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I want a mermaid protagonist. If I am to make this story realistic, how should the Merfolk's body structures be made to make them adaptable for both water (swimming) and land (walking, running)? Is this even possible?

I am asking, more specifically:

  • If my Merfolk had tails could they be separated into human legs without them having a human ancestor or sharing a ancestor with humans?
  • Could they walk if the tail was constructed into legs or would it take them too many years of therapy and not be worth it?
  • Would it be possible for a water based animal to swim with a tail that was structurally just human legs fused together?
  • Would it be possible for them to have both gills and lungs and the lungs work?
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The simplest answer would be to not have the legs fused at all. Underwater they would hold the legs together and swim using long flippers/flukes on the end. On land they would curl up the flippers/flukes and separate their legs to walk normally.

Any other solution is going to involve either biologically improbable mechanisms to separate the legs or them moving more like a snake on land.

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  • $\begingroup$ You need to address some of the other issues, like the lungs/gills problem. Nature has solved this, though, with extant examples like the lungfish. $\endgroup$ – PipperChip Nov 18 '14 at 20:02
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That I can think of there are 5 ways. Hopefully you find more:

  • The Naga: They use the same snakelike body to swim and walk.

from the wowwiki apparently

  • The Splash: Like the movie Splash or the little mermaid to a lesser extent, the mermaid magically becomes human but can turn back into a mermaid under certain circumstances. In the case of Splash, when exposed to water.

from imdb

  • The One Piece-two piece: One mermaids in the show one piece mature to a certain age, their tails split into two legs like fins. They can still squish them together like legs underwater. Like Sanji, I won't post this because who wants to see that.
  • Mechanically or magically assisted locamotion: Also from One Piece because they do everything and it is cool, mermaids can travel on land by using a levatating bubble that is lighter than air. You could more realistically use mecha legs or magic levatation dependind on your world.
  • The simple solution: Mermaids can have two legs. You can make a mermaid that walks like a person but clearly is not a person. This creature would be a more realistic solution but less complicated. I would focus on making this creatures legs clearly more useful in whater than on land despite dual functionality. This can be done by large flipper feet, necessity to crawl to walk, and/or a different joint configuration (likely the heel being used as a second knee for more power in the water).
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  • $\begingroup$ Just to clarify the difference between Splash and The Little Mermaid (the Disney movie). In Splash, her form automatically morphed - dry: legs, wet: fin. In The Little Mermaid, her form is fixed, but Ursula (using her witch powers) and Triton (using the trident) had the power to permanently change her form. $\endgroup$ – Donald.McLean Nov 17 '14 at 16:46
  • $\begingroup$ Also, for the last idea - a form that is more human in appearance, there was a TV series staring Patrick Duffy: imdb.com/title/tt0075533/combined $\endgroup$ – Donald.McLean Nov 17 '14 at 16:52
  • $\begingroup$ @Donald.McLean or simply Aquaman. $\endgroup$ – kaine Nov 17 '14 at 16:53
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Would it be possible for a water based animal to swim with a tail that was structurally just human legs fused together?

If mermaiding is any indication, yes. Real-world mermaids are humans who wear a swimsuit that covers both legs and ends in a monofin that functions as a fluke. The monofin clips onto the feet, and the swimmer uses a motion called a dolphin kick.

There's an online store called Mermagica that sells monofins and swimsuits designed to fit over them.

Could they walk if the tail was constructed into legs or would it take them too many years of therapy and not be worth it?

Milagros Cerrón and Tiffany Yorks were born with sirenomelia (fused legs). Their legs were separated surgically, and with physical therapy, they learned to walk. (No word on whether anyone in the operating room was named Ursula.) Ms. Yorks still needs crutches. Shiloh Pepin was born with the same condition, but because the blood vessels in her fused foot were more intertwined, she couldn't be given the same operation.

If my Merfolk had tails could they be separated into human legs without them having a human ancestor or sharing a ancestor with humans?

That depends on what you mean by "ancestor", as all vertebrates are believed to share an ancestor. If you just mean not sharing an ancestor within the order Primates, then yes, there are examples of other aquatic vertebrates swimming with specialized hind feet: seals among mammals, and loons and grebes among birds. If you're aiming for some measure of land agility, as your later question implies, you could consider elements of the physique of Olympic gold medalist Michael Phelps: long, thin torso, long arms, short legs, and large feet connected to hypermobile ankles. (See Phelps, No Limits, p. 67.) Then exaggerate them. But if you're looking to create mer-people, as opposed to yet an-otter animal species, you still may need a path to sapience other than that which produced Homo (tree dwelling -> prehensile hands -> tool use -> sapience).

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  • $\begingroup$ I really appreciate your science-based approach to this question. $\endgroup$ – recognizer May 21 '15 at 20:25

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