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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.