Why didn't underwater breathing re-evolve?

Nature Talk
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It’s not a question of relative efficiency; it’s a question of absolute quantity.

Consider this: Killer Whales (Orcas) prey on Great White Sharks. They’re bigger, they can swim faster, and they’re more intelligent. Or consider some of the other typical behaviours of cetaceans. Humpback Whales weigh around 40 tonnes, and yet they can jump clean out of the water! These animals aren’t just moseying along breathing through their ass - they’ve got energy to burn! It seems that one of the most productive ways to evolve a top marine predator is to leave the water entirely and become a warm-blooded, terrestrial air-breather. Given this, perhaps we should rather ask: why haven’t more fish evolved air-breathing organs?

22

Thanks for the explanation. I’ll trust you on this as I’m not that familiar with vertebrates. My main interests are early metazoan evolution and Tetraconata/Pancrustacea, and as you said, their body plans allow for more drastic changes to morphology to evolve more quickly. :D

I still believe though that, given sufficient pressure sustained for long enough time intervals, it is something that could happen somehow. I don’t think the constraints based on lineage would prevent it. Again, given enough time.
I do believe there will have to be many new apomorphies before a vertebrate descended line could potentially re-evolve water-breathing organs again. New traits don’t just impose new historic constraints, but also open up new possibilities. So the further into the future we look, the more uncertain it becomes what can and cannot happen.
(If the only animal we knew was the common ancestor species of all metazoa, the idea that at some point in a descendant line (or rather multiple) something like flight would evolve would seem quite impossible as well.)

23

I think evolving the ability to extract oxygen from water could re-evolve in vertebrates. Indeed, it has done so with the reptiles that breathe through their cloacas and the amphibians and sea snakes that breathe through their skin. However, there are serious limitations.

One problem is the inertia of water compared to air. Air is light. Pumping it in and out of lungs is practical. Water is heavy. Pumping it in and out of lungs might require more energy and oxygen than is gained. Also, the tissue would need to be strong enough to withstand the forces but gas exchange surfaces must be thin and delicate. So lungs themselves cannot be repurposed for water breathing.

One problem is a trade off between permeability and protection. A body surface thin enough to allow oxygen to diffuse through is delicate. It doesn’t offer protection and is easily damaged. But we animals need the protection our skin provides.

Another is a matter of surface to volume ratio. Sea snakes and lungless salamanders are long and narrow, maximizing surface area in relation to volume. Doing their gas exchange with their whole skin works. One reason it works is that they spend a lot of time exposed to moist air, where oxygen can quickly diffuse into the wet surface of their bodies. More rotund species need another solution.

Folding and refolding skin (making gills) improves the surface to volume ratio. It could theoretically solve this problem, though vertebrates haven’t gone this route (yet). And of course that delicate folded surface needs protection.

And then there’s metabolism. Getting oxygen from water through the cloaca works for hibernating turtles. (And cloacal “breathing” is a lung-like in-and-out system, not efficient; see above.) Using the whole body surface as an oxygen gathering surface works for quiet or hibernating frogs. More active vertebrates need a way to get more oxygen from water more quickly – gills of some kind. Very active aquatic vertebrates like whales can’t “wait” evolutionarily while gills evolve. I suppose amphibians could become more active in water gradually enough to re-evolve gills but they have a different, easier route to water breathing – becoming adult while retaining the gilled morphology of tadpoles.

So all in all, I think that Earth’s vertebrates will not evolve the ability to be very active while deriving their oxygen from water, though it’s not completely theoretically impossible.

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Yes, I do agree with what you’d said about the burn rate that can be usefully exploited by peak mammal metabolism and had already raised that point earlier (https://forum.inaturalist.org/t/why-didnt-underwater-breathing-re-evolve/56971/8) - though I posed it as a question instead of a statement of simple facts because I think there’s a lot of nuance to be found and explored in the full factual details.

But I think this is an overreach:

And doesn’t take into account the very different environmental conditions that we think were exerting pressure around the time the cetaceans appeared. Temperatures of both air and water were much higher than today, as was concentration of carbon dioxide, while oxygen concentration was much lower. Sea levels were much higher and water covered much more of the planet’s surface.

The ability to efficiently and constantly vent CO2 is at least as important for mammals as having a reliable supply of sufficient oxygen, and its solubility in water is finite (and greatly diminished as temperature rises) too.

If what is postulated about that time is correct, it would have been a very challenging time to become a large mammal if you remained strictly terrestrial.

Which again isn’t a function of the relative concentration of oxygen in air vs water so much as the ability to carry a reservoir instead of relying on an organ that requires constant flow in a region of sufficiently oxygenated (and sufficiently CO2 depleted) water. The period they can breath hold for on any single dive is highly variable and greatly dependent on the level of their activity during it.

So the point I was expanding on was that, like always, it’s mostly a question of what is the best tool for the job, and that always depends on all the details of the job.

We didn’t have trout-sized cetaceans evolve and populate closed water basins. The mix of lungs and gills in those environments is complex but quite different to the open ocean.

Which is indeed the question implicit in my suggestion that while gills and lungs in some sense “do the same job”, they cannot simply be substituted for each other. Each has costs and benefits that the other does not, and is optimal in niches where the other is not.

I have lungs, so I think lungs are cool. But that doesn’t make me better at everything than the things that don’t have them, or more able to survive for long in all the niches where those critters thrive in large numbers.

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But it hasn’t actually happened yet (hence the OP’s question) - so I was being deliberately conservative by qualifying it as unlikely :slightly_smiling_face:.

I was mainly addressing the OPs point about the limitations imposed by needing to return to the surface. But in any case, relative oxygen concentration clearly is relevant, since that will directly affect how long it takes to replenish the reservoir. Some whales make many deep dives every day, so more time spent at the surface means fewer dives (and perhaps greater exposure to predators, etc).

No one is claiming that one solution is better at everything, so I think this is an overreach :slightly_smiling_face:.

Even so, my previous post was meant to highlight the fact that cetaceans generally either directly out-compete fish or occupy ecological niches that aren’t exploited by fish at all. Oxygen intake must surely be one of the major limiting factors here, so why haven’t fish taken more advantage of air-breathing? There are (or have been), many species of fish that have both lungs and gills, or use other organs to increase oxygen intake (such as the swim-bladder). And fish have a very long evolutionary history. Thus, compared to most other large vertebrates, fish seem more likely (as a matter of historical fact) to evolve both air- and water-breathing organs. So why haven’t they exploited more of the (non-terrestrial) evolutionary opportunities this would appear to open up? The fact that they haven’t done this after several hundred million years underscores the low likelihood of the costs ever being worth paying.