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What adaptations are needed for human-derived sea-dwellers?

My people are not the classic half-fish, nor are they whale-tail-people - they wear ankle-length swimming skirts that bind their legs into a hydrodynamic shape, and flippers. On land they wear loose skirts and walk almost normally. (The skirts cause some misunderstanding and false rumors, but under their clothes the merfolk aren't that different from us. Bonus points for the possibility of hybrids not well-adapted to either life.) Decorative clothing is flat and clingy.

Genetic engineering technology was a few centuries advanced from ours, but is lost now -- the species was designed, created, and possibly tweaked a bit for a few generations, but now breeds true naturally. (Bonus points for occasional throwbacks who are not well-adapted to either water or land.) Root stock was carefully chosen for genetic diversity but maybe the mix needed tweaking with experience.

Magic is available, but is not something used in daily life. (Think of magic talent like musical talent. A lot of people enjoy singing. Some sing well enough that other people enjoy it. A few can make a bit of money playing gigs. Very few can make a living. A handful are important cultural assets. So important public works can be magicked a bit, but not your average home, reef ranch, or kelp farm.)

There's already a great question about their building methods, BTW. There can be a few storage buildings underwater, but people need to sleep where they can breathe air even if the entrances are underwater like beaver homes. Most human coastal cities have a sea-folk quarter, especially in tropical areas near reefs.

This needs to go further than the (heavily disputed) lake-dwelling shellfish-gathering waders proposed as human ancestors by Sir Alister Hardy in 1960, popularized in 1972's feminist The Descent of Woman and given a more scholarly treatment in 1982's The Aquatic Ape Hypothesis

So far I've got:

  • Lungs -- extra capacity and pressure. Bigger ribcage.
  • Gills -- aren't feasible, see this question
  • Hair -- something between human eyebrows and seal fur
  • Ears -- streamlined
  • Genitalia - retractable/covered
  • Body temperature -- cooler in water, warmer in air, but always lower than base stock humans (Camels use a similar scheme, probably to reduce water loss.)
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    $\begingroup$ A minor correction - Gills are not feasible as a sole oxygen source for homeothermic organism. But 1) does it need to be main source? Can't it be supplementary one? and 2) can't they be ectotherm, at least partially? Some ectothermic organisms can keep their internal temperature near to constant using environment. And you're already lowering their temperature anyways so you removed main reason against gills from acepted answer to the question you linked. $\endgroup$
    – Mołot
    Jan 28 '20 at 15:50
  • $\begingroup$ I feel like an important piece is missing here, where in the ocean? Like at what latitude and longitude and at what general depth? If the answer was along the equator, it would still change the answer drastically if the depth was 10ft instead of 400ft; and that's just on temperature alone, not to mention light levels. $\endgroup$
    – Culyx
    Jan 28 '20 at 16:16
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    $\begingroup$ @Molot The swim bladder of fish is evolutionarily homologous with our lungs. I think some fishes use their swim bladders to increase oxygen intake as well as maintain buoyancy. It stands to reason modified lungs could be maintained as a supplementary organ for respiration. Whales and dolphins collapse their lungs but they are adapted for rapid, deep dives. $\endgroup$ Jan 28 '20 at 18:10
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Lungs -- extra capacity and pressure. Bigger ribcage.

Nope. Serious sea mammals store oxygen in myoglobin in the muscles, as there's simply no useful way for the lungs to contain useful amounts of air at depth. Seals exhale before diving.

Hair -- something between human eyebrows and seal fur

Doesn't necessarily need to be either. Cetaceans get a long just fine without being fluffy, y'know.

Ears -- streamlined

The hydrodynamic issues of our ears are somewhat outweighed by the hydrodynamic issues of just about every other part of our body, starting with shoulders and arms. The ears can be the last to go, really. Having external ears might improve your hearing above water, too.

Genitalia - retractable/covered

As above. Though perhaps there's a little more justification here, as human male external genetalia expects to dangle around in a certain temperature to keep its contents in good condition, and spending a lot of time in the ocean will rather upset that.

I do also note that human male external genetalia are self-retracting in cold environments, to some extent.

Body temperature -- cooler in water, warmer in air, but always lower than base stock humans (Camels use a similar scheme, probably to reduce water loss.)

Neither dolphins nor seals need this, I'll note. This suggests that whatever benefits you though it might have are probably lower than you thought.


What else:

  • Crocodile-derived haemoglobin to improve oxygen release whilst breath holding, allowing longer dives.
  • Nictitating membranes to allow for sharp focus underwater and above water, for better underwater precision in all activities. Might also help protect the eyes, and if they are hairless (see above) reduce problems associated with water or sweat getting into the eyes unimpeded by eyebrows.
  • Ability to close nostrils before diving. It is just convenient!
  • Better insulation! Not enough to simply be "fat"... human fat doesn't insulate well, because of surface blood vessels. Most marine mammals are very "cuddly" for this reason. Unless they're exclusively tropical, your peeps are likely to be similar.
  • Resistance to skin maceration, to reduce skin damage caused by prolonged exposure to wet environments. No self-respecting mer-person should be suffering from trench foot, ever.
  • Enhanced diving reflexes, associated with better control of blood storage, oxygen release, maintenance of blood pressure and collapse and re-inflation of air passages if they're expecting to dive deep. If they're only shallow divers (say, <50m), squishable air passages aren't really useful.
  • Modified airway physiology. Diving mammals have somewhat different tracheae and bronchi than land dwellers, though these seem to have been poorly researched to date.
  • Sensitivity to water pressure, and hence depth. It'll help navigation and dive planning.
  • Assuming they're found in the sea: much higher tolerance for salt in their diet.
  • Dolphins get echolocation and seals get whiskers and sharks get a fancy sense of smell and electroception. All of these aid in prey location and navigation in dark or turbid waters. You might consider some of these too, though for shallow divers a pair of goggles or nictitating membranes will be just fine.

Honestly, the insulation is probably the most important bit. The sea in most places ( especially the places where it is most fertile) is cold and you're gonna need a lot of bioprene. Fresh water in most temperate parts of the world will be dangerously cold in the winter, too.

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Nostrils on the back of the head.

Starfish got all the low hanging fruit but I have this one. It is difficult for humans to lift the nose and mouth out of the water to breathe. Nostrils high on the forehead or on the back of the head would make this much easier; swimming now would be like swimming with a snorkel and much more energy efficient.

Princess Ariel might be somewhat less cute.

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Here are some more traits that seem useful for such merfolk:

  • Dolphin-like facial structure. Specifically with the nose expanded and raised into a blowhole at the top of the forehead. An extended jaw with fat above would also help make the face more streamlined. You could still have the eyes face forwards like in humans, though
  • Melon. A melon, like that of dolphins, would be useful for echolocation, which is a rather useful feature for aquatic animals
  • Trunk/siphon. A trunk on the blowhole, while it may seem superfluous, would be excedingly useful for breathing. With a regular blowhole, the merfolk would need to either turn on their back and lift their face out of the water, or stand upright and lift the top of their head out. With a trunk, they may simply move near the surface and twist their trunk out of the water to breathe
  • Barbels. Barbels at the mouth would allow these merfolk to smell things underwater, which is a nice feature to have
  • Dolphin-type ears. Dolphins have a very complex ear, which allows their echolocation in the first place
  • Shark-type ears. If you don't need to rigidly stick to tetrapodal anatomy, you could have an extra pair of ears like a shark, which would be attuned more to distant sounds. These ears would probably have to be quite high on the head to avoid interference. They would also need some way to close off when on land
  • Lateral line system. A lateral line is a useful sensory organ that can detect pressure gradients in water, and so they'd be a wonderful addition to your merfolk
  • Fish eyes. They should have water-adapted eyes like a fish. You should still include the irides, and have a more tetrapodal focusing system, but features like the shallow cornea and spherical lens seem quite important
  • Electroreception. Electroreception is the ability to sense electricity. This is useful underwater, and can be used for communication
  • Cetacean Pharynx. Cetaceans have a unique structure connecting the blowhole directly to the trachea. This would be quite useful for merfolk
  • Gills. While gills couldn't provide all the oxygen these merfolk would require, they would still help. The best solution would be something similar to the bony fish, but with a single pair of gill plates formed from the 5th pharyngeal arch (as it is not retained in mammals). This structure would naturally sit in the neck beside the throat, with paired outputs in the lower frontal part of the neck
  • Fish shoulders. It would be quite useful to have fish-like shoulders, as they are narrow and streamlined, but still in the right place for a humanoid
  • Flippers. They will benefit from long arms to do complex tasks. Having these arms be flippers as well should offset the hydrodynamic issues, as they'd add extra power to their motion
  • Dorsal Fin. A dorsal fin would be useful for stabilization. It probably couldn't be retractible, but many creatures do fine with this limitation
  • Pachyostosis. This is required for a diving animal with lungs. It will mostly be concentrated around the chest
  • Cartilaginous skeleton. Many fish, especially larger or more air-reliant ones, have a tendency towards cartilage skeletons, and so this could be used here. However, please note that it is only in fish; there are no examples of this feature in marine mammals or any other tetrapod
  • Salty tissues. It's a lot easier to osmoregulate when you don't have to; it seems plausible enough that, like starfish, these merfolk could keep their tissues at ambient salt levels. This, naturally, would be even more plausible in a more brackish sea. Either way, it'd still be good to have some level of osmoregulation
  • Moist skin. Having smooth skin that is kept moist will allow them to absorp oxygen from the water. While it will be very slow, it'll still get some oxygen
  • Strong immune system. They will be more susceptible to food-borne diseases, as cooking their food is harder
  • Larvae. A simple larval form would be an asset for these merfolk, especially if they could breathe water. A human-like tadpole form (or kidpole, if you will) would be a good base
  • Crocodilian heart. A heart like a crocodilian, with the doubled aorta, should allow these merfolk to recycle blood from the body directly back, bypassing the lungs. This would work well with gills or moist skin
  • Spiral valve. A spiral valve in the gut like a shark would be useful for absorping extra nutrients, due to issues with cooking food
  • Lungfish legs. Proper tetrapod legs are not good for swimming. Lungfish fins, with their segmented form, seem like a useful basis for leg-tails. Though they will need to be a lot more muscular

These features should make a rather nice mermaid

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