Creature plausibility check

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Whitewings
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Creature plausibility check

Post by Whitewings »

The concept is fairly simple: a creature that outwardly resembles a siphonophore. At some point in its evolution one of its metabolic processes began to leak moderately pure hydrogen into its bubble, making it slightly more buoyant. This provides a survival advantage due to the ones with it being less likely to be smashed against rocks or fatally torn on reefs during foul weather. This applies a selection pressure that favours more efficient floaters, and gradually results in the creatures becoming living balloons. They live on whatever their tendrils snare, be it fish, zooplankton, insect analogues, corpses, almost any sort of flesh. I'm not sure how they reproduce.

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Pabappa
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Re: Creature plausibility check

Post by Pabappa »

I dont think so, no. Hydrogen cant just sit around and stay hydrogen .... that's why we use helium in our balloons instead. Even if it is only, as you say, partially differentiated .... I'm pretty sure the hydrogen would steal atoms from the rest of the creature's body and change back into water or to some other organic compound. Also, even if they managed to do it, I'd think the animal would die of thirst almost immediately after they rose out of the water.
I'll take the theses, and you can have the thoses.

Whitewings
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Re: Creature plausibility check

Post by Whitewings »

I should have spotted the water problem myself.

Salmoneus
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Re: Creature plausibility check

Post by Salmoneus »

I don't understand any of the thinking here.

Why would the hydrogen "steal atoms from the creature's body"!? Hydrogen is not strongly reactive, and if it did react with atoms in the body, that wouldn't magically turn the body parts into gas - instead, the hydrogen would just bond with the solid (or liquid). It would be an issue, but controllable - after all, the body copes with having oxygen in it, and that's much more reactive than hydrogen! It probably would be a concern that the hydrogen would react with any free oxygen it came into contact with, forming water (and a lot of heat!), but that just means you need to keep them apart - or put them together on purpose to provide energy and regulate buoyancy.

And no, we don't not put hydrogen in balloons because it reacts with its container, because it doesn't. The Graf Zeppelin managed to circumnavigate the planet while filled with hydrogen! We don't fill things with hydrogen anymore because if you mix your hydrogen with a bit of oxygen, and then provide a great deal of heat (eg through an electric spark), it reacts explosively. However, explosive hydrogen-oxygen reactions only take place over 550 degrees celsius, which is unlikely to be an issue in an organic creature living underwater. Even ordinary reactions of hydrogen and water don't take place without a considerable energy input.

The more serious problem with this idea is that you'd have to have a lot of hydrogen. You can, of course, extract hydrogen from seawater... but it's difficult, and requires a huge amount of energy. As in, humans haven't yet worked out how to do it economically.

Now, to be fair, some organisms do produce hydrogen, including some algae. However, hydrogen-producing algae work on photosynthesis, so you'd have to be floating on the surface the whole time, and the reaction is stopped by the presence of oxygen and inhibited by carbon dioxide, which makes it hard to integrate into another organism (though not imposible). More importantly, the hydrogen yields are very small. Other organisms do produce hydrogen without photosynthesis, including several pathogens like clostridium and helicobacter, but they produce even tinier amounts. Now, I suppose you could handwave this, but the point remains: one way or another, producing this hydrogen will rquire a LOT of energy. So either you're floating in the sun all day, or else you're eating a LOT of food.

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The real issue, however, is that there doesn't seem to be any point doing this.

First, it should go without saying that you can't have a balloon under water, because the water would crush it - you're not going to be able to pump up your balloon with gas and still produce as much pressure as the ocean. Or rather: you'd need a very strongly reinforced balloon.

In other words, you'd have a nautilus. Now, nautiluses float up and down with gas-filled balloons (aka 'shells'), although they get crushed below about 800m. But they don't use hydrogen, because... why would they need to? Any gas at all is going to be lighter than seawater, and hence able to lift them. So it makes sense to just use ordinary gas (I don't know exactly what they're filled with, but I would presume mostly carbon dioxide?), since this is cheaper.

In particular, you don't want an expensive gas if you might want to go down again, because you'd have to vent your expensive gas and then make more of it when you wanted to rise up again.

I mean, even blocks of wood float to the surface of water, so bringing in hydrogen, the lightest thing in the world, is kind of using a cannon to kill a beetle...


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I don't understand, though, how hydrogen would stop things being torn or smashed. Indeed, quite the contrary - hydrogen would make them float higher, and the higher in the water you are of course the more at risk you are in storms! Waves happen on the surface (mostly). Of course, things underwater CAN be bashed against a cliff by a sufficiently huge movement of water, but mostly, if you're being hurled around by storm waves, thrown through the air into rocks and then pulled over the rocks by the force of the waves and torn, let alone being beached in the air on the rocks when the waves recede... it's because you're too near the surface!

Whitewings
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Re: Creature plausibility check

Post by Whitewings »

One correction: I imagined the creature to be like the Portuguese man-o'-war, floating on the surface of the ocean.

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Re: Creature plausibility check

Post by Salmoneus »

Well, the man o' war floats without needing any hydrogen (so far as I'm aware), so what would the hydrogen actually be doing, then?

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Re: Creature plausibility check

Post by Whitewings »

The hydrogen allows it to rise a little higher, which means it's less subject to waves. Basically, I have an image of a living balloon, like an airborne man-o'-war, and I'm trying to work out a somewhat plausible evolutionary path that leads to it.

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