The classic merfolk are creatures of simple form: Above the hips, they are human, but below they have the body of a fish. This anatomy seems typical enough, if you don't think about it, but it quickly becomes incoherent if you do

Humans, being tetrapods, have roundish bodies filled with organs, with only a band of bone and muscle surrounding the guts and the spine placed at the extreme rear. Fish, on the contrary, are filled to the brim with muscle, with the spine running right down the dead centre of the body. Swimming vertebrates, especially large, relatively fast ones like mermaids, need a complete spine, not one which hops around in the middle of its length. Furthermore, there are issues in connecting the tail muscles to the rest of the body, given that fish do not have pelves and humans don't have large muscle-blocks like a fish

Being essentially swimming humans in behaviour, these merfolk won't need extravagant bones for perfect swimming. But certain humanoid features, like being able to sit down, would be quite useful. Also, given their proximity to the vertebrate plan, I'd like to keep them within a tetrapodal anatomy: Try and complete the spine, rather than adding in exotic bones found in your lower digestive tracts

Given these criteria, how could I connect together the body of these mermaids?

  • 4
    $\begingroup$ Your last five Anatomically Correct questions got closed. Are you certain this one is going to do better? We have many merfolk questions, we even have a tag for it. I would suggest that you try to pose your problem without using ACS. $\endgroup$
    – KeizerHarm
    Jul 31, 2022 at 21:06
  • $\begingroup$ "Swimming vertebrates, especially large, relatively fast ones like mermaids, need a complete spine" seems to contradict partially with "Being essentially swimming humans in behaviour, these merfolk won't need extravagant bones for perfect swimming". Human swimming isn't "spectacularly" fast nor effective compared to most fishes. Therefore, how fast should your mermaids go? And how much do you want to keep the exact-fishy anatomy? If we think dolphins, they lost their legs with great success regarding speed... $\endgroup$
    – Tortliena
    Jul 31, 2022 at 21:43
  • $\begingroup$ I actually made this thread a month ago. Sadly, it got deleted although similar threads I made were upvoted and are opened just fine. $\endgroup$
    – ITM_Coder
    Jul 31, 2022 at 23:17
  • 1
    $\begingroup$ I’m voting to close this question because has all the same problems as this closed and deleted duplicate question. 7 of the last 10 AC questions have been closed despite the community claiming they want the ACS to continue. This strongly suggests that we want higher quality questions. $\endgroup$
    – JBH
    Aug 1, 2022 at 0:06
  • $\begingroup$ Humans do have a fully complete spine. Not even the smallest piece is missing. $\endgroup$
    – AlexP
    Aug 1, 2022 at 6:56

3 Answers 3


A whale like orientation (up and down flexing) is much more believable, mammal bodies are already built to flex horizontally which is why this form of swimming has evolved in all aquatic mammals. Skeletally it could not be easier, it involves loosing a lot of bones and reinforcing the spine.

enter image description here

enter image description here

your biggest issue is you need a lot of musculature on the dorsal side of the body so your merfolk will have very deep backs compared to a normal uhman more like an Olympic swimmer.

you do need to think about what kind of swimmer your creatures are, the shorter the tail the thicker the body needs to be. short and thick makes you fast, long and thin makes you maneuverable.


But certain humanoid features, like being able to sit down, would be quite useful

Clearly there are multiple solutions for the aquatic mammalian tetrapod design, but none of them (or the idea of "fish people") will give you this.

Tetrapods that sit do so using legs. Merpeeps, famously, lack those.

The closest example to a marine mammal that can sort of sit are the eared seals, which include the sea lions. They still have legs, and you can see a pelvis in their skeleton:

A render of a californian sea lion skeleton

(image credit: H. Zell)

Unlike regular seals, they can get into a raised pose that's a little like sitting:

California sea lion in profile, showing raise forebody, partially supported by forelimbs

(image credit Rhododendrites)

In humans there's a congenital deformity known as sirenomelia (CW: photos of preserved stillborn human fetuses and children who died in infancy). These may well have been the origin of some of more of our mermaid legends. You might imagine a less debilitating mutation that didn't cause fatal defects, but did cause fusing of the legs and down the line reduction of some of the unnecessary bones and changes to foot shape and musculature could arise to assist in swimming. Adaptation of the various bits of plumbing down there will also be required, but again: there are plenty of partially or wholly aquatic mammals who provide suitable examples, and you should feel free to pick one yourself, considering issues of streamlining and temperature sensitivity (and perhaps the strange compromises that sealions make in that area) and exactly how the babies get out I'm assuming live births here, because the other thing that merfolk are famous for is that mermaids are unambiguously mammalian, and monotreme merfolk seems like a weirdness too far.

They could swim and sit much like a modern human freediver using a monofin:

Human freediver with monofin

(image credit N00@flickr.com)

This is a reasonably efficient way to swim for humans, but it probably won't compete with cetaceans, pinnipeds or sirenians in terms of speed and efficiency... but those folk don't get arms and spears, and they're made of things that merfolk can eat, so they'd better swim carefully.


Human Skeleton

enter image description here

Fish Skeleton

enter image description here

Take the top half of the human skeleton and the bottom half of the fish skeleton. The mermaid skeleton looks like this.

enter image description here

The organs go in the human half. The fish half is your block of fish muscle.

For extra points have the human half's floating ribs change continuously into the fish ribs.

  • $\begingroup$ What about the muscles? And that isn't how the fish skeleton would attach: It's not oriented right $\endgroup$ Jul 31, 2022 at 21:20
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    $\begingroup$ @IchthysKing Whale orientation makes the anatomy a million times more believable than fish orientation. It is also the standard orientation for mermaids in fiction. $\endgroup$
    – Daron
    Jul 31, 2022 at 21:23

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