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How can one explain the ability of a creature to harden their skin to a solid strong enough to fend off predator bites?

The skin must go from being elastic and flexible to being a biological shield of hard tissue capable of resisting a shark's bite.

The creature is able to decide whether to harden it's skin only in localized areas or across all of the body's surface.

It mus also be able to restore the skin to it's natural elasticity and softness once it's safe from predators.

If such a process is possible, how can it be explained and how fast can it happen?
Days? Months? Seconds?

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  • $\begingroup$ I guess you're looking for shear-thickening fluids, though that still leaves the question of the container... $\endgroup$ – Mephistopheles Jun 5 at 19:58
  • $\begingroup$ @Mephistopheles I was thinking of muscles made of some metallic fibers which can be contracted to be a full solid ....but this seems the least energy efficient way and I don't even know if a muscle made out of solid metals would be flexible at all and probably it would not bee skin but chainmail. And I want the skin to be soft and vulnerable when the creature is not hardened. $\endgroup$ – user76252 Jun 5 at 20:12
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    $\begingroup$ So you're saying you want a souped-up version of the sea cucmber's skin? If I'm not mistaken, their skin can go from soft an squishy to as strong as some plastics. $\endgroup$ – ProjectApex Jun 5 at 20:24
  • $\begingroup$ @ProjectApex I don't think if Understood the article correctly but it's basically web that changes structure to be less penetrable, is it? Could it produce something even harder? $\endgroup$ – user76252 Jun 5 at 20:34
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    $\begingroup$ Well they do it through an association of collagen fibrils. Apparently they're making it to make a similar material which can go from soft as rubber to plastic as hard as a CD case, so while I can't say for sure, given we have plastics as strong as steel, I don't doubt it's possible to have something similar. $\endgroup$ – ProjectApex Jun 5 at 20:42
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Already exists

Echinoderm skin already does everything you want. Starfish are your best match, they have both fast hardening and very hard structures when hardened.

The skin must go from being elastic and flexible to being a biological shield of hard tissue capable of resisting a shark's bite.

Larger sharks can bite though starfish but nothing on that large a size difference is going to survive. this really comes down to how big the shark is and how big the creature is.

The creature is able to decide whether to harden it's skin only in localized areas or across all of the body's surface.

Enchinoderm skin hardening is controlled by nerve impulses so completely doable.

It mus also be able to restore the skin to it's natural elasticity and softness once it's safe from predators.

Again echinoderms already do this.

If such a process is possible, how can it be explained and how fast can it happen? Days? Months? Seconds?

Enchinoderms harden their skin at nerve conduction velocities, so on the order of seconds to minutes under the worst case, softening it takes slightly longer but not much longer. the speed varies quite a lot from species to species so you can pick the speed you want.

The downside is we don't completely understand how echinoderms do it, so you can't get to detailed in how in functions. We know their skin is made of millions of tiny calcium carbonate plates and the collagen fibers that connects them can somehow change its mechanical properties, going from flexible to remarkably stiff in seconds, but that is it. It is sad that these truly weird creatures don't get a lot of funding for study.

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    $\begingroup$ I love that half the time when someone wants to cook up a fantasy creature, the answer is "nature already does that" - TIL more about starfish, thank you :) $\endgroup$ – Syndic Jun 6 at 17:28
  • $\begingroup$ @Syndic often the choices are, nature already does it or evolution and physics will not allow it. I love this site for the very few that don't fit in either category. $\endgroup$ – John Jun 6 at 21:16
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You can engineer a very thick skin made up of very small and tough chitin scales, within a liquid/gelatinous matrix. Normally, this would be very flexible, since the scales do not touch each other and even when they do, they just slip past one another.

Inside, you have a layer of specialized suction organs with a rigid chitin "ring" skin-side, and a muscular sac on the inner side, strongly connected either with the skeleton or to one another through rigid tendons (both choices have pros and cons).

When one or more sacs inflate, they cause a depression and suck the matrix out of the skin layer immediately above (so, the effect can be localized). The skin compresses, and the scales lock against each other, forming a solid shield which is almost as hard as chitin, but not as fragile (it cannot "break"). The creature can even move while hardened, by briefly releasing as little pressure as needed and immediately locking it back. Not knowing beforehand which area is going to soften, a predator would have little chance of doing damage.

This is, in some ways, the opposite phenomenon to soil liquefaction. There, an abrupt dispersal of gel-liquid within "solid" ground turns it into a flexible putty; here, the dispersal is the normal state, and it is the abrupt removal of the filler that turns the skin into a "solid".

Something very similar happens with "Liquid Armor", which is turned solid by the application of a magnetic field that orients the liquid particles to increase their viscosity. Your creature's skin would be barorheological - it would respond to the inner pressure (you could have a magnetorheological armor if you went for a completely different biochemistry, with a ferrofluid-like-containing dermis and some organic magnets inside. If the magnets can rotate by 180° and are arranged in a Halbach configuration, the "skin" side could be hardened or softened almost istantaneously. How an organic creature comes up with this much iron and magnetite remains to be determined).

Another, slower way of obtaining the same effect would be through osmosis. The inner layer could "suck" and accumulate water out of the outer layer, turning it into a solid.

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    $\begingroup$ So its a slime that can turn into solid? $\endgroup$ – user76252 Jun 5 at 20:58
  • $\begingroup$ More or less, yes. Depending on the characteristics of the gel, it needn't be a slime - it could even be covered in some kind of thin leathery skin, not unlike human epidermis. In this case the teeth of a predator would cut through the epidermis, but would be stopped by the hardened dermis. $\endgroup$ – LSerni Jun 5 at 21:05
  • $\begingroup$ water/cornstarch mixtures (oobleck) are shear-harding fluids. In these they are normally liquid, but when hit or suddenly pierced they will harden. Unfortunately a slow piercing is unlikely to make it harden $\endgroup$ – CSM Jun 6 at 11:34
  • $\begingroup$ @CSM yes, but shear-hardening fluids need, in fact, shearing. They can't harden "at will" as was requested. This is probably the next best option. $\endgroup$ – LSerni Jun 6 at 12:49
  • $\begingroup$ @LSerni alternating shear will suffice, if I understand correctly. Mandrake plants can produce ultrasound via cavitation (and will readily do so when agitated), would (a reasonably louder version of) that be able to harden the skin? $\endgroup$ – John Dvorak Jun 6 at 16:34
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There is a structure that has evolved independently several times in nature that fits some your requirements. Whether it could be made practical for defense is something that has yet to be seen in nature as existing examples are notoriously fragile. Filling with fluid causes fibres to interlock and stiffen making a very soft organ significantly harder and very resistant to bending or twisting. Your creatures skin could be lined with similar fibres and push blood to the surface hardening its skin. Maybe additional adaptations could give it resistance to different forces like slashing or stabbing.

https://academic.oup.com/icb/article/42/2/216/652610

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