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In some environments (think of the Faerunian Underdark for a fantasy example, or a planet orbiting a pulsar for sci-fi folks), high-energy (UV and up, but I'm mostly concerned about UV and very soft X-rays here as hard gammas are going to make animal life impossible anyway) radiation predominates over visible light in the environment in question.

Assuming that other issues (food chain is solvable, see this answer for details)) are taken care of, would terrestrial animal (or better yet, sapient) life in that environment evolve melanism (i.e. very dark skin) as a result of the radiation-heavy environs? Or would this be counterproductive as an evolutionary adaptation? Am I already talking about a point where the environment is too radiation-rich for even a basic lizard to evolve, never mind intelligent, terrestrial life? (Oceanic life has it easier as water is a pretty good radiation shield, so I'm putting it out of scope for this question.)

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Fungi adapt to ionizing radiation by expressing more melanin - they are "melanized". This form of adaptation has been noted in a variety of environments.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2677413/

So if you are talking about a fungus, yes. But fungi might be doing something different - they might be using melanin to capture energy from the radiation and then use it for their own purposes.

http://www.asmscience.org/content/journal/microbiolspec/10.1128/microbiolspec.FUNK-0037-2016

In vitro work shows that introducing melanin into animal cells decreases damage from ionizing radiation.
https://www.ncbi.nlm.nih.gov/pubmed/10789895

This would lead one to expect a survival advantage for an organism with more melanin in the context of ionizing radiation.

Given what an easy experiment this would be to do, I was surprised how hard it was to find results. I finally did. http://www.rrjournal.org/doi/abs/10.2307/3578834?code=rrs-site

Melanin does protect against mutational damage from ionizing radiation and higher levels of melanin are associated with less damage in experimental organisms exposed to radiation over many generations.

/would terrestrial animal life in that environment evolve melanism as a result of the radiation-heavy environs/. They might, unless some superior method evolved that took away the adaptive advantage of melanism. Of note is the fact that UV does not penetrate very deeply and so the skin / integument is the place animals accumulate melanin. Because ionizing radiation does penetrate one might expect a more generic distribution of melanin throughout the cells of the body; i.e. they would be darker through and through.

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Short answer:

No.

Longer answer:

Firstly, melanism tends to develop within the natural world as a form of camouflage from other animals, predator or prey, rather than by light exposure, so it's more a matter of whether or not melanism would help the animal avoid detection.

Secondly, if UV is the dominant form of radiation, then it is more likely that, like bees, the lifeforms of this universe will see in that part of the spectrum, and as such will likely appear transparent or blandly coloured to us. If the radiation is strong, they will likely develop skin tissue of a 'colour' that reflects most UV light as white reflects most visible light. Given the high energy of UV, this seems quite likely to me. As such, melanism, or its equivalent within the UV band, is even less likely because they would be absorbing rather than reflecting the radiation.

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More than melanism it is likely that they would develop a suitable exoscheleton, to act as physical barrier against the radiation.

Life on heart developed first in water, where the UV radiation from the sun was reduced to a bearable level. Then, as ozone started to develop and filter UV, life migrated on dry land. If melanism would be a "tweak" to tolerate high UV, life on our planet would have already used it.

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