what limits how large a marine animal can grow? What biologically prevents a kraken from existing?
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Size is limited by food supply and the ability of the creature in question to be able to function.
Dinos grew so big because their food supply was huge and their growth was slow and steady over thousands of generations. Their bodies were able to adapt and grow stronger to be able to function at those large sizes.
Likewise, whales and elephants have slowly adapted to be larger and their bodies have adapted as well.
Sea animals, in particular, are limited by food supply mostly.
As to a giant squid/kraken, I’m sure there are giant versions out there, as the largest of whales have been noted to have massive scars from such creatures.
One reason is the food supply that have been already mentioned
The second reason why living beings can’t grow past certain limints is the limited toughness of tissues. No matter how large or how small a creature is, it’s living tissues are made from the same atoms (mostly carbon, hydrogen and oxygen). These atoms have fixed size and connect to each other with fixed force no matter big or small animal. If the piece tissue gets really really big, it starts to collapse under the weight of it’s own atoms basically, that’s why we can’t just build skyscrapers reaching orbit, for example. A very big creature would just collaps under it’s own weight or be very susceptible to forces like water currents.
The third reason is the [square-cube law](https://en.wikipedia.org/wiki/Square%E2%80%93cube_law#:~:text=The%20square%E2%80%93cube%20law%20can,the%20cube%20of%20the%20multiplier.). Basically, when you gradually make a creature bigger, it’s weight will grow at a higher rate than it’s muscle strenght, so compared to it’s own weight the creature will get weaker and weaker, until it will not be able to survive.
The latter two reasons are not as important for the sea animals, given that the mass of water supports the aquatic animals and allows for the largest animals on Earth to exist, but they are still important.
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