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So I'm working on an inhabited planet with around half the gravity of Earth (assume everything else is the same, just the gravity differs). While thinking about what kinds of terrestrial creatures this world could have, hydrostatic skeletons came to my mind and now I must ask: with a lower gravity, could vertebrate-sized creatures evolve with a hydrostatic skeleton instead of an internalized solid skeleton like ours?

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    $\begingroup$ It probably depends on the rigors of the environment. Even in half a gee, falls are easily lethal. Gravity might be weaker, but kicks/blows from predators might not be, and soft-bodied creatures would see a lot of internal damage in combat. It would be an interesting concept, though. $\endgroup$
    – BMF
    Jan 19, 2023 at 2:05
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    $\begingroup$ Given that there are terrestrial worms that are larger than some invertebrates what makes you think that this wouldn't be possible on a lower gravity planet? $\endgroup$
    – sphennings
    Jan 19, 2023 at 2:06
  • $\begingroup$ Actually, a low-gravity world sounds like a perfectly reasonable way to rationalize larger creatures with hydrostatic skeletons. Maybe not elephant sized, but I could suspend my disbelief with this. It would depend on the complexity of the creature. I'd have trouble believing birds or fast-moving creatures like cheetahs where substantial force must be applied against the skeleton. But I think you did just justify AD&D's Shambling Mound. $\endgroup$
    – JBH
    Jan 19, 2023 at 11:50

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yes, but not that much bigger - weight increases cubic, while strength of materials increases squared

So if you decrease gravity by 50%, then you still can't double the size of organisms. Also currently breathing seems to be the worst problem for that kind of "low" organisms on earth - so probably they'd need more oxygen in the atmosphere to survive?

Meanwhile planets with half gravity have other problems - like trouble keeping water (due to the hydrogen escaping to space)

Also our planet has that gravity due to it's metallic core - that's needed for different things, like building a EM-Shield for repelling radiation from space.

Earth has tons of variables set exactly correct to support our kind of life - adjust some of them and life would have to look totally different or not work at all.

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