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How big can an underwater animal really get before physics get in the way? (I'm sure feeding would likely get in the way first, but ignore that!).

The Blue Whale is currently the largest animal on planet earth the record holder 108 ft. long. The Megalodon topped out close to 70 ft. The Lion's mane Jellyfish has been measured to 120 ft. long though masses much smaller than either of the other 2.

So how big in ocean's as large as ours, could an animal conceivably be? And does the size of the planet make a difference? Would life inside Titan be able to be larger?

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    $\begingroup$ Your first question appears to be a duplicate of this one. $\endgroup$ – Frostfyre Nov 25 '15 at 19:41
  • $\begingroup$ @Frostfyre While the title is a basic dup, the actual question being asked (in that question) is what are the constraints, not how big can it be. I want to know is a theoretical limit of those constraints. Not just identifying what they are. $\endgroup$ – bowlturner Nov 25 '15 at 19:45
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On Earth

Depends entirely on the beast in question.

Blue whales are the largest mammals, and they develop quite severe spinal issues in their old age - even having water support their bulk, their skeletal system still takes a beating. Clearly a mammal can't get much bigger (not with a standard bone structure and composition), even in the oceans.

An animal that doesn't have bones, however, might grow to be quite a bit larger.

The problem becomes, as you've mentioned, how would that creature eat enough to sustain itself. The problem only grows as you envision a population of hundreds of such monsters.

Whales solve the problem by ingesting a large amount of water and then spitting it back out, filtering small creatures out in the teeth and then swallowing them.

Squids on the other hand, capture their prey and eat it the good ol' fashioned way. So a kraken (aka a giant squid) would probably go after proportionately large prey, such as a blue whale. There's not that many blue whales to go around though, nor really whales of any other kind.

Fish would be too tiny for the kraken to even bother with, at which point it's probably targeting seafaring vessels and snatching people off the decks - another time honored kraken tradition.

I think this is the reason why so often in fiction these gigantic animals are one of a kind - the results of experiments, or mutation triggered by some other-worldly event (radiation from a meteor, etc).

Alien World

Imagine a planet with lower gravity than earth, or on which creatures have developed on different principles than on Earth (aka skeletal structure and composition, etc.).

It would make total sense for mega-sized species to develop if they don't face the "structural" issues that Earth bound creatures do. (In Avatar, for example, the size of the local species is explained by them having carbon nano-tube naturally occur in their bones - it's basically hand-waving the size issue aside).

At that point you're making up your own rules though.

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  • $\begingroup$ My understanding is that giant squid are strictly deep-water animals. Is that a consequence of their invertebrate structure? Is a super-large ocean animal going to be limited to either surface or deep waters only, assuming it can somehow stay fed? $\endgroup$ – user243 Nov 25 '15 at 18:41
  • $\begingroup$ @JonofAllTrades Not really. Keep in mind that sperm whales, which eat giant squid, are equally happy at the surface. $\endgroup$ – ckersch Nov 25 '15 at 19:12
  • $\begingroup$ @JonOfAllTrades - again, it depends on the creature. Whales are able to dive to incredible depths and then surface. Some creatures, however, like to stick to the bottom of the sea/ocean (giant squids). If they can withstand high pressures, there's really no reason why they couldn't come up closer to the surface - not from that point of view. They might, however, not like the light. Or they might want to stick to colder water. OR, a predator of theirs might primarily stick closer to the surface and make life dangerous for them if they venture up. $\endgroup$ – AndreiROM Nov 25 '15 at 19:26
  • $\begingroup$ No need to define that it's the largest mammal, the blue whale is largest known animal to have ever existed. Further, I don't actually see an answer here. The closest you get is saying "An animal that doesn't have bones, however, might grow to be quite a bit larger". Well, back to the question, how large? It doesn't "depend on the creature", the question is only for two specific creature types, the leviathan and the kraken, so confine your answer to those two creatures. $\endgroup$ – Samuel Nov 26 '15 at 0:20
  • $\begingroup$ @Samuel - while the title of the question mentions a kraken, the actual question is: "So how big in ocean's as large as ours, could an animal conceivably be?". That "animal" could potentially be a mammal, since no other parameter is defined. I'm pointing out that mammals have already maxed out their size, or come pretty close. Next the author asks a question that takes us out of scope of planet Earth alone: "And does the size of the planet make a difference?" And I claim that yes, it does matter from a food source perspective, as well as other alien considerations. I think I'm on track here $\endgroup$ – AndreiROM Nov 26 '15 at 0:28
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Don't forget other lifeforms

I think the most heavy living being on earth was a mushroom that does have a mass of over 100 tons... Well, while this may not be a leviathan as described in your question, you should take in account that corals are animals too, If you consider a coral reef as a hive-like animal, they may exceed any other animal ever existed in size.

And it is an underwater-animal, so this somehow does apply to you question. Of course, coral reefs can sink ships, but usually they do not tend to be the most aggressive animals in existence (more like the most passive ones).

But if you are not afraid of handwaving, you can construct a coral reef of the size (and mass) of an aircraft carrier that is kind of intelligent and goes for new sources of food by itself. Add some tentacles and... wait, don't do this.

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