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It is somewhat common knowledge in the worldbuilding community that larger planets leads to smaller land creatures, because larger planets means more gravity gets bigger and that makes it harder to become big from the square-cube law.

However, water is different than land. Water creatures can become way bigger than land creatures on Earth, due to buoyancy holding up some of their weight. Buoyancy increases with gravity, but so does weight.

So would marine creatures be larger or smaller on a larger planets? Or would they be the same size as on Earth?

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  • $\begingroup$ I've always thought that people exaggerated the premise in your first paragraph. Remember, mayflies and brachiosaurs both evolved on the same planet. $\endgroup$ Apr 19, 2023 at 22:07

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The size of marine animals on Earth isn't really restricted by gravity... it is more about food supply and metabolism. The ultimate limit in size is probably quite complex, and a factor of various things like growth rates and predation and dietary requirements and so on.

If planetary gravity is too high, then it might be less likely that land-dwelling, air-breathing animals evolve that might then be able to return to the sea in the way that the likes of plesiosaurs and whales did, and sharks have metabolic issues getting enough oxygen from the water as they get very large so super-high-gravity worlds might have smaller things that Earth, but aside from that I'd expect things to be fairly similar, given availability of sufficient oxygen and food.

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    $\begingroup$ I would add that higher gravity makes higher atmospheric pressure likely (not guaranteed! depends on how much atmosphere there is) so more oxygen, carbon dioxide would dissolve into the water. Bigger animals, less corals and clams maybe? $\endgroup$
    – mart
    Apr 19, 2023 at 8:56
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    $\begingroup$ +1. Another factor is that higher gravity means pressure increases faster with depth, so it probably limits how deep air-breathing species can dive - but whether that makes much difference or not depends on what this planet's ecosystem is like. $\endgroup$
    – N. Virgo
    Apr 19, 2023 at 8:58
  • $\begingroup$ Creatures that are deeper where food is rarer are actually found to be larger. Whether this is surprising, I don't know. They need to use what they have very efficiently. $\endgroup$ Apr 19, 2023 at 22:15

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