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I'm making a species that turns the outer layers of its skin to stone, for defense purposes. It'll also have eyes that glow yellow, due to ingesting sodium vapor or helium. Is there some way to combine two processes, or do I just need to make the planet's crust or atmosphere abundant in one of the two?

Edit: would it be possible that life in mineral-rich water in caverns could lead to a build up that the creature takes advantage of? Because I've narrowed my ideas down to either that, or it forms chitin in rocky formations both as defense and as camouflage.

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  • $\begingroup$ What are you actually asking? How to chemically justify stone skin and yellow eyes (which we can't do, you know that, right?)? Are we dealing with silicon-based life forms (rather than our own carbon-based life forms)? Are we trying to establish the chemical relationship between the creatures and the planet? What is your goal and why are you seeking it? $\endgroup$ – JBH Jun 15 '18 at 4:09
  • $\begingroup$ Possible duplicate of How could something like a Gargoyle evolve? $\endgroup$ – Aify Jun 15 '18 at 7:35
  • $\begingroup$ @Aify, that question asks about a whole stone body, this one only the skin. $\endgroup$ – L.Dutch Jun 15 '18 at 8:09
  • $\begingroup$ Do you want the skin to turn to actual honest-to-goodness stone, or just hard, grey and rugged? Knowing what kind of anatomy it has would be helpful as well. $\endgroup$ – SealBoi Jun 15 '18 at 9:33
  • $\begingroup$ @L.Dutch the linked question has an accepted answer which basically answers this question, making this one a dupe $\endgroup$ – Aify Jun 15 '18 at 15:43
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If you want to have stone skin, you need to put the proper minerals in the skin layers.

Of these minerals, sodium can be one, but helium not. This because helium is a noble gas, and has practically no reactivity with other atomic species.

Also, sodium vapors and helium do not glow on their own, but need to be excited to emit light. This usually happens by applying a strong electric field, which is tricky to achieve in living organism. You can pursue something similar to electric eel, but that would give pulsed glow, not continuous.

On top of this, sodium alone is pretty aggressive, and hardly compatible with living organisms.

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  • $\begingroup$ Well, this is an alien species on an exotic planet, and nothing is really set in stone (pun not intended), so if there's a plausible way i could have strong electric fields eminating from this creature, how? Also, could it be possible the species has adapted to take advantage of advantage of a form of Scleroderma, where only the skin hardens, but not with any of the issues commonly associated with this disease? $\endgroup$ – Geoff Jun 15 '18 at 3:10
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The problem with transformation is, in your case, that it must reverse. This calls for multiple instances of chemicals substances and a wasteful use of energy. It is just too complex and wasteful for being an evolutionary trait, which selects simpler and efficient.

So the answer to your question is: have your species' skin covered with a secondary outer layer of interwoven fibers, transpirant but capable to fend off the fangs and claws of predators.

As for the luminous eyes, another no good: If they become a source of light themselves, they will interfere with the person's vision. I'd suggest you go for augmented light receptors, so to give them a perfect night vision. Their weak point would be, when used in this way, a sudden lightning that would temporarily blind them.

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  • $\begingroup$ Good answer. I think if you compare the complexity of a "hardening" system and the huge energy it needs to use and the possibility of just being faster and running away then i see little evolutionary benefit in the first one. If it's about defense against a predator the first is definitely a weak option. $\endgroup$ – ArtificialSoul Jun 15 '18 at 10:08
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There is already a group of animals that do this, echinoderms specifically the starfish.

Their skin is composed of thousands of bony ossicles mixed with a special connective tissue called "catch connective tissue" which can turn from soft and flexible to rigid in a few seconds. This is the reason when you pick up a star fish at the beach it is all stiff and hard, they use this for both defense and to make themselves stronger to pry open bivalves. How this tissue accomplishes this drastic change is not well understood but it is a real change in the mechanical properties of the tissue that goes down to the nanoscale. Cooler yet it is under nervous control, meaning your humanoid could have it under voluntary control.

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"Is it possible for the skin to harden into rock?"

I could only imagine this closer to an insect or alligator snapping turtle with light reflective eyes.

The skin could have been thick not made of stone, it may look like stone, or it could form a rock like chitin. So it's possible I think. Human skin could also underwent Keratinisation for example if that what you meant by hardening.


"Can its eyes glow?"

Eyes can glow, but they will be blind to the light spectrum they are emitting. Since vision works by absorbing light spectrum. Eyes that are reflective to light are pretty common among predators however, they called it Tapetum Lucidum.

Still, if it must to sacrifice their sense for that spectrum, then they may glow by means of either photo luminescence (absorbing light), or bio luminescence, they both do not require Helium. Sodium compound may emit light, but may only do so within ultra violet spectrum.

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Your species could turn the outer layer of its skin to stone by sticking sand and stones to its skin. The caddisfly larva does this.

http://bugguide.net/node/view/1302204 caddisfly larva

This could be reversible too.

If you want this process to be fast, your creature could store small stones and sand under the skin in pockets then push them out onto the skin surface when needed - sort of like a cats claw is pushed out when needed.

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