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Quite simply question. In my story I am depicting merfolk as having lighter skin than even Caucasian people, contrasting to my human society which bears resemblance skin tone wise to Hispanics.

I've done this as from my understanding, darker skin color naturally forms when a group over long stretches of time is exposed to higher amounts of sunlight (as near the tropics.) It was my understanding then, that the merfolk living under the water (but still in the sunlight zones) would likely have at least lighter skin than the human population.

It has come to my attention however from speaking to someone that this may be an incorrect assumption. Further wrinkling this is that the merfolk are still living in the tropical zones of the planet.

So would merfolk have lighter skin due to less sun exposure? Would it have a negligeable effect, is it not this set in stone?

For the purposes of the question, let us ignore aquatic aspects of this question, imagining basically if normal humans were able to breathe underwater, how it would effect skin pigmentation.

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  • $\begingroup$ "Lighter skin than even Caucasian people": Actual Caucasians, that is, Daghestanis, Circassians, Ingushes, Chechens and so on are about "average white" people -- they have a lighter skin tone than Arabs, and a darker skin tone than Swedes. Caucasians sensu lato, that is "white people", have a very wide variety of skin tones, from quite dark in North Africa and Arabia, to almost translucid in Scandinavia and Iceland. $\endgroup$
    – AlexP
    Aug 3, 2022 at 20:32
  • $\begingroup$ ... For illustration, see a video (on Wikimedia) showing President Barack Obama, who is supposed to be "black", and King Salman bin Abdulaziz and Prince Mohammed bin Salman, who are "Caucasians". $\endgroup$
    – AlexP
    Aug 3, 2022 at 20:38
  • $\begingroup$ @AlexP I wanted to use a more academic term than simply "white." I apologize if you disagree. $\endgroup$ Aug 5, 2022 at 3:20

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The short answer is most likely.

There are two selection pressures that control the color of human skin: the amount of UV light received and the need to produce vitamin D.

Darker skinned human races have invariably formed near the equator. This makes sense, as the Sun's rays are generally much more perpendicular to the Earth's surface, resulting in high sun exposure. The addition of melanin protects the skin from UV damage.

Lighter skinned human races invariably form near the poles, however, this isn't due to simply to the lack of UV exposure, but instead, due to the need to produce vitamin D. Because there is less UV exposure near the poles, and on average just less sunlight in general, human races near the poles have faced a selection pressure opposite that to those near the equator. Namely, they have decreased melanin production to allow more UV into the skin for calciferol production.

This can be seen in the health data as well when humans are moved out of the environment for where they are adapted. Namely, when races from higher latitudes move near the equator, they have higher rates of skin cancer. They also dynamically adapt (i.e. they tan) to the new environment. Conversely, vitamin D deficiency rates are much more prevalent in those of dark skinned races when they move to higher latitudes (i.e. closer to the poles).

So what does all of this mean for your merfolk? Well, if they have to produce vitamin D then they are likely to be much lighter skinned. Water is an extremely good blocker of UV light, as a large amount of UV is reflected at the surface, and what makes it into the water itself is quickly absorbed and scattered.

Since merfolk live predominantly underwater, they are not as likely to be exposed to UV, so there would be no selection pressure for them to develop darker skin. However, if they, like humans, need to produce vitamin D, then they will face a strong selection pressure to develop lighter skin in order to maximize their usage of the available UV.

This would create a problem though, in that merfolk would not be able to spend much time at all near the surface of the water where UV exposure is the strongest. Humans with albinism (a complete lack of melanin production) are forced to take extensive measures to protect themselves from sun exposure, even for small amounts of time in the sun.

It is important to note, however, that even with complete lack of melanin production, albinos are not "snow white" in complexion. The color of human skin isn't simply a function of the amount of melanin available, but instead comes from complex scattering interactions through multiple layers of skin. Skin is actually quite translucent, and a lot of the color you're seeing is from light scattered through blood and viscera under the skin. It's one of the reasons a huge amount of research was required before CGI rendering of humans looked correct and not "rubbery."

That said, the ratio of Red to Green in human skin is highly conserved, regardless of their skin color, and computers can be easily trained to pick out humans in images by this ratio, regardless of their skin color, as even though blacks and whites look very different, if you look at the RGB ratios, you find the relative values of the numbers are more or less the same.

So if you want your merfolk to be "milk white," you will likely need another mechanism to cause it. However, you can use any number of selection pressures to push this. Blue eyes and blonde hair became prevalent in European nations not because they provided any sort of survival advantage, but because there was a strong selection pressure for them as their "exoticness" made them desirable. Similarly, you could say at some point in the merfolks' past, a single mutation was seen as "beautiful" to the locals, and strongly selected for from a sexual standpoint.

Or, just as easily, you could come up with some sort of survival situation where being "milk white" provided an advantage. Fish often have silvery scales that reflect light in order to confuse predators, especially when the fish are in a school. So perhaps your merfolk faced some sort of predator that was confused by "flashing white."

Others have also pointed out the bicolor pattern that is prevalent in fish. This pattern is not simply to confuse predators; it is a camouflage, which benefits both predator and prey. When seen from below, a bright white body creates no silhouette with the bright sky above, and when seen from above, a dark body creates no silhouette with the dark ocean underneath. Such a pattern has obvious advantages if you are trying to hide, as well as if you are trying to ambush. So if you are ok with your merfolk having dark backs but light bellies, then it wouldn't be out of place at all, as it would give them a survival advantage.

One thing to keep in mind, however, is that merfolk are presumably intelligent. And evolution doesn't work the same way on intelligent species as it does other animals. If you have an animal capable of creating culture, cultural evolution happens far more rapidly than natural selection, and so it is far more likely that your intelligent merfolk would have developed bicolor patterned clothing long before any selection pressures kicked in. This is all the more likely if your merfolk evolved from humans who spent long periods of time in the water, rather than "climbed up the foodchain" like humans did.

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  • $\begingroup$ worth noting cholecalciferol production is basically a non-issue is piscivores. Their diet supplies abundant sources. this is how cetaceans get away with basically no production. $\endgroup$
    – John
    Aug 4, 2022 at 4:31
  • $\begingroup$ Yes, I'm trying to decide what I want to really describe their skin color as. I don't think it's realistic for them to have like vampirically white pale skin, especially because I am going to have merfolk characters come to land in the story and while I will address sun exposure, I don't want them to be unable at all to be in sunlight. I am hesitant to make them just normal white people, as the circumstances of the story, where one can frame the merfolk as being oppressed by darker skinned people, may not go over well, and I'd rather avoid that if possible. $\endgroup$ Aug 5, 2022 at 3:22
  • $\begingroup$ @WasatchWind If you're not writing your story to fit the current sociopolitical narrative of the day, then you've already lost if you're tailoring because of perceptions. The stories that stand the test of time are relevant beyond the few years when they were written. Unless your goal is to make a social commentary of some sort, make your merfolk whatever color you want them to be. $\endgroup$
    – stix
    Aug 5, 2022 at 16:46
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They evolved underwater, mostly.

Most fish follow the pattern of lighter belly, so that from below they are partially camouflaged against the sky, and darker back so that from above they're camouflaged against the dark deep; thus:

Manta ray with a pale belly

Huffpost 2012, unknown photographer, fair usage.

manta back with darker skin, slightly mottled

Marine Megafauna Foundation 2022, fair usage.

That is for the case where a species is accustomed to being picked-off by predators from both directions.

If they've been top of the food-chain for some long time, then there's a chance that they may have gone slightly more piebald, but even the biggest sharks and whales retain the bi-colour pattern.

If they've spent a deal of time on-land where their sensitive bellies catch the rays (so to speak) then they'll likely have a base-colour darker than the white of the manta above - giving them a dark-ish belly and a darker back (possibly with mottling).

I needn't point out that the mottling and occasional stripe like the manta above might act to distinguish individuals, be a sign of fortune or have other cultural significance for them - whatever the writer choses.

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  • $\begingroup$ Very interesting. I will consider this. I don't desire to have hyper biologically accurate merpeople - I'm mostly going with the traditional design - but perhaps having them be lighter on the bottom and darker on the back can be a subtle choice :) $\endgroup$ Aug 3, 2022 at 4:03
  • $\begingroup$ The selection pressure in this case isn't just for prey. The reason sharks and other apex predators still have bicolor patterning is because it hides them from prey, which gives an advantage. Both great whites and orcas are ambush predators, so it makes plenty of sense for them to have the pattern, as an individual from either species that has a hunting advantage will succeed more often at hunting and be more likely to pass genes onto their offspring. $\endgroup$
    – stix
    Aug 3, 2022 at 17:59
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Because they evolved far from the equator/near the poles.

Gloger's Rule – states that warm-blooded creatures near the poles are lighter in colour than equatorial species.

The 'why' of Gloger's Rule is up for debate, but the empirical correlation is not. (Which is what a 'rule' means in science; it's not a theory.)

Add to that the fact that the water shields them from most UV light most of the time. Only 10% of UV light penetrates 5-25m under the sea. (Depending on which kind of UV light and which kind of sea, see: Tedetti, M., & Sempéré, R. (2006). Penetration of Ultraviolet Radiation in the Marine Environment. A Review. Photochemistry and Photobiology, 82(2), 389. doi:10.1562/2005-11-09-ir-733).

Melanin in humans is largely for UV protection – so in an environment with little UV, why would they produce melanin?

Do you have more details about where in the sea they're living? What depth, latitude, and conditions?

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  • $\begingroup$ The OP is asking "Would ....?" you are answering "Why ....?" $\endgroup$
    – L.Dutch
    Aug 3, 2022 at 10:23
  • $\begingroup$ "...so in an environment with little UV, why would they produce melanin?" Because they originally evolved it and it has no effect on their survival. Light skinned humans aren't light because there's no need for melanin, but because there is a need for vitamin D. The Inuit are somewhat dark skinned, but that is because they originally evolved in an area where they needed darker skin and moved to the arctic, but because their diet is naturally rich in vitamin D (primarily meat based), there was no selection pressure that could cause them to get lighter. $\endgroup$
    – stix
    Aug 3, 2022 at 18:42

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