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In my setting, some humans hundreds of years ago became aquatic due to magic (the same setting has vampires pretty much like in the original Dracula novel, who descend from one guy who became a vampire due to same magic).

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What should the aquatic humanoids look like in order to be believable? They live in lakes up to at least 425 meters deep (possibly up to 1642 meters which is the depth of Earth's deepest lake, i haven't decided), across Europe and possibly continental Asia. Maybe they'd also be somehow viable in salt water?

I thought they'd have webbed fingers and toes and can't breathe air, but beyond that i'm not sure. Even though their ancestors became like that due to magic, i still want their appearance and abilities to make environmental sense. Coloring, fins, etc. They're ultimately still humans, enough to reproduce together and produce propably infertile hybrids, so they can't be too alien. They'd propably still be omnivorous humanoids too.

Would they also propably need to have superhuman strenght/toughness in order to be able to survive and casually move under all that pressure, and generally be able to tread water constantly?

Also, ideas on explaining how they've spread across the mostly unconnected water bodies despite being unable to survive on land?

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    $\begingroup$ As written you're asking a lot of questions at once. Please limit yourself to one question per post. It looks like you haven't completely fleshed out these creatures, can you provide a specific description of your creatures, their capabilities and culture. Without knowing that we can't answer your questions without engaging in wild unsubstantiated guessing. $\endgroup$
    – sphennings
    Jul 22 at 15:58
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    $\begingroup$ Don't tell us what features they may or probably have. edit the question to tell us what features they do have. Don't tell us what you assume, your creatures may need. Tell us about your creature. A question with a lot of maybes, isn't conducive to us being able to write good answers. We're not a discussion site we're here to ask and answer specific questions. $\endgroup$
    – sphennings
    Jul 22 at 16:13
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    $\begingroup$ VTC: (a) We don't answer questions about aesthetics ("what should they look like?"). (b) We answer specific questions. (c) You're allowed one and only one question. (d) There's no objective measure to determine whether or not something is "believable." (e) Stack Exchange is not a discussion forum. (f) According to the tag wikis, you can't use science-based and science-fiction together. Please carefully read the following Help Center pages: help center and help center. $\endgroup$
    – JBH
    Jul 22 at 16:52
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    $\begingroup$ I think I understand the question. I would suggest removing any "I think..." and "they would probably..." conjectures from the question. You'll get far more interesting answers if you aren't guiding people like this. If there are hard requirements for your story (like them being non-amphibious and living in lakes all over the continent), then state them as such. $\endgroup$ Jul 22 at 17:59
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    $\begingroup$ Welcome Nuke, sorry you didn't get a quick and dirty answer to your problem just yet, but we do have a special place reserved for people to fine tune their questions. It's called the Sandbox, and you can copy this question there to get help on wording it well. Check it out! When we have a suitable question, you can edit this and ask to open it up again. Don't worry, many new members do this! $\endgroup$
    – Vogon Poet
    Jul 22 at 19:08

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Not a single land animal that has become aquatic has gained the ability to breathe under water. Even dolphins still breathe air. They can hold breath for a very long time (an hour maybe?), but they can't breathe in water, even after 50 million years of evolution.

That's because there is not enough oxygen dissolved in water to provide energy for a mammal brain. Or a bird brain. Or even a reptile brain.

Your aquatic humanoids in order to survive would need to be as dumb as fish.

They would also have to be cold blooded (poikilothermic)

Even if water breathing humanoids would be possible, your humanoids are supposed to breed with humans, so they are genetically very close. An aquatic and even water breeding organism needs to undergo a lot of genetic changes. There is no chance they could interbreed with humans.

But since they are magical beings, design them as you want to.

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Assuming these humanoids breathe water, they may have a physiology similar to Reinhold Messner Who peaked Mount Everest for the first time without an auxiliary oxygen supply. You might consider such a body superhuman.

From a practical perspective, their highly oxygenated water might be viewed the way we view our drinking water, and their water may have the value of our oxygenated air.

The chest cavity would be much thinner than Homo sapiens, and the upper abdominal organs like the heart and stomach may move up into the space where our lungs are. Ribs will be much thinner and the diaphragm much smaller or absent.

If these are quite a humanoids are still going to locomote like bipedal humans, they would need to be A) significantly more dense than water or B) able to grip the ground to prevent walking motion from taking forever. In any case, legs would be weaker.

These humanoids would survive using their intelligence rather than sheer strength, speed, or endurance. Camouflage would be unnecessary. That being said, humans are exceptional among all animals when it comes to endurance since our intelligence allows us to be adapted to a lower-stress life. We may see similar adaptations in aquatic humanoids: exceptional dive depth limits and ability to acclimate to very low oxygen levels. They would rule over most of the aquatic world.

These aquatic humanoids spread across the aquatic world during the great deluge. If they develop advanced technology, they will likely develop 'wet suits' which allowed them to walk on land. This development may even make them competitive with land humans, since land humans currently do not have the ability to die that extreme deaths without scarcely-available, expensive equipment.

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