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In my world, there is a species from the Homo genus named Homo maritimus (which means marine human) (there are still humans, just not Homo sapiens). They are commonly named merfolk. However, they do not look the typical descriptions of merfolk: they have blubber (vascularized fat), they do have hands and feet, both with digits, but they are webbed (I want to go on a realistic way), and adults who do not have dwarfism, neither gigantism, nor who are morbidly obese, neither gaunt, are as large as beluga whales.

Compared to humans, they have a better sense of hearing, of smell (they can smell underwater like star-nosed moles), and of eyesight (they have four types of cones: red, green, blue, and ultraviolet), but a worse sense of taste, and of touch.

The reason why they can see ultraviolet light is because of a duplication of the OPN1SW gene (also known as the blue-sensitive opsin).

So, I wonder if there could be any advantages for being able to see ultraviolet light for any mammal from the Homo genus.

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    $\begingroup$ The novel opsin is both unnecessary and insufficient to see ultraviolet. Our plain old opsins work just fine in ultraviolet. The problem is that the human eye lens absorbs ultraviolet; ordinary people acquire the ability to see near ultraviolet light if they have their ultraviolet absorbing lenses removed during a cataract operation. The homines maritimi need is a lens which lets ultraviolet light through. This, of course, has side effects, meaning that more changes would be needed to avoid having their retinas burned. $\endgroup$
    – AlexP
    Commented Sep 29, 2022 at 16:27
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    $\begingroup$ related worldbuilding.stackexchange.com/q/202958/30492 if not even duplicated. $\endgroup$
    – L.Dutch
    Commented Sep 29, 2022 at 16:29
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    $\begingroup$ Trivia: both babies and people with cataract surgery see further into the violet. As we age, the proteins in the lens denature some from the UV $\endgroup$
    – UVphoton
    Commented Sep 29, 2022 at 23:05
  • $\begingroup$ Do you have a specific goal in mind with this question? Any number of evolutionary pressures, including random luck, may result in humans comprehending UV light. Is there a point to needing millions of years worth of insight? In what context will you use this information? (I've never had a reason to know why humans evolved tonsils. Even medical doctors might not need to know.) How would you judge a best answer? Can we edit this question to be more in line with the help center rules? $\endgroup$
    – JBH
    Commented Sep 30, 2022 at 2:30
  • $\begingroup$ Sight is either about perceiving predator threat or about seeing food sources. Identifying which could help drive the story. Personally, I think that seeing heat sources would be of better use. $\endgroup$
    – David R
    Commented Oct 1, 2022 at 15:04

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Ultraviolet Fish

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Some of the fish look the same to visible light but look different in the UV spectrum. Some of the fish are poisonous. Some are tasty. Some are tasty and not poisonous. The mermaids need to know the difference.

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  • $\begingroup$ I agree this the logical answer, though it does beg the question why the fish are ultraviolet themselves. Poisonous fish would want to broadcast that fact to as many potential predictors as possible and that means using a wavelength that most marine creatures can see to indicate it. $\endgroup$
    – dsollen
    Commented Oct 27, 2022 at 16:22
  • $\begingroup$ @dsollen I don't know the answer to that one. But there presumably is a good answer because in the real world some flowers and birds have patterns in the UV spectrum that are invisible to humans. $\endgroup$
    – Daron
    Commented Oct 27, 2022 at 16:27
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I partially agree with Daron's answer, the only reason to evolve ultraviolet sensitivity is if it's being used to either avoid predation or find food.

However Daron's explanation of figuring out if an animal isn't poisonous by using UV light doesn't work because poisonous animals want to broadcast that fact in a manner most marine animals can see, ie traditional visible light spectrum. In fact most animal coloration is intentional and driven by the eyesight of other animals, so it's unlikely for an animal or plant to evolve an intentional UV coloration since there is no point to wasting resources creating coloration most can't see.

So instead of intentional broadcasting I'd go the exact opposite, failure to hide. I imagine the adaptation for seeing UV light would be due to a species that relies on camouflage, either a prey species hiding from your merfolk or a predator actively hunting them. This species is hard to see with visible light spectrum due to their evolving coloration that blends into their surroundings. However since most species can't see ultraviolet light your camouflage species hasn't evolved proper camouflage in that range. In other words they 'blend in' in visible light, but stand out when looked at under UV light.

This would imply a few other thing about the species. First they likely live in shallow waters, since if you go too deep in water UV light is blocked by the water above. Second they would likely be bottom dwellers, living on the seabed near the coast, since they need something to blend into to hide. This species would also need to be a staple of merfolk life, either something that they regularly eat or that regularly hunted them in the past, for them to evolve UV sensitivity just to find this one species. On the other side of the spectrum the species is not entirely dependent on humans and either is hunted by, or hunts, many other species otherwise the species would have evolved UV camouflage to combat the merfolk's ability to see them.

If you really don't want to make up a species like this you could always imply the species has gone mostly extinct precisely because their primary prey, or predator, humans evolved an adaptation that made it possible to see them and that the merfolk's UV sensitivity is now mostly vestigial and simply hasn't adapted away yet I suppose.

Going a step further in theory the merfolk may have adapted UV sensitivity due to a species that is long extinct, but after evolving it found other uses for it. I said most species don't have UV coloration because most other species can't see it, but if merfolk had already evolved the ability to detect UV light then it now makes sense for merfolk to likewise evolve coloration in the UV spectrum. The primary reason UV sensitivity is important now may be because other merfolk have UV coloration that they use either for communication or, more likely, as a sexual display somewhat similar to a peacocks tail. Modern merfolk may see UV precisely so they can fully 'see' the intricacies of their fellow merfolk even if the original driving motivation for the UV sensitivity is long extinct. Imagine the merfolk ladies admiring how ultra violet the chest of a mer-man is the way you may hear homo sapiens admiring a man's abs.

As mentioned in comments humans explicitly don't see UV to protect our eyes. However, when underwater the water above a merfolk would already help to filter some of the more intense light so it's possible merfolk could see UV light underwater safely. I imagine they likely would have something like a see through eyelid that closes shut when they come above water to provide protection against the stronger light above water. This would of course imply they can't see UV light above water unless they intentionally 'open' their eye to see it, and in so doing they would risk pain or being blinded by bright lights.

As an aside be careful not to make your merfolk superiors to humans, evolution is about trade offs, they can't be better then homo sapiens at everything. For example better hearing doesn't seem like a necessary evolution for a marine species since sound already propagates much better underwater then above. Merfolk likely would be able to hear 'well enough' for their evolutionary needs underwater even if they had worse hearing then humans above water. Of course you can handwave this away by saying they use sound over long distance underwater to communicate, much as whales do, with an implication that their improved hearing would likely be slanted towards a very specific type of hearing like infrasonic. But my point is less hearing specifically and more a general caution to avoid the standard fiction mistake of creating species that have only advantages over humans. Make sure your merfolk have disadvantages and things they would struggle with out of water or you have to explain how they didn't outcompete regular humans if they are superior to us in every way and able to survive in more niches.

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Sight being a sense that Homo sapiens heavily relies on, improving it to include ultraviolet will provide an advantage. For Homo maritumus, the advantages will depend of the light conditions, which depends mostly of the depth this species lives. The deeper, the less useful sight will be, even ultraviolet ones.

However, the advantage is to balance with some drawbacks. Ultraviolet light is energetic and can damage tissues, leading to cataracts, corneal burns, macular degeneration, pterygium and skin cancer around the eyelids. UV-A cause damage to the central vision while UV-B damage the cornea.

To conclude, UV-vision does not provide a tremendous advantage and has serious drawbacks. This probably explains why it restricted to few species, obviously including the world champion of eye complexity: the famous Mantis Shrimp, who even manage to see polarized light on top of UV on its 16 types of photoreceptors.

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Different wavelengths of UV are absorbed to different degrees by water. About 60% of UVB that is responsible for sunburn is stopped by 1 meter of water. Some wavelengths penetrate more, some less.

It means that, during daylight hours, merfolk might be able to sense depth by the degree to which the UV penetrates. Water at 1 meter would be this color, and 10 meters that color. (The colors being colors we humans can't see and so have no word for.) Knowing you are in range of a surface critter's claws might be very valuable when swimming along looking predominantly down. Down where the food is.

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