I've read many posts here and elsewhere for example:

  • Wikipedia where breathing is well described, but does not answer the question (or anything close.)#
  • Wikipedia Gills, more specific, but still doesn't answer the question
  • This blog post
  • Several Quora's blogposts including this one.
  • Also some Youtube videos like this one

and I do not think that the fact that gills surface / contact area is larger with water wrt to air fully explains the reason why they can not breathe in air.

I wonder if one possibility is that they do not control the bones that open the gills (I don't know the name.) since they seem to use the water pressure to some extent for that matter.

Is there a more detailed explanation that you can offer, or point to?

  • $\begingroup$ What about how fish gills collapse in air? And they have no way to pass air over their gills? Or how gills dry out? $\endgroup$
    – DKNguyen
    Nov 28, 2023 at 21:42
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    $\begingroup$ Surface area doesn't mean much if your gills collapse. $\endgroup$
    – DKNguyen
    Nov 28, 2023 at 21:53
  • 4
    $\begingroup$ Drying out is not a separate question. $\endgroup$ Nov 29, 2023 at 1:36
  • 4
    $\begingroup$ The microstructure, too small to see with the human eye absolutely collapses to a small fraction of its surface area. Just to note: 1) Prior research is a requirement when posting, 2) the purpose of comments isn't to be "interesting", or to make derogatory remarks, it is to request clarification from the OP and to provide direction to new contributors those who may lack familiarity with the site's ways and with the topic. $\endgroup$ Nov 29, 2023 at 13:22
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    $\begingroup$ What you've cited is blog-posts and Quora threads, not scientific papers or from scientific journals. I've requested that you show your prior research, please do so - bearing in mind that we accept scientifically credible sources (Wikipedia is a secondary source but a good place to start as they have references you can follow-up on). I've also explained to you what comments are for, they are not for answering the question (with or without references). $\endgroup$ Nov 29, 2023 at 16:23

2 Answers 2


From the Wikipedia page on gills that you link:

The density of the water prevents the gills from collapsing and lying on top of each other, which is what happens when a fish is taken out of water

Wikipedia's source for this statement is a biology textbook: M. b. v. Roberts; Michael Reiss; Grace Monger (2000). Advanced Biology. London, UK: Nelson. pp. 164–165 though I'm not able to access this source.

You need a lot of surface area for gas exchange. If the gills collapse as they do if not submerged in water, there's not enough surface area for gas exchange. If you did somehow spread them out mechanically, they'd dry out. You might think about how a net can sit expanded in water, but if you pick it up it all bunches up; same as gills.

Other sources:

Gills are very delicate, almost paper-like structures that depend on the buoyancy of water to remain open. The fragile gills will collapse if the fish is taken out of the water because air doesn’t have the same density and buoyancy water has to keep the gills open and functioning. This is why a fish cannot live outside the water.

https://blog.limnology.wisc.edu/2016/09/14/at-home-in-the-water-condemned-to-life-on-land "This essay originally appeared in the Lakeland Times in Minocqua, Wisconsin. Carol is an Aquatic Invasive Species expert and outreach specialist with the CFL and Wisconsin DNR."

A paper on a fish with modified gills that allow it to breathe in air:

The bowfin, Amia calva is a facultative air breathing fish restricted to North America and is reported to estivate. ... The secondary lamellae are fused to form a lattice-work of rectangular pores, a gill arrangement unique among freshwater fishes. This highly modified gill structure imparts considerable rigidity such that these do not collapse upon air exposure.

Daxboeck, C., Barnard, D. K., & Randall, D. J. (1981). Functional morphology of the gills of the bowfin, Amia calva L., with special reference to their significance during air exposure. Respiration Physiology, 43(3), 349-364.

  • 1
    $\begingroup$ What is being said here, is not how one should reason imho it's at best suggesting that they correlate. Yes, they collapse and the surface area decreases, we all know that, it's even included in the post, so doesn't add anything new. Maybe the source has some extra details. Currently, this isn't at all an answer. Feel free to downvote again, since that's how people reason here apparently. $\endgroup$
    – Mah Neh
    Nov 29, 2023 at 17:50
  • $\begingroup$ I will also add, that I do not find that quote, or even an equivalent statement, in the linked article from Wikipedia (not a book as you said, and we should include the source if it is open source): evodevojournal.biomedcentral.com/articles/10.1186/… $\endgroup$
    – Mah Neh
    Nov 29, 2023 at 17:54
  • $\begingroup$ @MahNeh The source is the one that I posted in my answer (ref "#4" on the Wikipedia page), which is a book, not the link you have ("#7") - not sure why you're looking at that one. And no, this is not saying that they merely correlate. Surface area is critical for gas exchange and causal for sufficient gas exchange. Surface area of gills decreases massively in air, to the point of insufficient gas exchange. That's the answer. If you want a different one, that's not likely to work out well. $\endgroup$
    – Bryan Krause
    Nov 29, 2023 at 18:05
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    $\begingroup$ I hadn't downvoted your question before but now I will as I see it has not been asked in good faith. Good luck. $\endgroup$
    – Bryan Krause
    Nov 29, 2023 at 19:02
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    $\begingroup$ @MahNeh Should she re-read the comments where you're confused between reference 4 and reference 7, and what should she conclude from those? $\endgroup$
    – Bryan Krause
    Nov 29, 2023 at 21:44

Fish can't exchange gas efficiently in air with their gills. However, many more fish can breath air than people often think, even those you wouldn't normally think of as "air breathing", but they don't use their gills. All bony fish have a swim bladder, a sort of internal water wing that fills with air and allows them to remain suspended in the water column at rest. Fish can change their buoyancy by diffusing gasses into or out of the blood, changing how full the swim bladder is. So, you actually have something that does almost all the stuff lungs do. It holds gasses and allows for them to be exchanged between itself and the blood stream. Give it a way to be filled from the outside and you have a simple air breathing system, and many fish do just that. They gulp air and have some way of getting that gulped air into their blood, either from the stomach or the swim bladder. A lot of fish who live in murky water or water likely to be somewhat de-oxygenated not only are able to do this, but need to. Beta fish will actually drown if they are blocked from surfacing to gulp air. So yeah, lots of fish, even ones that you don't think of as "air breathing" can breath air, but they don't tend to do so with their gills. Some gas exchange can still happen through the gills so long as they remain moist, but you need something that won't collapse or dry out to actually be able to breath out of the water.


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