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Mermaids are commonly depicted as mammalian, human-like creatures from the waist up and scaly fish-like creatures from the waist down.

What properties (color, texture, etc) would their mammal skin likely evolve to have while living under deep water?

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    $\begingroup$ Are we assuming a mermaid species that has evolved from terrestrial humans who moved back into the ocean? $\endgroup$ – Arkenstein XII Jul 4 '19 at 20:22
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    $\begingroup$ given that human skin is not meant to be soaked for extended periods of time and in fact breaks down when immersed in water for more than a few days its likely mermaids have scales not skin. see here: sciencefocus.com/the-human-body/… $\endgroup$ – BKlassen Jul 4 '19 at 20:55
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    $\begingroup$ (a) Bringing the politics of discontent into the question reduces the value of the question. The site is focused on helping you build your fictional world. Everyone else's opinion about someone else's world is irrelevant. (b) The depth and nature of the ocean contributes significantly to evolution. Please provide a location (pref. longitude/latitude) as a reference for this discussion. (c) Like every other "movie monster" in history, there are a lot of definitions for "mermaid.", please describe your mermaid. $\endgroup$ – JBH Jul 4 '19 at 21:13
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    $\begingroup$ Finally, note that asking specifically about a commercial third-party world is off-topic for this site. Such questions would be better asked at Science Fiction & Fantasy or Movies & TV. $\endgroup$ – JBH Jul 4 '19 at 21:14
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    $\begingroup$ worldbuilding.meta.stackexchange.com/questions/7334/… The question does not ask about how that franchise handles mermaid anatomy, so it does not violate the 3rd party exclusion. That only applies to questions about cannon, and since JacobPariseau is not asking why mermaids evolved to be those colors in the Little Mermaid, it seems on topic to me. $\endgroup$ – Nosajimiki Jul 5 '19 at 13:55
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Assuming you're asking about human skin, not a fictional blubber.

It depends on the depth at which they live

Melanin (a brown pigment found in human skin) is a natural form of sunscreen, it is also the only colouring in human skin (pinker skin comes from oxygenated blood near the surface of the skin).

The graph below shows the absorption of different frequencies of the electromagnetic spectrum. Melanin aims to block UV (the left most section) where we can see that, per meter of depth, water absorbs a lot of this.

Water absorption of radiation

So, just as different skin colours naturally developed depending on sun exposure on the surface it would also depend on what depth the mermaids lived (much less exposure the deeper you go as seen by the graph above). If the mermaids regularly swim above the waves, however, they may well need more melanin to combat not just the sun from above but also that reflected from the surface around them.

So if the mermaid needs to surface regularly they would have darker skin, if they live below the surface they would likely be pale skinned.

The answer would be different if you want your mermaids to have blubber other adaptations but, given the question is about the little mermaid, my answer focuses on human skin.

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You say they live in deep water? Well, A) there's only so much I can do if I don't know the depth since it depends on depth, and B) it's likely mermaids will look a lot different than they are commonly depicted.

Seals are mammals, but they don't live deep underwater. In fact, bottlenose dolphins (another species of marine mammal) "typically don't go deeper than 150 feet." (teacher.scholastic.com) As far as I know, marine mammals do go deep underwater (like the sperm whale) but they do not live there.

That being said, even with my little understanding of biology, I can make some guesses. As Lio said, your mermaid's upper bodies will be pale. The skin there will likely be smooth and rubbery, like a dolphin's, and be just as thick-15 to 20 times thicker than a regular human's, with a third layer of modified skin serving as blubber.

So while these mermaids will be streamlined, as they need to be hydrodynamic, don't expect them to be skinny-bitty! But what about the color of their scales? It turns out, deep-sea life follows a strict color pattern:

  1. Blue near the surface
  2. Slightly deeper: blue on the back, white on the underside (like a whale).
  3. At great depths, most creatures are transparent, with red stomachs
  4. Below those great depths, animals are entirely red or black
  5. Those at the very bottom are a pale red or cream color

Considering mermaids would have evolved to live in the sea, they would likely follow this pattern. That being said, there are plenty of colorful fish in Levels 1 and 2 that don't, so who knows? They might look like mandarin fish!

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