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I'm wondering whether atmospheric pressure (high or low) would have any impact on the favorability of hydrostatic skeletons. Obviously hydrostatic skeletons work well in water, but I'm not sure if a gaseous medium would have different effects. Could anyone give any physics-based insight on this question?

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    $\begingroup$ If it has gasses dissolved anywhere in its body, don't forget the bends with any sudden change in pressure. $\endgroup$ Jan 13 at 5:04

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The "hydrostatic skeleton" idea basically revolves around using incompressible fluids (usually water) to maintain a structure, like in big trees or hemichordates. In fact, as silly as it sounds, mammalian penises, including those of humans, are hydrostatic organs, and use the same concept to remain stiff even under pressure. It's especially useful in high pressure environments, so you're on the right track.

Essentially, the idea is that if you don't have some sort of pressure on the inside pushing out, then the big outside pressure will squish whatever needs the skeleton. Generally, for things like organs or creatures, being squished isn't great. But water, which can't be compressed, essentially acts like a brace: it's really, really hard to squish your organ or creature if its skeleton is hydrostatic because water "tubes" inside the skeleton exert an outward pressure of almost-equal magnitude. It would take an extremely large amount of pressure to crush a creature with a hydrostatic skeleton; in fact, the pressure would be great enough that the creature would've died of something else (like suffocation or incineration) long beforehand.

If you're evolving life on a planet with high atmospheric pressure, and big oceans aren't viable for life (so that creatures don't choose the evolutionarily-easier path of just growing gills), then a hydrostatic skeleton might show up at some point.

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  • $\begingroup$ Thanks! I should have mentioned I'm trying to design life on planets with atmospheric pressures varying between 0.5 to ~5 atm. 5 atm wouldn't be a terribly crushing pressure, so I'm not really factoring that in. How do you think atmospheric pressure would affect the size and movement of a hydrostatic creature? Would it be easier or harder to move? Would hydrostatic land animals be more or less able to support their own weight? etc. Thanks again!! $\endgroup$
    – Elhammo
    Jan 12 at 23:42
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    $\begingroup$ @Elhammo That depends on a couple more specific parameters, but generally, I would think it not impossible. Certainly, the bigger the creature, the more difficult it would become to keep the skin from breaking from the air pressure, so if you want a big creature, it should have a low surface area (so more amorphous, with fewer detailed bits) to minimize the amount of skin it'd have to produce. But yes, with HS skeletons, I would think it would be possible to evolve standing life. $\endgroup$ Jan 13 at 1:19

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