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I have devised an alien species for a universe I am working on. They originate from the same planet as my high-pressure creatures (What would happen to a high-pressure creature in a low-pressure atmosphere (50atm)?), but can survive comfortably in a much broader range of environments due to their robust nature (they have survived nearly every mass extinction that the planet has to throw at them and can even survive for extended periods of time submerged in boiling water, freezing water or exposed to the vacuum of space. Think of them as being a little like tardigrades).

Although they did evolve on this particular planet they have since been transported around the universe and are currently taking up residence inside of ships and space stations that are made of metal.

The idea behind them is that they are essentially a pest species - not dangerous to any organic life form, but making their living eating away at the hulls of such ships and stations, often causing massive damage and even hull bursts.

They possess a tough exoskeleton made of aragonite (like a chiton) and bear a vague resemblance to some kind of arachnid or crustacean. Is such a creature possible? Can any biological organism (carbon-based or otherwise) survive by chewing up metal, and metal alone?

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There are many kinds of bacteria that damage iron as they grow by producing various kinds of acid, such as sulfuric acid, as waste. At least one kind of chemoautotroph, ferrobacillus ferrooxidans, actually uses the oxidization (rusting) of inorganic iron as a primary energy source. So yes, an organism that eats metal is certainly possible.

As you might imagine, there isn't a whole lot of energy to be gained by rusting iron, so I wouldn't expect to see high-energy organisms (animals) chowing down on metal. A contagious, corrosive mold would certainly be a threat to space travelers though, and isn't that far-fetched.

If you want to have the creatures be arachnid-like, perhaps there could be a form of symbiosis, where the animal 'farms' the iron-eating mold by spreading its spores, goes into hibernation for a while, and then harvests the mold after it has spread.

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Energy is not the only reason to eat stuff. Metal may be a bad energy source but it makes a decent building material.

At the crudest you could have creatures that just break up the metal to make homes, similar to a caddisfly larvae making cases or a paper wasp making its nest.

You could also have something like a mollusc or crustacean like that consumes the metal to make its shell/exoskeleton .

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It depends. On Earth, when the atmosphere was a reducing one, before the first Stromatalites, the seas were full of iron, dissolved into seawater. Consuming iron in solution was possible at this time. Many organisms lived in the sea, albeit very simple ones. Stromatalites began photosynthesizing, in the shallows, and wiped out those other organisms with oxygen, but this took a couple billion years. Those organisms were anerobic, but they definitely thrived in the iron rich seas. So it's not only possible, it happened. Hell, we eat iron, we use it to bind oxygen, as hemoglobin, just in compound. Certainly aliens might.

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Can any biological organism (carbon-based or otherwise) survive by chewing up metal, and metal alone?

This is a hard no. Apart from being poor fuel source, biology requires a number of non-metallic elements. That said, you could make your creature far more reliant on metal as a part of its biology.

Your creature could be based on limpets

Limpets are a kind of mussel who's teeth are made of an iron rich super materials called goethite. Limpet teeth have evolved to be stronger than stone so they can spend their days grinding away at rocks looking for microbial snacks in the pores. Apart from being one of the strongest substances known to man, goethite is made mostly out of iron.

For purposes of your species, these pest animals have taken goethite one step further and decided not just to make their teeth out of it, but their whole exoskeletons too. This would make anything containing iron worth consuming not necessarily for the caloric value but because it needs the extra iron to grow its shell.

The best thing about it for your purposes is that it would further reinforce the idea of your pests being nearly unkillable. Not only is goethite much stronger than aragonite, it is even stronger than spider silk or kevlar. So, apart from being resistant to hot, cold, etc. they could also be able to shrug off small arms fire if you make their shells thick enough.

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