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This is a submission for the Anatomically Correct Series

Giant seahorses are often portrayed in fiction as being ridden by merfolk or other underwater people.

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I'm wondering if such a creature would be biologically possible. I"m aware that simply increasing an animal's size isn't that simple (given the square-cube law), but is there any way for giant seahorses to exist?

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Enlarged Marine Animals need a reason to be large

I believe large seahorses are more than possible. Anatomically, seahorses are simply fish, they have no relation to horses let alone any mammal or land animal.

As they are simply fish, we have seen enlarged fish and sharks to enormous scales - the largest currently being the Whale Shark:

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You seahorse does not need to be this big though, so perhaps just a sunfish size is adequate as follows:

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So given we have examples, we know it would be possible.

Now for evolutionary pressures: there is likely a reason why Seahorses haven't grown to this size yet. That's because larger animals need much more food to sustain themselves, and much more vulnerable to ecological and environmental change. Larger animals tend to have to travel further, to abundant 'small filter food' areas that have a seasonal variation, so the size becomes an advantage as you can support longer endurance-style swims in comparison to smaller fish.

So your seahorse will need to eat lots, swim far, and likely be a filter feeder.

Unfortunately, sea horses are not efficient swimmers - in fact they are the slowest fish in the world. This is because they are vertical, and use their dorsal fins for propelling, in lieu of their tails. That makes sense though and is ok, as the appeal of being a sea horse is being sexy amongst the safe coral, not in long endurance swimming.

So if it does happen, to be large your sea horse may actually start looking like a normal fish instead.

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  • $\begingroup$ I completely agree with your answer, with one minor correction. Saying the whale shark is the largest fish is true. However, saying the whale shark is not a shark is false. The whale shark is most definitely a shark. While all sharks are fish, not all fish are sharks. And yes, before I wrote this comment I looked to see of there was new science that says the whale shark is not a shark and found none. $\endgroup$
    – Keltari
    Apr 15, 2020 at 13:55
  • $\begingroup$ @Keltari - thanks for the heads up - have adjusted the answer - thanks! (Funny - I had that misconception in my mind...) $\endgroup$
    – flox
    Apr 15, 2020 at 13:58
  • $\begingroup$ No problem. Maybe you are confusing them with killer whales? Killer whales are not whales but dolphins. People make that mistake all the time. $\endgroup$
    – Keltari
    Apr 15, 2020 at 14:01
  • $\begingroup$ What if the seahorse looked more like a Leafy Seadragon? Could it be a larger size then? $\endgroup$
    – Richard Lindahl
    Apr 15, 2020 at 15:42
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    $\begingroup$ This question may be worth a look as a giant seahorse is essentially a mythical hippocampus, Greek for horse sea-monster, (not to be confused with the part of the brain that processes memory): worldbuilding.stackexchange.com/questions/34235/… $\endgroup$
    – TitaniumTurtle
    Apr 17, 2020 at 16:27
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A giant seahorse could exist in a seahorse-like shape if there were something like trees underwater, with the seahorse being like a large chameleon but without legs

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  • $\begingroup$ Quite right, else why would they evolve to be such terrible swimmers and good at hiding in trees.... Hang on, doesn't that also suppose giant sea-horse eaters; someone needs to tell the mermaids. $\endgroup$
    – Escaped dental patient.
    Apr 21, 2020 at 15:32

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