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The Eternal Arms Race Between Weapon and Armor

This world has a race of giant crabs who have developed armor sufficiently strong that the top predator's teeth can't puncture the armor. In response, the top predator has taken the next evolutionary step of developing specialized hammer shaped teeth and extremely fast closing jaws. The jaws close fast enough to create shockwaves in the crustacean's armor that generate high speed fragments that bounce around for a bit. (In modern tank armor, these high speed fragments are called spall). Spall damage to the crab's muscles and organs slow it down enough for the predator to finally kill the crab.

This is somehow familiar...

We've seen this kind of evolution in the development of HESH ammo in WWII by the British and the use of warhammers against late medieval armor plate. If puncture attacks don't work, go for percussion. For human carried plate armor, there was no counter as firearms made armor obsolete. In response to HESH, tanks and other armored vehicles acquired spaced armor and spall liners.

How likely is it that these giant crustaceans would develop the equivalent of spall liners inside their shells to deal with the spall generated by the predator's bite? If they don't develop spall liners, what would be the simplest and cheapest evolutionary change combat the predator's bite?

Out of Scope:

  • We are ignoring the scaling problems of getting giant crustaceans. There are lots of reasons why we don't see these on earth but we are ignoring those problems. These crustaceans just get big.
  • How the predator bites so quickly. It just does.
  • None of these creators are designed, nor can the changes made to them be designed. Whatever countermeasures the crab develops must work and provide benefit at all times during the evolutionary process.

Originating Question

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    $\begingroup$ ""Why do we even HAVE the square-cube law? I wish giant monsters were as impossible as they're SUPPOSED to be!"— Atomic Robo $\endgroup$ – Schwern May 16 '18 at 18:27
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    $\begingroup$ Why did you link to the wikipedia article on crabs? Are you assuming the general audience has never heard of a crab before? $\endgroup$ – Sydney Sleeper May 16 '18 at 22:09
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    $\begingroup$ Theres something so entertaining about many of these questions. As for the square cube law, just assume some kind of metamaterial is used for the shell. Living creatures consist out of carbon for the most part, imagine if the creature can lock in large amounts of Carbon Nanotubes for high-strenght shells and relatively lightweight because of it. Continuing that trend, spiderwebs are definitely biological and can be made for 90% out of water. This could be your spall liner as well as strengthen ligaments and muscle attatchments. $\endgroup$ – Demigan May 16 '18 at 22:52
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    $\begingroup$ I'm not even sure why you bring in the size at all. Why can't they simply be normally sized crabs, protecting against the largest surviving top predator in your world - the weasel! (or a seal, maybe more apt for crabs) $\endgroup$ – pipe May 17 '18 at 8:16
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    $\begingroup$ If you're curious, apparently I'm creating something to eat your crabs. Battle on! $\endgroup$ – Anoplexian - Reinstate Monica May 17 '18 at 16:02
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Crabs already have composite armor. So do you.

The jaws close fast enough to create shockwaves in the crustacean's armor that generate high speed fragments that bounce around for a bit. (In modern tank armor, these high speed fragments are called spall). Spall damage to the crab's muscles and organs slow it down enough for the predator to finally kill the crab.

Spalling occurs when a rigid surface armor has only air behind it, a free-surface. An armored vehicle is mostly air inside. With nothing behind the armor to absorb the shockwave, it sort of "splashes" out. A spall liner is something tough but yielding, like Kevlar, to absorb the shockwave. Or composite armor with layers of different material to break up the shockwave.

enter image description here

Source

Unlike a tank, which is mostly air inside, a crab is mostly fluid and flesh inside (citation: your local crab restaurant). The arthropod cuticle is really like composite armor with several layers.

enter image description here

Source

  1. The epicuticle, a waxy layer to hold moisture in.
  2. The exocuticle, "the shell".
  3. The endocuticle, a laminated structure of layers of interwoven fibrous chitin and protein molecules.
  4. The epithelium, a layer which lines organs, muscles, and cavities.

This, plus the squishiness of the organs behind it, acts as composite armor to absorb the shockwave and prevent spalling.

Vertebrates already have a spall liner, of sorts, inside our skulls. The brain isn't in direct contact with the skull, instead there is cerebrospinal fluid and various other cushioning material between it and the hard skull. This provides cushioning for light impacts. A concussion is when you get hit too hard for the cushioning to absorb and your brain bangs against your skull.

enter image description here

Source

Finally, this paper title I found while researching this is too good to not mention: Strength Analysis of Human Skull on High Speed Impact.

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Schwerin gives a great explanation of how the internal arrangement of most creatures provide protection, but you are also looking at ways to help defeat what is essentially a smashing attack against the outer shell.

Probably the fastest evolutionary response would be for crabs with more elaborate shells to survive these attacks, since the protruding ridges and arabesques will deny the hammer a flat surface to strike, deflecting the hammer entirely or reducing the blow and increasing the time the impulse will be received (as the bits and pieces get crushed or broken off by the strike).

enter image description here

You can elaborate from here

This can be doubly selected for, if male crabs use elaborate shells as signalling mechanisms for their mates. Female crabs will be attracted to males with elaborate shells, and since elaborate shells provide more protection by deflecting or absorbing the hammer blow, crabs with the most elaborate shells will survive to reproduce, bringing generations with even more elaborate shells into the population.

Of course, evolutionary arms races are, well, evolutionary, so the predator will likely strengthen the skull, increase muscle mass and leverage to the jaws and exchange the "hammer" for penetrating "spikes". The last time this happened in Terrestrial history, we got this:

enter image description here

All hail to the King

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Soft shells.

Your predator has traded cutting / piercing teeth for teeth that produce a shockwave through a rigid surface on impact.

The defense: dispense with the rigid surface. The crabs are flexible and soft. The hammer teeth lose their energy with a slowed deceleration into the softness.

hulk punces Blob http://www.killermovies.com/forums/archive/index.php/t-574442-blob-vs-punches.htmls

And better yet: the crab is sticky. Once the predator has triggered the spring that releases the energy to produce its shockwave attack, its jaws are just jaws - that are lodged in a lot of blubbery crab flab. Just behind the jaws is the neck. The best defense is a good offense, and crabs have a good offense too.

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  • $\begingroup$ However, there comes a point where your former crab is no longer what you could sensibly call a crab. Also, with the soft skin instead of a hard shell, why would the predator now have evolved the shockwave-spring attack? That would just be silly and ineffective. $\endgroup$ – pipe May 17 '18 at 8:54
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    $\begingroup$ @pipe: read up on the Red Queen Hypothesis - en.wikipedia.org/wiki/Red_Queen_hypothesis. Evolution is dynamic. Crabs have hard shells. A predator with the shockwave attack does well against these and has higher fitness than its conspecifics and so leaves more offspring. A crab with a mushy shell is resistant to the shockwave predators and so it has higher fitness and leaves more offspring. Maybe there is a stable balance between the mushy shell crabs and the hard shell crabs because each resist different predators: the crabs are splitting into 2 species. $\endgroup$ – Willk May 17 '18 at 22:41
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Spall liners actually aren't that unrealistic.

Having a soft covering that will stop shrapnel can be done pretty easily with muscle fibers covering the inside of the shell.

The main issue I see is that you don't just need a way to save the crustacean, but also a way to escape/injure the attacker. Maybe make the shells poisonous, or spiky and poisonous. Maybe the crustacean evolves to be faster than the predator, or able to reach places the predator can't.

Protection is good, but a deterrent is better.

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    $\begingroup$ Good idea, but instead of muscle they can use a thick layer of bristles. Some species already produce them, and they can become irritants like a tarantulas bristles. $\endgroup$ – Dan Clarke May 17 '18 at 0:18
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How likely is it that these giant crustaceans would develop the equivalent of spall liners inside their shells to deal with the spall generated by the predator's bite?

Unlikely. There are too many mutations which would not be immediately selected along the way involved. It would take millions of years, and in the meantime other mutations might arise that would render it useless, so they might never get anything close to those liners.

If they don't develop spall liners, what would be the simplest and cheapest evolutionary change combat the predator's bite?

A symbiotic relationship with a particular wild and powerful type of cavities bacteria. Crabs have no teeth so they get no cavities, and the bacteria might thrive on the food leftovers that the crab rubs on its carapace.

So a single crab will not deter an attack from a predator. But once the predator bites a crab, they will be infected. The bacteria could destroy the predator's teeth faster than they could regenerate, and in a few days could cause them to become unable to hunt. Other crabs would thus be spared and the crab species would thrive.

Of course, natural selection will favor the more resistant predators. But keep in mind that bacteria will become an extra player in the game, and they evolve faster than more complex organisms.

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What the heck - let's imagine that the crabs develop reactive armor. The predator goes to bite down on the crab, and the portion of the crab's shell being bitten blows up in the predators mouth, knocking out teeth, eyeballs, and brains (such as they are) in one go. To paraphrase the famous radio message from the battle of Midway: "Scratch one T-Rex!!!" :-)

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  • $\begingroup$ That's not how reactive armor works, but +1 for the image of a very confused and sore T-Rex! $\endgroup$ – Schwern May 17 '18 at 6:05
  • $\begingroup$ I was going to post reactive armor but got ninja'd. For further fun, I was going to suggest that the explosive material was an evolutionary jump from a previous skin layer that was poison - much like some South AMerican frogs have IRL. $\endgroup$ – Carl Witthoft May 17 '18 at 17:48

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