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The popular image of a mermaid is a half-fish, half-human creature breathing fine above and below the surface. The only real-life analogy to this are the amphibians - frogs, toads and salamanders.

But here lies the snag. Mermaids are often portrayed as marine while there are no species of amphibians capable of breathing the salty seawater. In order for the mermaid to breathe easily in air and in seawater, we need to speculate a species of amphibian who evolves major measurements to still breathe fine in the air but also tolerate the saltiness of the sea. What changes would I need so that any amphibious animal can breathe both air and seawater?

For the sake of this argument, don't bring up lungfish, mudskippers and tarpon, for although they do breathe air, the circumstances are very limited.

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    $\begingroup$ You'll need to think carefully about heat transfer and waste buildup. Gills allow high concentrations of waste products to be dumped into the surrounding water directly, this doesn't work in air. Gills also rapidly bring the blood of a creature to the same temperature as the water (unless you are large enough and have some counterflow mechanism like some sharks and tuna, though they're only a few degrees above their surroundings). Sea mammals can remain warm blooded because heat transfer through air is much lower than water. $\endgroup$ – Troyseph Jun 9 '16 at 9:20
  • $\begingroup$ As far as we know, mermaids were most likely imagined based on sea-mammals. Do you really need to have them breathe underwater, when their "precursors" were air breathers? And if you do, why not have both lungs and gills? That's how all the land animals evolved from fish started. The problem with gills on air is that they no longer work when collapsed and dried out - but that's reversed as soon as you go back underwater. $\endgroup$ – Luaan Jun 9 '16 at 15:47
  • $\begingroup$ See also this question. $\endgroup$ – JDługosz Jun 9 '16 at 16:36
  • $\begingroup$ I don't think the mermaid legend actually says that they can breath air easily. It usually takes very special circumstances for it to happen. I'd say that making them capable of holding their breath on land for 20 min is not far fetched for most aquatic animals. Maybe make it possible for them to do controversial surgery\magic for more the more permanent stays on land. $\endgroup$ – krowe Jun 9 '16 at 23:18
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Gills and lungs are a lot like each other, from a physiological point of view. They are highly vascularized, and have a large surface-to-volume ratio, so as to have as much gas exchange efficiency as possible.

Lungs work well for air because they allow ventilation through air pumping. Expand to pull oxygen-rich air in, contract to push oxygen-poor air out. This fails underwater because the latter is (approximately) seven hundred times as dense, and one hundred times as viscous as air. It is also practically incompressible. Pumping water in and out of a lung takes so much strength and energy that underwater breathing becomes inneficient, which is why we drown.

Gills, on the other hand, are adapted to the characteristics of water as a fluid. From Wikipedia:

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.

How about giving your mermaid some kind of flexible-fluid lung?

Let her have a throat system much like ours, but instead being a collection of air pockets, her lungs are hollow cavities. Inside these cavities there is a large set of gills. Let the mermaid also have one or more pair or slits on her sides, right under the lower ribs and connected to those lungs. These slits can vary in size, say from five to twelve inches in length according to your creature designing taste. They can also be located more frontally if you like.

When underwater, she breathes like a fish, just with a longer circuit for the water to flow through. She takes water from the mouth and nose, let it go through the gill/lung system ("gillung"?), then it is pumped out through the rib slits.

In order to breathe air, she first expels water through the rib slits, then she closes those slits. Now she can pump air in and out of her "gillungs" (I really like this neologism) just like a human. She just has to keep a little water in to keep her internal gills moist, so that they don't collapse over each other (this is actually how land crabs breath). She might evolve some bodily fluid, maybe some mucus, to make sure that her gills won't dry out fast.

This would be much to her advantage. Quoting Wikipedia again:

In fresh water, the dissolved oxygen content is approximately 8 cm3/L compared to that of air which is 210 cm3/L.

For her to have a metabolic rate like that of a human, if she's only breathing water, she will have to rely on ram ventilation. That is, she will have to be constantly swimming to get enough oxygen, just like a shark. That would take a lot of energy. But the fact that she can breath air as well means she can go to the surface for an oxygen boost every now and then.

Add a very large concentration of myoglobinin to her muscles, and she would be able to keep that boost for long dives just like whales do. With the advantage that she won't drown if she spends too much time diving deep - she'll just revert to her slower metabolism until the next time she surfaces.

Oh, and about salt...

Just use the same solutions that nature has already found. A large tolerance for urea, like sharks; and super kidneys like those of cetaceans. This is actually the part that requires the least suspension of disbelief IMO.

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  • $\begingroup$ Is your water oxygen concentration corrected for density? 0.8% of water volume is appreciably more mass than 21% of air volume. $\endgroup$ – March Ho Jun 9 '16 at 9:57
  • $\begingroup$ @MarchHo The density of water doesn't matter in this case. Regardless of the density of each medium, you will find less O2 molecules in a liter of water than in a liter of air. $\endgroup$ – Renan Jun 9 '16 at 10:12
  • $\begingroup$ Is "wather" a misspell of "water"? $\endgroup$ – haykam Jun 9 '16 at 16:08
  • $\begingroup$ @Peanut it is. Thanks for spotting that. I typed this answer from a cell phone, so I got lots of grammar errors (which I've been fixing). $\endgroup$ – Renan Jun 9 '16 at 16:11
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    $\begingroup$ "Expand pull oxygen-rich air in, contract to push oxygen-poor air out." you're being mammal-centric without realizing it. Some animals have lungs where air passes through in one direction, and that allows for much more efficient gas exchange. Though birds still have air sacks to serve as buffers. A flex-fluid organ could have separate intake and outlet openings, more like the way gills do work. $\endgroup$ – JDługosz Jun 9 '16 at 16:40
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1- A creature doesn't have to be able to breathe underwater to be successful in water

There are many examples of creatures which hold their breaths for so long that they can spend more time underwater than above it. Examples include whales, dolphins, turtles and hippopotamuses. Instead of giving your creature the ability to absorb oxygen underwater, you can simply give it large lungs and allow it's blood to absorb much higher concentrations of oxygen than it has, now.

2- Some turtles can absorb oxygen through their anuses

Here is the reference to the statement above. Your creature does not have to have its/their oxygen absorption organ at the back end of its body, but anywhere where you can afford to give it some large enough surface area (I suggest the abdomen area). Oxygen absorption through this way is a passive process, meaning that the creature does not have to consciously do anything to absorb oxygen. The process carries on by bodily functions carrying on by themselves (regulated by backbone, instead of brain).

3- Gills

Fish have gills and they are very effective in underwater breathing. You can use the same apparatus for your creature.

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    $\begingroup$ Although having gills on their a... On their rear end wouldn't make for a Disney-like romantized version of mermaids, that is nonetheless another possibility. Since we imagine mermaids as being very active, they need as much oxygen as they can get, and that's another viable source. +1. $\endgroup$ – Renan Jun 9 '16 at 12:54
  • $\begingroup$ Honestly, the first answer is the vest. The sensible answer is: they don't. They're air breathers, just like seals, whales, porpoise, etc. These air breathing creatures have successfully invaded the niches of animals with gills so there's no reason to believe it's a significant disadvantage (I think it's actually an advantage). $\endgroup$ – Jack Aidley Jun 9 '16 at 15:52
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breathing basically comes down to gas exchange across a semipermeable membrane. Typically, this membrane has to remain moist to function properly, so above water, you are limited by keeping the entire creature moist (lungless salamanders, lots of inverts like flatworms) or keeping a moist membrane enclosed in a chamber within the body while the rest of the body has a barrier to loss of moisture (lungs of vertebrates, gills in cheek pouches of mudskippers, book lung of spiders, etc.).

This is a bit different from the salt vs fresh question. In water, semipermeable membranes are not only going to allow the movement of dissolved gases, but also of water, which is going to want to reach equilibrium with the water on the other side on the membrane. So if on one side we have a dense fluid and on the other side a less dense fluid, water is going to flow from the less dense side to the more dense side. The challenge of the organism is to balance this out so that it doesn't either pop from the water trying to dilute its internal fluid or dessicate as water is sucked out to equilibrate with the surrounding saltier sea.

In fish for example, in freshwater, they have to pump water out to keep the balance, so as water permeates in through the gills, the kidneys are pumping it out. Damage the kidneys and the fish gets dropsy and swells up like a pinecone. In marine fish, they have to drink water and them filter out and excrete the excess salt. Both of these strategies work partially because fish aren't completely permeable across their whole body. Fluid and gas exchange is limited to small regions (gills, gut, lung or other modified organ in some species).

The other strategy is trying to match the density of fluid inside and out (osmoconforming) which is sort of what the crab-eating frog is doing by storing urea to make its internal fluid match the density of the saline water around it. Most marine inverts are osmoconformers to some extent.

All this is a little bit off topic and a gross simplification, but just wanted to elaborate a little on why some critters are limited to either freshwater or saltwater. It isn't specifically linked to breathing air or underwater, but both do involve exchange across membranes so maybe there is something useful in briefly having this out.

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I know I'm breaking your rule from the start but I think air breathing fish are informative and any answer is going to involve the same basics that they share with amphibians: absorbing air across a moist membrane. For the amphibians it is their skin, for the fish it is internal, cheeks/gills or primitive lung. I imagine this would be the most reasonable mechanism for a vertebrate like a mermaid: lungs in addition to gills or internal gills in a chamber that can hold water that can keep saturated with dissolved oxygen by gulping air when above the surface.

Alternately, look to arthropods. Plenty of crabs and crayfish are amphibious to varying degrees.

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  • $\begingroup$ Is this meant to be posted as two separate answers? $\endgroup$ – Tim B Jun 9 '16 at 12:12
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Amphibians' skin needs to stay moist as well as permeable. Therefore salt would enter the organism.

Source: https://www.reddit.com/r/askscience/comments/3isp06/why_are_there_no_saltwater_amphibians/

My proposal is to either make them have the mentioned frog's ability to tolerate the salt water, or instead just not let them be amphibious creatures but instead allow them to hold their breaths for a very long time. Either by means of low-oxygen requirements, or by storing air or concentrated oxygen somewhere inside their bodies.

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  • $\begingroup$ How "very long" do you propose? $\endgroup$ – JohnWDailey Jun 9 '16 at 1:11
  • $\begingroup$ Records for humans is almost 20 minutes (with no movement tough). Imagine a biological built-in oxygen tank inside the body, so to speak. Could also contain compressed air and/or oxygen rich air. This would be quite stressful in over-water situations, but in deep water, under great pressure, this might be a fairly small strain to take. $\endgroup$ – InstantMuffin Jun 9 '16 at 1:23
  • $\begingroup$ Although the higher the pressure the more air you need to fill your lungs. I'm not sure how far you can go introducing changes to the anatomy. Half an hour (at least) would sound believable to me as a reader. $\endgroup$ – InstantMuffin Jun 9 '16 at 1:30
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    $\begingroup$ I'm going to have to add another annotation to this. First, I could not find out for how long exactly that frog species can tolerate salt water. Second, you'll have other issues with mermaids: Temperature. Homoiotherm animals in sea would need a completely different build in order to isolate themselves from the possibly very cold water. Wales do this by having a very thick layer of fat. So to speak, realistic Ariel would not be a size 0. $\endgroup$ – InstantMuffin Jun 9 '16 at 1:57
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    $\begingroup$ "Ariel would not be a size 0" <- that was priceless. $\endgroup$ – AndreiROM Jun 9 '16 at 3:17
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the crab-eating frog has some tolerance for saltwater, it has specialised kidneys and skin to cope with the salt.

I'm not sure why you're resistant to look at the lungfish, I would have thought it would be the best example of something working close to how a mermaid with gills and lungs would in theory.

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  • $\begingroup$ Except that a lungfish doesn't spend enough time on the surface for the mermaid analogy. $\endgroup$ – JohnWDailey Jun 9 '16 at 1:10
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Basically steppingonants is right about copying the design of the crab gills so that they breathe in and out of the water. More explanation on this can be seen here. https://www.thoughtco.com/how-do-crabs-breathe-2291887 There is, however, one catch about the breathing system: while the gill's outlet can be wide enough between the ribs, the nazal holes are still too tiny. Don't forget that oxygen dissolved in the water hovers around 1% compared to 21% in air. http://www.lakeaccess.org/russ/oxygen.htm

You should alter the inlet location to accommodate bigger water inlets. Maybe two openings atop the shoulders, between the top chest rib and the back rib? The diaphragm can still fulfill its action. The inlets and outlets open and close in alternation and allow flow in one direction. Just like the crabs, the openings shut outside water to retain humidity and they breathe through their nose. They must eject excess water when taking the first breath of air.

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