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How do floating aliens work? They are portrayed as having a large gas bladder or sack that keeps them neutrally buoyant while they propel themselves through the air. A notable example of this is Doctor Krill from “Humans are Space Orcs,” who floats thanks to a hydrogen sack on his back, occasionally deflating the delicate sack for better mobility or to avoid rupturing it. This was novel to me despite it being somewhat intuitive. A floating creature needs to fill and empty its gas bladder on a near regular basis.

But how should this work? How does this affect the creatures anatomy? From what I can tell, hydrogen is impractical for a creature. Hydrogen-producing organisms are poisoned by O2, and yields of H2 are often low. Instead, methane seems like the most logical alternative: an odourless, colourless gas that's lighter than air and is a byproduct of gut bacteria. But knowing this raises other technical questions. How does the creature make sure the methane ends up in the gas bladder? What kind of seal does it use to close off and deflate the gas bladder? How long does it take for the bladder to refill?

I'm thinking of applying this information to a floating herbivore.

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  • $\begingroup$ Just some additional information: a sack for storing hydrogen needs to be much more hermetic than a sack for storing methane because the methane molecule is bigger and will leak less. Unless, of course, that your biological process refills the sack with a higher ratio than it leaks. $\endgroup$ Nov 7, 2021 at 2:12

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Hydrogen isn't as implausible as you'd think

Not all hydrogen-producers are damaged by oxygen. In fact, some oxygen producing species can be made to make hydrogen instead, given the right conditions. This ability seems to be quite efficient, and it could only be more efficient for a hydrogen-floating species

It seems plausible that a species could filter out hydrogen from other dissolved substances, though I can't be sure. On the specifics of the gas-bags, then it seems like the best solution would be a sort of air-space, as found in man o' wars or cephalopods. The air-spaces would be filled with hydrogen, and would have to make up a large portion of the body. Vertical motion could be handled similarly to how fish ascend and descend with their swim bladders

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  • $\begingroup$ Interesting. Do you have a citation or link to the hydrogen-producers you’re talking about? $\endgroup$ Nov 6, 2021 at 19:50

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