Yes, thanks Siromi Samarasinghe !  We shouldn’t get derailed from acknowledging the excellent and informative post.  I plan to continue breathing on dry land, however.

  17. Rajini Rao says:

    Tom Willingham just posted a link to the scene from The Abyss on breathing liquid..utterly terrifying, even though we all had liquid filled lungs for 9 months: ABYSS. Breathing the Fluid

    Apparently, there are liquids we can breathe..yikes. So, I may stay dry on land too, Gaythia Weis , and enjoy watching reruns of Jacques Cousteau’s shows. 


  18. Samir Patil says:

    Rajini Rao Ur posts are always worth reading. Most show that science can’t be bracketed in narrow disciplines. Ur posts help me keep in touch with those disciplines of science I have interests in but don’t deal with in my professional life.

  19. An awesome post to read & know.

  20. Beautiful world. Thanks Rajini Rao

  21. Bill Collins says:

    Bonus points for the “What the Fakir” line. Neat article and thanks for the really good explanations. I’d say we’ll see novelty artificial gills as toys for those who can afford them, however for air capacity scuba gear will still outshine them for a long time.

  22. Kevin Clift says:

    Samantha Andrews recently pointed to an article explaining that a part of our hearing apparatus is evolved from spiracles: http://goo.gl/xBJwEf

  23. Gary Ray R says:

    Excellent explanation of how the diving beetle gets its oxygen. And a really good analogy using the Fakir.  

    You really should post more, you are an exceptional writer. I always read and learn from your work.



    You must have seen the physics and physiology  explanation of why the Triton might not work at 


  24. ahhhhhhhhhh,tres excellent!

  25. When I started reading the article about artificial gills, I was breathing easily, and everything felt fine. As I got deeper and deeper into it, however, I felt more and more pressure and soon, my understanding began to dissolve, and I couldn’t think clearly anymore. I had to come back out of it and take a deep breath before trying again.

  26. Adit Morey says:

    The concept of gills to alow humans to stay underwater for longer periods is revolutionary, though the implementation seems next to impossible…..but I especially liked the explanation of the underwater breathing processes of some creatures using basic concepts of physics.

  27. Rajini Rao says:

    Samir Patil I like your point about science covering many disciplines. I love that aspect too..bringing chemistry and physics into biology, in particular. Thanks 🙂

  28. Rajini Rao says:

    That was a delightful video of the underwater spider building an air bubble home (and food pantry!) under water, Vincent Peralta . Thanks for the link!

  29. Kevin Clift thanks for that link.

  30. Rajini Rao says:

    R Prakash Prakash , thanks 🙂 Bill Collins , I have to thank physicists for naming the drops formed on micropillars (as in a lotus leaf), fakir drops. Love their sense of humor. 

    Kevin Clift , thanks for the link. The fossils of Panderichthys apparently show the transition from fish to early tetrapods, and there is a bone next to the cavity where the spiracle would have been, that appears to have transmitted sound. 

  31. Thomas Kang says:

    First earworm good. Second earworm not so good. Cool post!

  32. Rajini Rao says:

    Thomas Kang second earworm is classic bad pop from late seventies. A classic! Therefore, orders of magnitude better than bad pop from Miley Cyrus or Justin Beiber 🙂 

  33. Rajini Rao says:

    stefan jeffers , Stay Calm and Keep Your Plastron On 🙂

  34. Rajini Rao Great and informative  post!

    There’s an interesting article about Homo aquaticus:


  35. I have read that the fetus shows filogenetic traces of branquias…

  36. Rajini Rao says:

    annarita ruberto the story of Jacques Cousteau’s vision in your link was fascinating. A lot of things can happen in 50 years. Well, we are not quite breathing water through gills as Cousteau predicted, but perhaps in another 50? 🙂

  37. Rajini Rao says:

    Alfonso Ramirez around 4 weeks, the human fetus develops pharyngeal arches, which are equivalent to the branchial or gill arches in fish. 

  38. Stuti S A I says:

    Thank you for the informative post Rajini Rao  🙂

  39. Rajini Rao Yeah! Who can know it?:)

  40. Thomas Kang says:

    Thanks, Rajini Rao. The earworm had disappeared, and now it’s back again in full force as this post cropped up in my notifications. I’m so glad that that’s the only part of the lyrics that I can, or care to, recall.

  41. Rajini Rao says:

    I would hate to do anything to revive that earworm, Thomas Kang  😉

  42. Thomas Kang says:

    Ugh. Round 3. I will have my revenge one of these days.

    For some reason, another earworm has entered my head, most likely through a time period association: “Heard it from a friend who, heard it from a friend who, heard it from another you’ve been messing around.” This one is so virulent that it’s actually pushing out the oxygen ditty — a real-life “good news, bad news” deal.

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