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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 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 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. $\endgroup$ – user2352714 Jan 28 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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