Underwater breathing is the anestral condition in vertebrates, seen in the basal elasmobranch and fish lineages. Obligate air breathing has evolved more than once, for example, in the Anabantoid fishes and the fish lineage that led to the tetrapod clade.
Now, when we consider two lineages of marine vertebrates, namely, the cetaceans and the ichthyosaurs, we see that they re-evolved fish-like structures, including secondarily evolved caudal fins not derived from the hind limbs and dorsal fins. Cetaceans and icthyosaurs resemble fishes more than their terrestrial ancestors, and have returned to the sea so completely that they do not return to the land even to reproduce. And yet both remained steadfastly air-breathers, having to surface for every breath. This would seem to be a limitation, as surfacing requires interrupting their other activities. If the selective pressures of life in the sea can lead to fish-like convergent structures, why has it never led to secondarily evolved underwater breathing?
Evolution can only improve on what already exists. It may not be possible for air-breathing lungs to evolve back into water-breathing ones, so it simply just hasnât happened.
Another way to think about it is that evolution never goes backwards. Think about it perhaps like a landscape, with valleys, and mountains. When creatures evolve, they can only move to a higher point. For water-breathing creatures, it might be beneficial to climb a hill that leads to breathing air from the surface. But once they are on top of that hill, it would longer be beneficial to evolve back down.
Simple answer: Dolloâs rule. This âruleâ (which is more a really good suggestion than rule IMHO because of an ever-growing number of exceptions) states âan organism never returns exactly to a former state, even if it finds itself placed in conditions of existence identical to those in which it has previously livedâ.
The underlying reason is because of genetic erosion of the original trait that cannot be easily reversed. For example, turtles, which lack teeth, have stop codons in at least five genes known to act in tooth formationâŚthe gene for dentin formation, enamel formation, etc. Instead turtles have a hardened keratinous beak that serves the function of teeth (at least as far as shearing goes). So, the genes have for tooth production have already degraded to non-functionality. However, some turtles that eat really slippery slimy things for which teeth for grasping would really help have re-evolved tooth-like structures. But these turtles have serrations in their beak (not true teeth) that serve the purpose of grasping. Same function, different way. Incidentally, some birds have serrated beaks in this same way. The genes for teeth are still there, just either inactivated or co-opted for another purpose. In other words, irreversibility.
itâs not breathing exactly, but i thought some sea snakes and some freshwater turtles have been found to be capable of gas exchange through skin and other organs?
Softshell turtles (Apalone spp. and maybe others) are known to use tissues in the cloaca for gas exchange, so they can do some âgill breathing.â But it only supplements their air breathing and doesnât replace it.
is this actually still true based on the latest science? i know that a lot of freshwater turtles were supposed to have been capable of cloacal gas exchange, but i think only a few in Australia have been proven to do so, and some others do gas exchange through skin, tongue, mouth, etc.
Only if you discount the limitations that would be imposed by the sort/size of gill structure that would be required to support the gas exchange needed to support Cetacean-level metabolism?
in short, it would be very difficult to reevolve that. you forget that gills are not an isolated organ. in that ancestral state, there are gill slits in esophagus and gill arches, with blood vessels running from a single chambered heart (one ventricle, one atrium) in one way - a vein entering the heart, then the gill arches, then oxigenated blood from them to the rest of the body.
in vertebrates already this is being repurposed for another functions. cranial arches become jaws. and then as you stop needing the gills, all of its structures are used for something else. for instance, aorta with its arch is a remain of gill arch.
there is no evolutionary pressure to change that backwards, because all the hypothetical intermediates are non viable and we call that lethal congenital defect.
and to build a de novo structure that would support the required blood flow and to radically change the way blood is running through the vertebrate body has the same problems with non viability. in short, to breathe air is a solution that already works.
Fascinating question. For vertebrate evolution, others have pointed out that ârepurposingâ some other organ or trait to regain underwater breathing is apparently a nearly impossible pathway.
That said, the existence of several genera of aquatic moths (like aquatic Crambids in the subfamily Acentropinae) suggest that, if you have an exoskeleton and spiracles, re-inventing gill breathing has apparently not been too big of an evolutionary leap!
I think this is the main reason we havenât yet seen it. For already aquatic organisms with terrestrial ancestors having to breathe air isnât a big enough disadvantage for it to be selected against. Usually these organisms live near the water surface anywayânot only because of air, but also because of food (which is directly indirectly dependent upon light) and probably because going deeper would require additional changes to the physiology and morphology (different feeding strategies, adaptations to higher pressures and to lower light/complete darkness, etc.).
One big driving force of evolution is inter- and intraspecies competition, or rather the advantages of avoiding it. Therefore when organisms evolved the ability to breathe air, there were significant advantages to colonising terrestrial habitats, as there were a lot of free potential niches yet to be filled.
This is not the case for the way back. Most aquatic niches are already filled, and those which arenât are likelier to be filled by organisms with a âshorter way to evolveâ.
That being said, I could imagine scenarios where underwater-breathing would develop in previously air-breathing lineages, but they are all very hypothetical.
In any case, it is safe to assume that in order to prevent this:
âŚwater-breathing organs would evolve as separate organs, so that the more basal taxa would have both. Truly water-breathing organisms would then evolve by atrophy of the lungs. So, before obligate water-breathers, thereâd be an amphibious stage.
in vertebrates, separate organ is a problem. it is a problem of body plan. other phyla, namely arthropods, are way more like jigsaw puzzle like, versatile, in that matter. separate segments means you can play with that, try to swap some, add some new. in vertebrates, there is no space and no convenient building blocks for something new. gills would require massive blood flow and very thin interface between the blood and water. but in mammals, there is no space inside and you cannot develop outside gills by some skin outgrowths because you have the skin, that is airtight and thick. and the size for whale-sized organism would be not good anyway, for hydrodynamic and safety reasons (pathogens! predators!). you could try to develop anal breathing, like turtles, but mammals have been around for so long that just the fact that whales do not shows that it must have its own limitations.
Because re-developing a new breathing organ able to catch oxygen in water with all the physiological consequences is much more complex than adapting the body and its appendages to life in water in terms of mutations. So, it is much more unlikely to occur and/or requires much more time to occur.
I agree, those limitations make it unlikely for water-breathing to evolve in air-breathing mammalian or vertebrate lines, but I wouldnât say it is impossible, especially given long enough time scales.
As I said, this is all wild speculation, basically just crypto-zoology, but what I had in mind were a pair of big branched organs filled protruding from the mouth (stemming from epithelial tissues of the mucous membrane). Those could theoretically be sucked into the mouth for safety.
I have not done nor do I know any calculations on how big these would have to be, but I agree that it would not work for a creature the size of a whale or even of a dolphin. And itâd probably be a filtration feeder organism.
A lot must happen for these changes to occur, and Iâd assume it would take a few hundred million years at least. But I do not believe that âit hasnât happened yetâ is sufficient proof for it being impossible.
mouth is a bad place for that. for two main reasons: mouth is already in use, and second, there is no significant oxygen exchange through that mucosa. there is simply not. that means no evolutionary pressure to increase that by increasing surface area for that exchange.
anal breathing at least has a significant exchange, so there could be an incentive here to improve upon that, and yet, turtles are the only known example.
Metabolism is probably the most important factor. Creatures with relatively high metabolic rates need to burn more oxygen. Air has a much higher concentration of breathable oxygen than water, so for creatures that already have lungs, the cost of evolving water-breathing organs is unlikely ever to be worth paying. Some species of whale can hold their breath under water for well over three hours, whilst most can manage around one hour. So it seems the limitations imposed by needing to return to the surface are relatively easy to overcome.
since these creatures are probably the closest to transitioning back to fully aquatic respiration, letâs take a look to see whatâs holding them back from losing the lungs altogetherâŚ
among the freshwater turtles, there are a few that can get 100% of their normal daily oxygen needs through cloacal respiration, but most of them â whether exchanging gas through cloaca, skin, or oral tissues â need to enter a reduced metabolic state to survive without breathing air.
but what happens when itâs time to reproduce? as far as i know, all turtles lay their eggs on land. whereas amphibian adults can lose their gills and still lay eggs in water, itâs harder to lack lungs as an adult and still be able to lay eggs on land.
most sea snakes solve the problem of laying eggs on land by giving live birh. so then whatâs holding them back from ramping up their skin respiration from 30% of their needs to 100% of their needs?
probably cutaneous respiration doesnât provide oxygen efficiently enough for their active lifestyles. i would guess it doesnât provide enough oxygen to digest their prey and support reproduction, but iâm not sure.
so then i would guess that losing the lungs would require developing other organs akin to gills and not relying solely on skin respiration. in my mind, the likeliest way to do that is to adapt the tongue or other oral structures for respiration (as some turtles have done in a limited way), but that would require some of their specialized sensory functions to move off of the tongue first.
and then what would the process of losing the lungs look like? lungs in sea snakes arenât just for breathing. they also are there for buoyancy like the swim bladder in many bony fish. would there ever be a condition that would enable a lung to evolve backward into just a swim bladder? and if such a condition doesnât exist, then would it ever make sense to develop fully aquatic respiration if you need the lung for buoyancy anyway?
sharks lack the swim bladder and instead rely on lipids to maintain neutral buoyancy. so then some mechanism like this would probably also have to develop alongside lung atophy.
Amphibians donât even need the water, just a moist environment. Thatâs what direct developing salamanders do. Lungs make it more difficult, but not impossible, which is what plethodontid (lungless) salamanders do. While most plethodontids are direct developing, there is at least one reversal in desmognathine salamanders where they reverted to aquatic larval form and gave up the direct development. The key is to recognize that the egg is the aquatic environment and that they didnât really ever lose the gills, just retained them only through egg development and hatching out having already transformed. This is in essence what direct developing gymnophiona do.
Also, keritanized scales are largely gas impermeable and most cutaneous respiration happens at the exposed skin in-between scales so to ramp up theyâd simultaneously have to down-regulated scale development. No reptile has ever truly lost their scales in this way (birds just modified them, but retain them in their legs, turtles turned them into scutes), so probably defensive function outweighs respiratory benefits.
Gills can extract it more efficiently and completely - though that also makes you vulnerable to pockets of water that become oxygen depleted, and presumably to water with a high concentration of carbon dioxide too âŚ
Lungs give you a reservoir that can be (almost) instantly replenished, which then continue to support diffusion/perfusion until the partial pressure of the oxygen in them falls too low, or that of excreted carbon dioxide rises too high and needs to be vented.
They both do the job of gas exchange, but they are not equivalent organs that could simply be substituted without other significant consequences beyond just where you source the oxygen your body needs.
Some terrestial/aquatic organisms have actually partially evolved a means to respire underwater vis a vis Elseya albagula the âbum breathing turtleâ of southern Queensland. https://animalia.bio/index.php/elseya-albagula