Continuing the discussion from Will Musk's starship reach orbit this year?:
Not to hijack the thread on Musk’s starship reaching orbit this year, answering post #494, I wonder what animals would feel comfortable in zero gravity. The launch, of course, would be stressful. But once up there? Yes, I agree with Dr.Strangelove, a seal might feel fine, particularly with extended flippers. The air is a tenuous fluid compared with water, but the available space is restricted anyway and it has the advantage of being breathable without having to surface.
A cat, perhaps? It always lands on its feet, it is claimed. After an adjusting period I can imagine they would love it. Specially if there are enough surfaces in the orbiting module where it can sink its claws into.
A squirrel would have a field day!
How would a snake manage? Coiling is fine, but there would be no traction without it. But a sea snake could swim in the air too.
An octopus should have endless possibilities if drying out can be avoided.
Perhaps a gekko would do best.
According to Arthur C. Clark’s “Rendezvous with Rama” future astronauts on-board assistants will be genetically enhanced monkeys. In the gravity-less environment of space, the small creatures with four hands and a prehensile tail are quite at home, not needing to walk on a floor.
Cats are not particularly happy in zero-g:
Screws up their righting reflex. Though maybe if they were raised from kittens, they would get used to it.
A cat in lunar gravity would have a lot of fun, I suspect.
Those parabolic flights last much to short for the cats to get used to the new normal of zero G. In proper orbit I guess they would adapt.
That sounds plausible, but I would consider genetical enhancement kind of cheating the premise of this thread.
Bees were pretty screwed up at first, but after a few days they learned new techniques for flying that worked better in zero g.
Water bears (tardigrades)! They don’t even need a capsule.
I’ll go with cats. I thought about writing a short story about a guy tasked with rounding up feral cats on a lunar base, and it does not go well for him.
I think as a broad generalization smaller animals should do better? Gravity is a less significant force for them on earth because of the square-cube law.
I would also think that aquatic animals might be less affected by the absence of gravity (in a suitable spaceship compartment full of water, of course)?
Turkeys. In low gravity they’d actually be able to fly.
Air has insufficient viscosity for swimming creatures to exert any control, and most aves and ‘flying’ mammals are dependent upon gravity for gliding and orientation. The same is true for Lepidopterans, but many Holometaboles and some Hemipterodeans could probably fly controllably. Dogs, cats, et cetera which lack grasping appendages will never be comfortable or be capable of controlled locomotion in freefall conditions.
However, the bigger problem with animals is hygiene. On Earth, wastes fall down and evaporate, desiccate, or are consumed by other scavengers, fungi, and microorganisms. In freefall or very low felt acceleration, waste will just float around, covering all surfaces and promotion an explosion of biota, at least some of which are likely to be pathogenic or produce toxic byproducts. Even humans have difficulty keeping habitats in freefall clean of wastes notwithstanding all other bodily emissions (saliva, mucus, sebum) and all of the dead tissues (hair, skin) that are constantly being shed and serve as nutrient and anchorage for a wide array of microbial and fungal pathogens.
Orbital space habitats are actually a horrorshow of uncontrolled biota, and dogs, cats, goats, parrots, et cetera would just serve to make it an unmaintainable cesspool, which is one of the many issues with long duration space missions and habitation that only the scientists and engineers working in the field really appreciate. Without a large rotating habitat to simulate something reasonably close to Earth surface gravity, having a pet or farm animals in space just not practicable.
Stranger
I’ve often wondered how well sloths would do in zero or low-gee. Would they be more active than on Earth?
Uplifted octopuses in space.
Not quite as good as the first novel (spiders), but still remarkable.
Fully aquatic animals that breathe underwater would do best. Most of them live in an effectively 0-G environment already. Assuming limited spacecraft size then the smaller the better, most of their environment needs to be dedicated to plankton growth for food, sufficient oxygenation of the water for the astroaquanauts to breathe and to break down waste. Given a enough power the water could be reoxygenated by hydrolysis and cleaned to some extent with ozone and UV light.
Fish with swim bladders depend upon gravity to maneuver.
Blennies do not have swim bladders, and depend upon gravity to spend most of their time sitting.
Rusty gobies spend most of their time in rock, zipping out, sometime upside down to eat, but they too depend very much on gravity to navigate.
I believe most fish wold find it very difficult to swim in zero G.
Google provides more about genetic changes than maneuvering.
Gobies to use their ventral fins to form a suction cup to attach themselves to objects; they may do better in weightlessness.
Bodies of water are not “effectively 0-G” environment, and in fact many nutrient transport processes require the gradient of pressure and gravitational forces. Almost all Osteichthyes (‘true’ or ‘bony’ fish) use a swim bladder to control their depth and motion, and a wide variety of sealife use the polarization of sunlight on a flat surface to orient and navigate. Of course, water is very dense (one metric ton per cubic meter) and will flow under any acceleration so it would basically have to be fully contained without any exposed surface lest any impulse result in sloshing.
Stranger
Some fish do ok in space, after an adjustment period:
They learn to use the light direction as up/down vs. the gravity vector.
One suspects that it is the most adaptable critter is the one that will do best. Humans are pretty adaptable, but we only have two grasping hands. A critter with more is likely to do better. An octopus might do very well as it can anchor itself with just about any part of any appendage, and can stretch and contract remarkably.
A spider that learns to use its web spinning ability to useful effect might do really well. Ability to create an instant tether would be quite an asset. Freed from the iron grip of the square cube law for supporting themselves, spiders might evolve quite interestingly.
One constraint may be that a successful critter needs to have some mechanism, even if inefficient, to get itself back to a surface if it is cast free. In a space with moving air, so long as it can wait things out, a critter might be OK. But in a large space, the time needed might prove fatal.
I’m going to go with a slightly evolved octopus.
Has a bird ever been flown to space?
A pterodactyl would be a thing to behold. They came in all sizes and they have prehensile digits on the wings and on the feet. Plus the beak.
ETA: Of course Stranger’s objections are a bit of a party spoiler. As they seem to be pertinent and true, let’s ignore them. 
Of course they do. Their swim bladder is what makes their normal life in the water effectively 0-G and causes no problems in the absence of gravity. They are no more dependent on gravity to exist than other animals and can adapt as easily as any non-aquatic animals who are far more dependent on gravity.