Define Mermaid...

Okay, so what do I mean by 'mermaid' here? A half-fish, half-woman creature, AKA a typical, standard mermaid.

Why did this happen?

Long ago, there was an event called the Trifold Clash involving a certain magical structure called the Source Gem. This Gem takes essence from those it contacts and uses it to transfer traits from the creature it last contacted to the next creature it contacts, and the energy released in the aforementioned Clash caused an awful lot of essence to go around.

This resulted in a lot of otherwise impossible hybrids and chimeras, including mermaids. However....

Magic Has Rules

Despite their being magical creatures, mermaids still play by the rules of genetics. In other words, natural selection is still a thing for them. What I'm wondering is how this would change their 'final form,' their design. Restated, my question is How Would Natural Selection Influence Mermaid Physiology?


  1. What I'm looking for is a realistic mermaid concept, based off the 'traditional' mermaid portrayal but advanced by natural selection into an optimal form. Yes, some parts will be mostly (if not totally) handwaved, like breathing air and water (covered by this question.)

For example, please take first the chimerical design of the typical mermaid: normal skin above the waist and scaly skin below said waist. It seems obvious natural selection would change this, no? But, seeing as my mermaids will start out like this, is it more likely that they'll simply develop tougher skin (like a dolphin's) on their upper half or end up covered entirely in scales?

Second, the fishtail. Normal depictions of mermaids have just a caudal fin, but this seems highly unlikely; fish have more fins than that, for good reason, so why not mermaids? As logical as that is, I lack the knowledge necessary to determine which fins they'd gain (result) and why that'd happen (to explain why).

Basically, I'm asking how natural selection/evolution would adapt a population of 'typical' mermaids over time, and what the final result would be.

Please Note: I have decided (for now) that mermaids will be a female-only species, viable with regular humans (this bit is handwaved, don't worry about it), which means that things like long hair will be akin to a peacock's grandiose tail. As this will reasonably impact answers, I felt it best to add this now.

Also Note: Yes, mermaids are as intelligent as humans, just with a 'fishy' influence. This means they have extremely good long-term memory and spatial awareness, which is good for navigation, and is a prime factor behind their highly social, close-knit and cooperative culture.

  • 1
    $\begingroup$ Are the mermaids intelligent like humans or like fish? Because for humans natural selection doesn't exist no more, now we only have sexual selection through looks, behavior, economics and politics. $\endgroup$
    – user89947
    Sep 8, 2021 at 9:24
  • $\begingroup$ @SteroidSandwich: thanks for your input, please see the edit at the bottom of OP. $\endgroup$
    – Alendyias
    Sep 8, 2021 at 9:27
  • $\begingroup$ Just curious, do mermaids get their tongue eaten by tongue esting louses? $\endgroup$
    – user89947
    Sep 8, 2021 at 9:58
  • 2
    $\begingroup$ @SteroidSandwich: I haven't thought of that, and will probably post another question accordingly. $\endgroup$
    – Alendyias
    Sep 8, 2021 at 10:38
  • $\begingroup$ I would rename this question to "Make-a-fish" just for the pun potential. $\endgroup$
    – Mermaker
    Sep 23, 2021 at 2:46

3 Answers 3


Your big event is the Trifold Clash, isn't it? So, what about taking that concept of "trifold" and running with it for the Mermaids?

A Mermaid isn't a half-human, half fish. It is a human-orca-fish trifold chimera.

For starters: mermaids are quite a bit more believable and "biologically possible" if one goes with a marine mammal as a base for the tail instead of a fish. Pick a creature like a killer whale or a dolphin, and you have a far better starting point - not only by being evolutionarily closer, but also by having a body plan that is way more compatible with those of a human.

That said, while Killer Whale Mermaids are an amazing concept, they aren't quite what we need. Thankfully, since we're doing a trifold chimera, we can throw in some regular fishes in the mix to spice things up.

A trifold human-orca-fish chimera would take the best from the three parts. The resulting being could be more or less something like this:

  • The overall skeletal and muscular structures would derive mostly from the human+orca parts. This enables one to create a more functional body plan with less hassle, since it is easier to translate the bones and muscles from a marine mammal to a human, and vice-versa than it would be to translate to and from a regular fish. Same goes for internal organs.
    • The number of fins is more or less correct already for a creature with that body plan, with maybe a dorsal fin evolving overtime for better control underwater.
    • Adding to this: the hands will eventually get webbing between the fingers. It's also possible for them to eventually to grow fin-like structures out of the upper arms and forearms, for increased mobility.
    • Fish don't usually sing, while killer whales do.
    • The killer whale part would also provide a better dental equipment, for improved sailor-eating.
    • Making the reproductive system work is leagues easier. Killer whales are already mammals, so less adjustments are necessary.
  • Fishes would contribute greatly to the overall aesthetics of our mermaid, alongside some extra features that would be hard to find on a marine mammal otherwise:
    • Scales. A killer whale mermaid wouldn't have scales normally, but with our trifold chimera, that becomes a possibility. The scales would be mostly decorative, used to entice and seduce sailors for their doom - it is easy to spot a mermaid from afar if she has a shiny golden or red tail with colorful accents instead of a boring black, featureless one.
    • Gills. The fish bits would give our mermaids a few sets of gills - probably on their throats - that they could open and close at will, to enable breathing underwater. They would still keep their mammalian lungs for out-of-the-water action - specially singing and sailor-hunting.
    • Extra fins. Fishes have so many different types of fins, with so many different functions that is hard to determine a single path for the fins of our trifold mermaids to take. Their fins would probably be colorful, and probably bigger than they need to be - they aren't only swimming aids but also tools for seduction. The prettiest mermaid gets the tastiest pirates!

But what about the evolution?

They wouldn't change much.

Humans are incredibly slow to evolve, and that would probably translate to our mermaids. Humans exist for surprisingly little time compared to the rest of the planet, and while we have been changing as a species, it isn't really because of natural selection and more of a mix of cultural, political, and economical contexts. Those would translate more or less in the same way for our mermaids: Their evolution would be driven by nothing else but themselves.

This allows you to go mostly in whatever direction that you please, but whatever physical changes they experience would be minute in the grand scheme of things. In fact, I would argue that - since they are a female-only species that "hunts for men" in order to reproduce, they would probably select themselves to be either more attractive to male humans or better in hunting them force them to "cooperate". So, if having fish scales on just half of the body and the rest being a fair-skinned, red-headed beauty is what works, then that's the direction their species will go. Those are the mermaids that are reproducing, in the end.

If you ask me, mermaid evolution isn't about how their bodies adapt over time, but instead it is about how their culture does so. Whatever is the best hunting/seducing technique available to them at the moment will guide how they will change their habits, mannerisms, and overall culture.

If that means waving their lite arms to sailors or becoming ruthless 8-feet tall amazonian-like huntresses, then that is what will become of them.

Of course, if you want to go wild, you can always have different tribes of mermaid evolve in different directions. Humans are different from one another. Nothing blocks you from your mermaids being very different from one another, too.

For an overview on what I believe would be a nice take on Mermaid Architecture, check my answer on this question.

  • 3
    $\begingroup$ Great, I love your perspective on the Trifold Clash and I must say, I like the idea of trifold mermaids! As for your points on evolution, thanks for being thorough and clear, now I just need to determine whether sirens or huntresses will be more common.... $\endgroup$
    – Alendyias
    Sep 14, 2021 at 22:17
  • 1
    $\begingroup$ @Alendyias My suggestion: Make several sects of mermaids, each one with their own twist. For a few quick examples: The Clan of the Bloody Fin would be Dolphin-like, the Sisters of the Song would be Orcas-like, and the Witches of the Depth could go with a octopus-like build. Depending on what you need for the story, you can touch on this or that tribe. $\endgroup$
    – Mermaker
    Sep 14, 2021 at 22:38
  • $\begingroup$ Very good idea, thank you! The Sarcastic Fringehead (Gaping Dreadmaw) tribe should be especially terrifying.... $\endgroup$
    – Alendyias
    Sep 14, 2021 at 22:42
  • $\begingroup$ @Alendyias For a quick source of inspiration on mermaid variants, check Nami's cosmetics page. $\endgroup$
    – Mermaker
    Sep 14, 2021 at 22:47

Sunlight adaptation

Humans, like most animals used to be able to produce vitamin C, the animals who do are able to produce what we would consider incredible megadoses of this vitamin.

Humans and most mammals lost this ability and instead developed a starving gene that allows us to survive with the lowest ammount of vitamin C possible, it makes us more disfunctional than we were before but we are still capable of surviving long enough to have sex and reproduce.

Similarly your mermaids, the branch of mermaids who decide they prefer the depths of the ocean over the coast will starve themselves off vitamin D, which they have to either megadose in their diet to survive or develop a starving gene like we did with the vitamin C. Consider vitamin absorbtion by ingestion is very low, usually always lower than 80%, take that into account when designing their diet if you do. Hence why in hospitals they give vitamins and nutriets either through the blood or the butt, less stress on the liver and kidneys and more efficient.


Coastal and surface fish have scales for sexual selection and mild protection from bacteria and parasites and also to protect against the sun's radiation.

Depth fish usually lack scales as they only weight them down and become dangerous at high pressures as the water could break them or push them inside the skin.

Humans have melanin and the human skin doesn't have any active dna which forces the skin to grow scales, we can grow calluses at best. So coastal mermaids will just get darker skin.

Depth mermaids might lose their scales instead and opt for thicker and rougher skin.


Humans are wide, the wingspan with arms open of a human is equal to their height, and the back of a male human is the largest part of their body.

Both the shape of the body and the muscles are decent for breaststroke swimming but useless and hindering for other types of swimming.

While fish are taller and longer than they are wide therefor their swimming is by flexing their body side to side.

If you want to just cut a person in half, cut a fish in half and then stick them together then they would look like an arrow and probably swim by rotating on themselves. Or just choose if you want mermads flat like a fish or fish flat like a human.

If you make mermaids tall and long like a fish, which basically at this point is just a fish which barely resembles a person who was killed on the road by a giant truck, flat. Then you need to balance their body, if the fish part still has the fins then they are enough otherwise theyde have to learn how to flap their arms to not go upside down.

  • $\begingroup$ Great points overall, welcome to the site! I must say, though, I don't understand what you're saying for swimming.... $\endgroup$
    – Alendyias
    Sep 8, 2021 at 10:25
  • $\begingroup$ These are interesting points. But this is also for evolution of a species that mates semi randomly. If mer people are intelligent. There will be detailed choices to mates that could lead evolution to be less advantage and more sexual $\endgroup$
    – Trevor
    Sep 8, 2021 at 18:01

It depends on how fire works

Specifically, on if fire, or some other substitute for cookery, can be sustained underwater

If not

If there is no submarine cookery, then mermaids could not survive underwater. They would have to stay in the shallows at a minimum. But, the shallows are still not a great place in general, so evolution will help increase their range

A plausible progression would be the acquisition of a more serpentine form. This serpentine form would affect most of the body, causing the body to be tubular from the shoulders down to the tapering part of the tail

The scales would likely expand to cover the entire body, excluding a few parts, such as the face, breasts, and hands

The arms and hands would adapt to bear weight, to aid in its motions along the ground, though I don't see a reason for them to lose the simian traits, and they will have to maintain their fine motion and prehensility; fist walking seems like a good solution. The 'sole' of the hand (as in the part that touches the ground) could likely also develop scales for traction

The neck would have to bend up, and the foramen magnum would also have to move up the skull so that they can see forwards as they walk/slither

There would also be some more general adaptations for land, such as keeping water well, adapting for breathing air over water, etc

If so

If they can cook food underwater, then they could easily extend into the sea. While some could follow the terrestrial pathway, it'd be much easier to live underwater where there are less humans

The first thing selection would eliminate would be the arms, which would shrink down to small flippers to reduce drag. These flippers would still end with hands, which would likely be small, unwebbed, and unscaled. The rest of the body would be scaled for protection

The hair would also be a source of drag. While the question does claim it'd be useful for attracting human mates, there isn't much stopping the mermaids from skipping that step in the mating process, and so the hair would go

The face would also likely end up adapting for streamlining, most likely taking on a typical 'fish-like' profile, mostly made up of the nose; the nostrils will have no reason to move, as these mermaids can breathe underwater. It's plausible that the increasing nose would push the eyes outwards, giving them a seal-like look in this regard

The body as a whole will continue the face's streamlining, so the mermaids will have a profile like a dolphin or fish. The breasts will also shrink inline with the body. It's not implausible that a dorsal fin would develop, but it would be more delphine, unlike the ichthyoid caudal fin

  • $\begingroup$ Very interesting, I especially appreciate your points on land mermaids. However, your "if so" mermaids seem more than a little likely to scare off sailors, and therefore go extinct... $\endgroup$
    – Alendyias
    Sep 14, 2021 at 22:19
  • $\begingroup$ @Alendyias There are other methods of getting a mate $\endgroup$ Sep 14, 2021 at 22:28
  • $\begingroup$ I am aware, but at the same time, I feel like this won't end well for the mermaids...your marine mer will probably end up christened as "selk-demons" and get hunted down by crusaders.... $\endgroup$
    – Alendyias
    Sep 14, 2021 at 22:37

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