[Ships of Wood, Men of Iron]

I like the general idea of this general_re calculation, but a similar calculation would seem to show that large fish (e.g., large sharks) could not exist (when it uses its muscles, it would need too much oxygen).

[general_re]

Ah, it's much easier for fish - they're heterothermic (cold-blooded), so they don't need to expend lots of energy maintaining a high body temperature, among other things. Nor do they have large brains with the attendant fuel requirements. Etc., etc. - fish have much, much lower oxygen requirements than people, so they're perfectly happy with 5-7 ppm oxygen for their environment, versus the 160,000 ppm your body likes to have :)