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This question already has an answer here:

I have a character in the expanse of my various fictitious worlds, who is a genetically modified version of a peculiar creature. This creature visually resembles something somewhere in between a squid, a ray, and a cuttlefish. However, its physical appearance isn't the most defining feature; these creatures are capable of flight.

Well, more specifically, they contain bladders of lighter-than-air gasses. They can adjust their weight through an unknown process, allowing them to quickly become much lighter or heavier than their surroundings. While they aren't particularly fast, they can also use their tentacles to push off of the ground and other anchors, allowing them surprising bursts of speed at times.

Now, as I'm developing this concept in my head, I've come across a fairly significant question: How biologically possible is this? Can a hypothetical creature excrete, or even adequately contain, lighter-than-air gasses, to the point of actually being capable of controlled flight? What would the size restrictions of such a creature be?

A few stipulations:

  • Ideally, the body of the creature would be able to fit inside a space roughly the size of a human skull; this character covers up his appearance by way of a powered exoskeleton, which he rides inside the head of.
  • Evolutionary compliance is not mandatory; the setting world is full of the table scraps of an overly creative god, so pretty much anything goes.
  • I'm hoping for something that can put on a decent amount of speed, but even getting it to float aimlessly a la Engineers is enough.

EDIT: I've been told that my question may be a duplicate of another one. However, the question in question (heh) is asking about man-made devices, whereas mine is looking for purely biological means.

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marked as duplicate by SJuan76, Mołot, Hohmannfan, Frostfyre, Samuel Dec 13 '16 at 19:05

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

  • $\begingroup$ If evolutionary compliance isn't mandatory and you can handwave it, then answers to the linked question adequately answer this one, as the same concerns apply. $\endgroup$ – Frostfyre Dec 13 '16 at 13:24
  • $\begingroup$ There are a few similar questions here, here, here, here, here, and here. It's called the "living gasbag" on TV Tropes. $\endgroup$ – Anonymous Dec 13 '16 at 13:32
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Air is about 1000 times lighter than water, and the most buoyant gas is hydrogen, about 13 times lighter; most animals have a density around the density of water, so if the total size of the animal is similar to the size of human skull of around 1200-1300 cm³ then about 1/1100 of that, say about 1cm³, can be the living tissue of the animal and the rest must be filled with the buoyant gas; of this one cubic centimeter a not insignificant part must be dedicated to gas generation and containment.

(On the other hand, as Lu22 observes, the gas sacs could be inflated on demand; in this case, an animal the size of a human skull would need to inflate a gas sac of about 1.5 cubic meters to be able to float in the air.)

There already are certain biological processes which generate hydrogen.

Animals of different kinds, such as many fish or some hydrozonans, have gas generating glands and gas-containing bladders, which they use to obtain or regulate their buoyancy in water; so an animal which floats in the air using hydrogen sacs is not completely unbelievable.

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    $\begingroup$ Size need not be an issue at all. If it's floaty sacs are inflatable and can stretch (Like a frog's throat), then its terrestrial form could fit in a skull, while it can balloon up to fly, deflating when coming back down. Also for propulsion its respiratory system could take in air from the front, but have a moveable rear jet to expel waste gas. I didn't want to give an answer based on yours, so just tacking this on. $\endgroup$ – Lu22 Dec 13 '16 at 11:27
  • $\begingroup$ @Lu22: I have incorporated your observation about external gas sacs in the answer. $\endgroup$ – AlexP Dec 13 '16 at 11:38
  • $\begingroup$ Awesome. Forgot to upvote when I was commenting. The sacs would have to be pretty big though to contain 1500 litres of gas. This critter is gonna have a ton of loose skin..... $\endgroup$ – Lu22 Dec 13 '16 at 11:50
  • $\begingroup$ @Lu22 You say This critter is gonna have a ton of loose skin. Sooo.... it'll be a "[mix] between a squid, a ray, a cuttlefish" and a Neapolitan Mastiff! $\endgroup$ – Mrkvička Dec 13 '16 at 13:36
  • $\begingroup$ @Mrkvička Hahaha I actually have to do the math first to enforce that comment. If it could fold the skin under fleshy covers (similar to a beetle's wings) though, it's normal appearance shouldn't be affected much. $\endgroup$ – Lu22 Dec 13 '16 at 13:51

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