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Anatomy wise, how could growing legs or having your legs melded together to form a large fin happen in a matter of hours? How could the change work in reverse?

I want this to be able to work without flesh, bone, or extra tissue being gained or lost magically.

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    $\begingroup$ Um, I'm seeing a lot of problems with clothing when changing. $\endgroup$ – Xandar The Zenon Dec 22 '16 at 23:08
  • $\begingroup$ So what exactly are you asking? If you can magically change, then can't your legs just change their anatomy? As long as you have the same mass before and after, your entire body may just be a slightly different size. $\endgroup$ – Xandar The Zenon Dec 22 '16 at 23:11
  • $\begingroup$ I got the clothing part down, I dont want it to be magically. Scientifically, how could mermaids gain legs for land or go back to the ocean and regain their fins? $\endgroup$ – Isillome Dec 22 '16 at 23:29
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    $\begingroup$ I think your best bet is to consider a sort of cocoon and metamorphosis: this is a fairly significant anatomical change, and that is simply not the sort of thing that happens overnight. This probably isn't what you're looking for (slow, obviously rather unusual, and likely tied to environmental factors like season, age, etc., rather than at will), but it would be the most realistic. $\endgroup$ – Palarran Dec 22 '16 at 23:38
  • $\begingroup$ Do you mean over generations (evolution) or within a couple hours to a year? $\endgroup$ – Zxyrra Dec 22 '16 at 23:50
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Let's steal some techniques from other creatures and apply them here. It's unlikely that mermaids would evolve these methods, but then again, it's unlikely that a humanoid mermaid would evolve.

Texture and color change

The mimic octopus has an external organ system that allows it to accurately portray one of more than 15 aquatic species! It's composed of two main parts, both of which will be useful to you:

  • Chromatophores are small sacs of pigments that can be controlled by some octopuses to adjust color. Mermaids don't need more than two groups of pigments - one for scales, if you so desire, and the other containing normal skin pigments - but you are welcome to include more if you want color-changing tails.
  • An additional organ or system of muscles that allows the skin to completely change shape and texture. Your mermaids can have an adapted version of this system, which - instead of being used for disguises - will switch between two states, scales and flesh, depending on which they prefer.

Physically making fins

Flying fish extend large, wing-like appendages only when needed - otherwise, they're drawn relatively close to the body. While this is not the best example, you could consider keeping the end of the tail all along, but retracting and folding it into a crease in the skin (or a slit-like organ) when not in use. While you could probably find a way to make a new tail fin every time, it's most feasible to hide it elsewhere.

For this process, it will be easiest to have thinner, flexible feet - or more poseable ankle joints.

Joining the legs

There aren't many animals that physically "join" two of their limbs - besides some octopi, which push them close together, but do not fully "join" anything.

Consider combining the ability to change texture, described above, with powerful reflexes and muscles to keep the legs as watertight - and streamlined - as possible.

Breathing

This is relatively simple. Three approaches will work well:

  • Combining both lungs and gills on the same body. This will allow efficient breathing in either phase, and Australian lungfish are able to do just that.
  • Relying only on lungs. Similarly to whales, dolphins, and several other species, you may not need to develop gills at all.
  • Breathing through the skin, which may work both above and below the water. Several amphibians are able to do so; it's not unrealistic to adapt that system, although it may be visible from the outside.
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Maybe for the "joining of legs"-part you could use a mechanism similar to velcro, which was inspired by burs, so that they are hold together. Then you could let the mermaid control if the mechanism ties the legs together and therefore act like some sort of fin or allows them to be used as separate legs.

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