I'm developing a planet populated by giant sea creatures (think Megalodon, but bigger.) While I know that gravity has an effect on size, the sources I found only made reference to land creatures, who can't grow past a certain size, or else their legs/bones would snap. Legs are obviously not a problem for sea creatures.

What things determine the upper size limit on aquatic creatures? How much effect does gravity have on aquatic size? Or is size more a function of available food (which is what I've heard about blue whales)?

(First-time asker, so if I've done something wrong, please let me know and I'll do better next time!)

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    $\begingroup$ Welcome to WorldBuilding! If you have a moment please take the tour and visit the help center to learn more about the site. Can you simply scale up animals? might be helpful for you. The accepted answer briefly talks about aquatic animals and the general problems. Have fun on the site! $\endgroup$ – Secespitus Sep 6 '17 at 19:30

As far as I know, the things determining the size for swimming creatures are breathing, and feeding. As long as you manage to get oxygen and nutrients to every bodypart, they can end up in sizes that we would consider islands.

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