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This anatomic conception of merfolk (mermen and mermaids) is an amalgam of biological particularities found in in sea mammals (dolphins, whales, manatees, sea lions) and sharks. Some are not found on Earth, but are still plausible. My idea is separated in four part for simplicity.

First, the metabolism, body strength and internal anatomy:

  • The vascular system would be made in such a way that the body could stay warm without the need of an overly thick layer of blubber (marine mammal fat).

  • The inside of the lungs would have two layers of oxygen-carrying capillaries and twice the number of alveoli found in those of humans, permitting long diving sessions of fifteen to twenty minutes.

  • The muscles of the body provide a specimen with a strength ten times greater than the best human athletes; however, because of the body shape of a merfolk, it would be inhibited on a land mass (shore, rock on which they get heat from the sun and seduce sailors).

  • While in water, if a merfolk cannot have access to air on the surface, they can slow their metabolism to rely solely on their hair for oxygen (explained in part 2).

  • The throat has a larynx able to emit sounds in the same frequencies that can be heard (explained part 3).

  • Salt glands expell the excess of salt in the body via the anus, keeping the orifice clean in the process.

  • The blubber contains organohalogens which prevent the body getting infections in case of gaping wounds and acts as a buoyancy agent. The cellular regeneration and immune system is so efficient that even open wounds takes just weeks to heal.

Second, outside body features:

  • The teeth would look like ours, but be sharper because their diet is comprised of bones and shells. A tooth is able to regrow when lost.

  • The hair (and beard for the males) would be very long, able to absorb oxygen like gills and detect pressure changes due to movement. The color could be dark blond, black, white, grey. It would gain a tint or become completely green over time because of the presence of algae. It could become a small ecosystem which both attract prey and serve of camouflage for the mermaid.

  • The hands have retractable webbing and hard nails that can be used as claws if sharpened and not pedicured.

  • The iris colors could be either silver, amber, emerald green, dark orange, light and dark brown, nearly black, or dark or light blue. The pupils of the eye would glow red or silver if exposed to light in dark water because of the presence of a tapedum lucidum in the back of the eyes.

  • The tail, based on the lower part of the dolphin, would have caudal fins for reproduction purpose. A urogenital slit placed over a anus would be located between the caudal fins.

  • Nasal membranes placed deep in the nostrils can close at will, stopping water from reaching the lungs while letting smells in.

Third, the senses:

  • The sense of hearing is seven times more developed than human because the head possess three time more ear cells, making them able to perceive frequency from 20 hertz to 150 kilohertz.

  • The sense of smell has a range of hundreds of meters.

  • The sense of touch is very developed, making a individual more skin-sensitive than a human.

  • The range of taste is limited to the basic (salty, sour, sweet and bitter) but the taste buds detect potential food over many kilometers in water.

  • While the eyesight is in the same range as humans, the perception of their surroundings is superior. They are capable of echolocation like dolphins, which provides them with a bio-sonar, and electroperception like sharks, because of the receptive organs called ampullae of Lorenzini.

Fourth, the drawbacks:

  • Because of the body shape of merfolk, their strength is near useless on a land mass (shore, rock on which they get heat from the sun and seduce sailors).

  • Because of the particularity of the vascular system, bleeding would take longer to stop, forcing the wounded merfolk to go on the surface to speed up coagulation.

Things I learned after this post :

  • Great concentration of myoglobin in the muscles would extend diving time because it is good at retaining oxygen.

  • The hair cannot be used to supply the body in oxygen. Their length could lead to entanglement, letting them to be cut, and development of algeas on the filament would block the absorption of oxygen. It would let body heat escape, while body heat is crucial to keep in a marine environnement.

  • The urogenital slit and anus would placed just below the waist, not between the caudal fins.

  • They either use echolocation or electroreception, not both of them. If it is echolocation, it would be the shorter range kind.

  • They could consume some crustacean with their carapace, but for the mollusk it would be only the flesh and not the shell.

  • Too much muscle mass would compromise floatability. Only if the lower part of the body that has a strength ten times greater than the best athletes and the upper portion has the strength of a normal human torso(albeit very fit because of near constant physical activity), it would keep a reasonable amount of floating capacity.

  • No need for salt glands, the body of marine mammals can balance it salt concentration by the use of very efficient kidneys and super-concentrated urine.

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    $\begingroup$ If you write so pedantically that your readers wants to pause to find fault, that's one thing. Else, if you want to write a story using those parameters, what proportion of readers is going to challenge anything? $\endgroup$ Mar 7 at 22:20
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    $\begingroup$ @RobbieGoodwin this is what I always think of when people post very scientifically detailed depictions of merfolk, or other fantasy creatures. Who in the story is going to be thinking extremely scientifically about your creature? Perhaps if the story is a sci fi, where we find this creatures and they're being studied as part of the plot, yes, but for most stories, I think only a sampling of these details are needed for a good story. The rest can be left to a wiki. $\endgroup$ Oct 5 at 18:13
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The vascular system would be made in such a way that the body could stay warm without the need of a overly thick layer of blubber

This sounds like a bit of a "...and then a miracle happens..." part of your plan. Conductive and convective heat loss is still going to be a serious problem, both on land and at sea, for an endotherm.

If you want skinny merpeeps, have them live somewhere warm.

The inside of the lungs [have some stuff] permitting long diving sessions of fifteen to twenty minutes.

If you want to increase diving time, improving gas exchange efficiency will do little to help. There have been posts here (here are some of mine: one two, but there are plenty of others, I'm sure) which cover this and other issues around warm-blooded merfolk. Do what diving mammals do: have more myoglobin in muscles.

The muscles of the body provide a specimen with a strength ten times greater than the best human athletes

This is just weird and magic, not to mention unnecessary. If you're going to just magic stuff up, why are you worrying about plausibility?

Also note that you can't have super strength in isolation; you also need super-connective-tissue to hold the super-strong body together, and you also need super-bones to withstand the much greater forces put on them. You're going to end up with really heavily built merfolk, with a really inconvenient density for swimming, and it isn't obvious what the gain is. Humans are already reasonably strong, and we get tool use by way of a bonus.

Salt glands expell the excess of salt in the body via...

Lets gloss over the reasoning behind this, and simply observe that marine mammals don't have salt-excreting glands, but do have a biochemistry that tolerates higher salt levels, super-kidneys and super-concentrated pee (to maximise salt removal rates) instead.

Its not that salt glands are unrealistic, but they don't seem to appear in mammals and aren't necessary.

The hair (and beard for the males) would be very long, able to absorb oxygen like gills and detect pressure changes due to movement.

This is "...and then a miracle happens..." part two. If you want gills, they need a blood supply. How else do you transfer oxygen in and metabolic waste products out? If you have some kind of heat-exchanging filter organ, it would be odd to put it on the head, cos that's where the brains are.

A gill-cloud big enough to supply the oxygen demands of a magically super-strong endotherm is going to be huge, making it both a tangle hazard and also a source of considerable vulnerability. If it develops a coating of algae and whatever other marine biofilm might be attracted to it, how are you expecting stuff to diffuse in and out of it? It would have to be kept scrupuously clean.

Moreover, to use as a sensory medium the hair will probably need to be stiff. Look at whiskers: they don't droop, because if they were soft they would not be able to communicate small movements to sensory nerves down at the follicle. You'd probably also want them to be relatively short, because very long stiff hairs with sensitive nerves at the bottom are going to be very uncomfortable in a whole range of circumstances.

The hands have retractable webbing

How would that even work? It isn't really that necessary either. I'm sure you're thinking of sexy mermaids here, but lets be honest: horny sailors are going to be looking at boobs, not manicures.

[capable] of echolocation like dolphins, which provide them with a biosonar

Odontocetes have quite complex anatomy to support echolocation, and that sort of anatomy is not necessarily compatible with the sexy mermaid style you seem to be going for.

It also isn't strictly necessary... seals don't need it, after all, and they can dive into very deep dark water and hunt for prey just fine. With electroception too it just seems a bit like you're trying to pile superpowers onto them... its all very cool, but difficult to justify and they don't need whiskers and sonar and electroception and superior chemoception and so on and so forth, especially if you're going to be eating a lot of shellfish.

Aside from that, seems OK.

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  • $\begingroup$ So, more myoglobin in the blood for a higher diving time. $\endgroup$ Mar 8 at 13:19
  • $\begingroup$ @TimothéLépine not blood, muscles. Like I said ;-) Make sure you read the linked posts for more information and some references. $\endgroup$ Mar 8 at 15:26
  • $\begingroup$ Oh, sorry I misread. $\endgroup$ Mar 8 at 16:37
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Could this idea for an anatomically functional merfolk “sound” plausible?

In short: no. Several items on your list contradict the known laws of physics.

Starfish Prime already covered several, but here's a few additional comments:

The muscles of the body provide a specimen with a strength ten times greater than the best human athletes; however, because of the body shape of a merfolk, it would be inhibited on a land mass

This is just pure handwavium. If anything, they're liable to have less (effective) strength in the water due to lack of leverage and the greater viscosity of the environment. Also, I'm pretty sure we know of no way to give a roughly-human-shaped creature "a strength ten times greater than the best human athletes". Not, at least, without giving them much greater muscle mass.

Now, that said, their tails basically are a huge mass of muscles, so if you're talking only about their tail strength, that's a bit more reasonable. And, yes, it's probably going to be hard to use their tails out of the water. Basically, though, you're looking at something like a dolphin or a shark; you should do some research there to get a better idea what sort of strength your merfolk would actually have. Also, look at performing dolphins, or maybe seals / sea lions, to get some ideas what they might do out of the water.

Simply saying they can't use this impressive strength on land "because of the body shape" needs more thought.

The cellular regeneration and immune system is so efficient that even open wounds takes just weeks to heal.

This is fairly miraculous, though perhaps more likely to pass "willing suspension of disbelief" than some of your other ideas.

The teeth would look like ours, but be sharper because their diet is comprised of bones and shells.

I doubt very much any creature with biology anything like that of terrestrial animals can survive on "bones and shells". Shellfish, maybe. Just the shells is extremely unlikely.

The hair [...is...] able to absorb oxygen like gills and detect pressure changes due to movement.

As already noted, this is just pure baloney. I won't rehash SP's answer, but this is probably the single most ludicrous trait you've proposed. Biology simply doesn't work like that.

The tail, based on the lower part of the dolphin, would have caudal fins for reproduction purpose. A urogenital slit placed over a anus would be located between the caudal fins.

While not completely unbelievable, this is still fairly implausible. No mammal (no vertebrate, AFAIK) has a GI or reproductive tract running down its tail, and if you were planning a mammalian-style reproductive organ at that location, that seems particularly implausible. I recommend doing some research into where real marine animals keep there orifices.

the taste buds detect potential food over many kilometers in water.

The more I think about it, the less I'm convinced I can say this is impossible, but it strikes me as "unlikely", and has a fair chance of having the same effect on your audience. I would have expected smell to be used for long distance detection of potential food.

Because of the particularity of the vascular system, bleeding would take longer to stop, forcing the wounded merfolk to go on the surface to speed up coagulation.

This, OTOH, sounds plausible. In fact, you might be overthinking this. Just being in water will tend to wash away any blood, which might also wash away coagulants. Even without "particularity of the vascular system", I'd buy this.

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    $\begingroup$ Good spot on the caudal fin thing. I mentally parsed that as ventral or anal and didn't notice the major anatomical misunderstanding. Tasting food many km away isn't implausible, but stuff that you can detect that far away is probably really rotten and the OP doesn't seem to be describing a scavenger species. $\endgroup$ Mar 7 at 13:46
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    $\begingroup$ @StarfishPrime, yeah, I had to look up "caudal", because I had the same assumption but was suspicious enough to verify 🙂. I think we're probably on the same page with the taste thing; it's not that I can't believe it, it just doesn't seem likely/useful compared to smell working over longer distances. IOW, it could happen, but why would it happen? $\endgroup$
    – Matthew
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  • $\begingroup$ I didn't see my mistake about the genitals between the caudal fins, thank for pointing it out. $\endgroup$ Mar 8 at 13:17
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Here are some of my ideas:

The vascular system would be made in such a way that the body could stay warm without the need of an overly thick layer of blubber (marine mammal fat).

The inside of the lungs would have two layers of oxygen-carrying capillaries and twice the number of alveoli found in those of humans, permitting long diving sessions of fifteen to twenty minutes.

Salt glands expell the excess of salt in the body via the anus, keeping the orifice clean in the process.

Great concentration of myoglobin in the muscles would extend diving time because it is good at retaining oxygen.

These traits are found in real aquatic animals

The muscles of the body provide a specimen with a strength ten times greater than the best human athletes; however, because of the body shape of a merfolk, it would be inhibited on a land mass (shore, rock on which they get heat from the sun and seduce sailors).

This is actually half as strong as a gorilla, from what I've read

The throat has a larynx able to emit sounds in the same frequencies that can be heard (explained part 3).

I don't see how a larynx could work underwater. It'd be better to use some sort of resonant structure inside the respiratory tract, as in cetaceans

The blubber contains organohalogens which prevent the body getting infections in case of gaping wounds and acts as a buoyancy agent. The cellular regeneration and immune system is so efficient that even open wounds takes just weeks to heal.

Many marine species can produce halocarbons, and using them as an anti-infectious agent seems feasible. The regeneration speed also seems believable

The teeth would look like ours, but be sharper because their diet is comprised of bones and shells. A tooth is able to regrow when lost.

There are no animals that can regrow teeth specifically when they are lost. You could have the teeth be replaced constantly, or perhaps you could give them some novel way to tell if a tooth is lost and to quickly react

The hair (and beard for the males) would be very long, able to absorb oxygen like gills and detect pressure changes due to movement. The color could be dark blond, black, white, grey. It would gain a tint or become completely green over time because of the presence of algae. It could become a small ecosystem which both attract prey and serve of camouflage for the mermaid.

If the hair is to be used for respiration and sensing, then it needs to be made of living tissue. It seems like having the hair as exposed gills should be pretty close to what you'd want. In this case, the gills would have to be red (or whatever colour the blood is)

The hands have retractable webbing and hard nails that can be used as claws if sharpened and not pedicured.

No animals have retractible webbing, as far as I'm aware, and it doesn't seem like it'd be useful or functional. The claws seem OK

The iris colors could be either silver, amber, emerald green, dark orange, light and dark brown, nearly black, or dark or light blue. The pupils of the eye would glow red or silver if exposed to light in dark water because of the presence of a tapedum lucidum in the back of the eyes.

I don't see any problems here

The tail, based on the lower part of the dolphin, would have caudal fins for reproduction purpose. A urogenital slit placed over an anus would be located between the caudal fins.

While vertebrates don't have their anus at the end of their tail, there is no reason I know why it couldn't be. However, I'd say most of the guts will need to be near the torso, with only a narrow tube going through the tail

Nasal membranes placed deep in the nostrils can close at will, stopping water from reaching the lungs while letting smells in.

While this seems like a rather useful adaptation, it'd make more sense to use the epiglottis, as this structure is low enough to allow both smelling and eating underwater with only a single blockade

The sense of hearing is seven times more developed than human because the head possess three time more ear cells, making them able to perceive frequency from 20 hertz to 150 kilohertz.

The hearing seems good enough, especially given that they live underwater

The sense of smell has a range of hundreds of meters.

Some animals (though terrestrial) can smell over miles, so just 100m seems easily doable

The sense of touch is very developed, making a individual more skin-sensitive than a human.

There's nothing wrong here

The range of taste is limited to the basic (salty, sour, sweet and bitter) but the taste buds detect potential food over many kilometers in water.

A long-range sense of taste is not necessary: That's what smell is for

While the eyesight is in the same range as humans, the perception of their surroundings is superior. They are capable of echolocation like dolphins, which provides them with a bio-sonar, and electroperception like sharks, because of the receptive organs called ampullae of Lorenzini.

Many animals, such as shrews and bats, are adapted to use echolocation. Even humans can use it. Electroperception is also useful underwater

Because of the body shape of merfolk, their strength is near useless on a land mass (shore, rock on which they get heat from the sun and seduce sailors).

I don't see how their body shape could prevent them from using their strength on land

Because of the particularity of the vascular system, bleeding would take longer to stop, forcing the wounded merfolk to go on the surface to speed up coagulation.

This seems plausible

The urogenital slit and anus would placed just below the waist, not between the caudal fins.

There is no reason to keep with tetrapodal anatomy

They either use echolocation or electroreception, not both of them. If it is echolocation, it would be the shorter range kind.

There's no reason not to use both. In fact, there is evidence that some dolphins can sense electricity, alongside their echolocation

Too much muscle mass would compromise floatability. Only if the lower part of the body that has a strength ten times greater than the best athletes and the upper portion has the strength of a normal human torso(albeit very fit because of near constant physical activity), it would keep a reasonable amount of floating capacity.

Many aquatic mammals have dense bones to increase their density, so their's no reason why these mermaids would need such low muscle mass

No need for salt glands, the body of marine mammals can balance it salt concentration by the use of very efficient kidneys and super-concentrated urine.

Either solution will work, so their's no reason to choose one over the other

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  • $\begingroup$ Thank you for the feedback ! $\endgroup$ Oct 6 at 18:09

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