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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, 2020 at 15:50
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    $\begingroup$ 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, 2020 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, 2020 at 18:10
  • $\begingroup$ This takes me back. Thanks for going through your older questions and accepting answers! $\endgroup$ Mar 19 at 9:18

3 Answers 3

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

Respiratory

  • 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

  • Trunk. A trunk on the blowhole, while it may seem superfluous, would be exceedingly 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

  • Efficient brain. A more oxygen efficient brain, like that of octopodes or manta rays, could be quite useful in reducing oxygen requirements

  • 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

  • Moist skin. Having smooth skin that is kept moist will allow them to absorb oxygen from the water. While it will be very slow, it'll still get some oxygen

  • False hair. 'Hair' growths like that of the hairy frog will work well as gill surfaces, at least with the moist skin. These could replace the regular hair on the head and other hairy parts: However, it will likely have to keep short

  • Low oxygen requirements. Many organisms, including some animals, can survive without oxygen. While these organisms are often small and simple, using anaerobic methods where possible will greatly reduce their need for oxygen. However, they will still need it, and so will still need to do some gas exchange

  • Mesothermy. Heating the body requires energy and oxygen, and so reducing the amount of heat produced will also lower oxygen requirements. This shouldn't slow your merfolk down; some of the fastest animals in the sea are mesothermic

  • Oxygen storage. Cetaceans also have adaptations for storing oxygen. Specifically, they have large amounts of myoglobin and hemoglobin in the muscles and blood, which can store large amounts of oxygen. This obviously is useful for an animal that must surface for air

  • Open coelom. Allowing water to flow into the coelom, as in lancelets, provides more surface area for gas exchange. This could be achieved with one unused pair pharyngeal pouches descending down and opening into the thoracic cavity. This would allow water to be admitted into the coelom, and flow over the organs. The water would exit through an atriopore in the front of the pelvic cavity, and would be moved by cilia, perhaps with a few strategically place arthropodal hearts to aid in pumping. The exposed coelomic surface would need to be rich in blood vessels to make the most of the flow

  • Internalized Gill Stalks. Instead of (or perhaps even in addition to) regular fish gills, these merfolk could possess stalked gills like those of amphibians and lunged fish, which would be rooted in the throat and hang down into the open region of the coelom. These gills would have more space than the fish-like internal gills, but may be slightly less efficient due to having to push water through the entire coelom

  • Cetacean lungs. Cetaceans have many adaptations in their lungs, such as a double layered alveoli vessels, that are excellent at capturing oxygen

  • Digestive tubules. Having arachnid digestive tubules in the intestines would work well with an open coelom. While they wouldn't be much help for digestion, they would certainly increase the external surface area of the gut, which would aid gas exchange if the coelom is open to the water

  • Mantle cavity. It should be possible to reduce the length of the small intestines with a spiral valve (though I'd say they should still be longer than in sharks). This, alongside a shortened colon, should put some space in the merfolk's belly that could be filled with a molluscan mantle cavity. This cavity should be in front of and to the sides of the bowels, should be roughly filled with a pair of feathery gills, and would be entered or exited through an opening where the private parts should be

  • Cloacal breathing. There could be more respiratory organs within the cloaca, to extract extra oxygen from the water. While they may not be as efficient with a mantle, they'd still be worth it. The two main forms of cloacal breathing would be the turtle form, involving a lung-like chamber in the pelvic cavity, and the sea-cucumber form, with two long respiratory trees growing along the abdomen. These structures could likely coexist together in one species

  • Siphon. If you give them a mantle cavity, you'll also need to give them a siphon, like that of squid or clams, in order for them to use the mantle gills with their legs together. The best solution would be a double-siphon, like in clams, which is wide enough for lots of water to flow

Sensory/Communication

  • Melon. A melon, like that of dolphins, would be useful for echolocation, which is a rather useful feature for aquatic animals

  • Fish eyes. They should have water-adapted eyes like a fish. You should still include the irides, and could have a more tetrapodal focusing system, but features like the shallow cornea and spherical lens seem quite important

  • Forked tongue. A forked tongue with a vomeronasal organ would be a useful feature for these merfolk, as it would allow them to smell things underwater like a snake. It should be possible for the vomeronasal organ to be set into the roof of the mouth, as this arrangement is more-or-less how an elephant's mouth works

  • 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

  • Electroreception. Electroreception is the ability to sense electricity. This is useful underwater, and can be used for communication

  • Hygroreception. If they can't hold water well (which is likely for merfolk), it'd be useful to be able to detect air moisture, so as to avoid drying places

  • Osphradium. An osphradium in the mantle cavity would be quite useful in ensuring that the water flowing in this region is clean, given its distance from the rest of the senses

Digestive

  • Suction feeding. It is hard to get food into the mouth underwater, and so these merfolk will need some adaptations for this. Suction feeding seems like the best solution

  • Prehensile tongue. A prehensile tongue is another way to make sure food stays in the mouth. Both suction feeding and a prehensile tongue could coexist, either working in tandem or for different foods

  • Velum. A velum like a lamprey could help them drink on land. This structure would comprise of a pouch or tube in the throat, where the gills connect internally, with a slit at the top that can be opened and closed to allow or block flow through the gills

  • Gizzard. It seems like it'd be easier for food to fall out of merfolk's mouth, due to the effects of water. Hence, it would be useful for them to have gizzards or something similar, which might grind down their food deep in the body after it has been swallowed

  • Spiral valve. A spiral valve in the gut like a shark would be useful for absorbing extra nutrients, due to issues with cooking food

Locomotive/Musculoskeletal

  • Flexible neck. These merfolk would need a flexible neck that allows them to comfortably turn their heads to face forwards both while swimming and walking. This wouldn't require much alteration, as shown by various apes

  • 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 retractile, but many creatures do fine with this limitation

  • Swim bladder. A swim bladder will help these mermaids manage their buoyancy. It would likely need to be attached to the lungs as an extra lobe, but it wouldn't need to totally eclipse the lungs in size

  • High strength. A much higher level of strength should be important, for swimming, defense, and other needs

  • Pachyostosis. This is essentially 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 tetrapods

  • 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

  • Finlets. The legs should have a set of finlets, like a tuna, on the outside of the legs, as these would help them while they swim

  • Tail-flippers. The legs will need a large, fin-like surface for swimming. The best solution should be something like each leg ending in something like a fleshy ray-fin, which could work as both a foot and a tail-fin

  • Prehensile feet. Prehensile feet will make it easier to avoid drifting away in the water, as they can simply grab onto the floor

Defensive

  • Venomous bite. Having venom in the mouth will be useful in driving off predators. Ideally, the venom should be optimised to produce pain

  • Cnidocytes. The sea is more dangerous than the land, as dangerous animals could come from any angle. Cnidocytes would be a good first line of defense against predators, as they respond very fast and are quite effective at deterring predators

  • Contractile blood vessels. Being able to naturally clamp off blood vessels, like lizards can do with their tail, would make it much easier to survive traumatic amputations

  • Colour changing. Being able to change their skin colour will aid in camouflage, which makes it much easier to hide from predators

  • Poison. Toxic, bad-tasting flesh will be useful to keep predators away, as they will learn not to eat these merfolk

  • Cocoon. Forming a tough cocoon around a pupa is a good way to defend it against predation, though it will also impact respiration

  • Scales. Bony scales under the skin will protect these merfolk from attacks that pierce the skin

  • Shell. A shell like a turtle would be very useful in protecting against bites. This shell could be internal, as in soft-shell turtles, and shaped like a human ribcage to make sure they still look like merfolk

  • Ctenophore tentacles. A pair of sticky retractile tentacles could be useful in snaring prey. They could be placed somewhere in the groin or inner thighs, and would be soft and gelatinous (though they could likely have musculature to move them)

  • Ink sacs. Having an ink sac would be useful, as it could provide good cover to swim away from predators. The ink sac should be placed behind the intestines in the coelom, opening into the anus

Reproductive

  • Larvae. Expanded upon in the next section

  • Internal breasts. Having the breasts inside the body, as in dolphins, would be an easy way to reduce drag. They should be under the ribcage, so there is space for them to expand into the abdomen

  • Pouch. A pouch like in marsupials would be a useful adaptation for protecting the young from attacks, as the larvae could crawl fully into the pouch to avoid attacks

  • Eggs. If they have a mantle cavity, childbirth wouldn't be an option. They would need to lay eggs. These eggs should be fish-like, as amniotic eggs won't work in the water

  • Claspers. If they have a siphon (or even just the mantle cavity), then their genitalia will have to be remade to account for this. A pair of claspers beside the siphon would be a good solution

  • Fused fimbriae. If the coelom is open, then the uterine tubes will have to be solidly fused to the ovaries, to keep seawater (and any contaminants therein) out of the oviduct

  • No penis/clitoris. If there is a mantle cavity, then the original erogenous parts of the genitals will have to be eliminated from the genome, to ensure that no merfolk end up choking themselves trying to use them

  • Selective vagina. In water, it is much easier for sperm to leave the semen and swim away. This means that it is important that the vagina can be opened and closed to keep unwanted sperm out

Larval

  • Yolk. If they have eggs, they'll need a yolk to supply the young with food. The yolk only has to be large enough to supply them until they can drink milk, so it shouldn't be that large compared to other species

  • Suckers. Some form of attachment near the head, such as a sucker-mouth, would be useful in attaching to their parent's teats

  • External gills. External gills can be much larger than internal gills, due to their extended nature and the fact that they sit outside of the throat. They are also useful for stirring up stagnant water, which is useful for merfolk in anoxic seas

  • Pupa. A pupa could help defend the larvae as they metamorphose, and it would allow a quicker, more aggressive remodeling as in butterflies and other such insects

  • Tail. Tails develop before limbs, and so it'd be useful if the larval tails could be used to swim. A vertically compressed tail with a fringe/fin would be the most sensible solution. The legs, as they develop, would sit beside the tail, which would degenerate once the legs were fully developed

Other

  • 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

  • Strong immune system. They will be more susceptible to food-borne diseases, as cooking their food is harder

  • Hemocyanin. An oxygen-carrying pigment suspended in the blood plasma will greatly increase its capacity to absorb and store oxygen, which is very useful to these mermaids

  • 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

  • Strong kidneys. Strong kidneys will aid in osmoregulation, which is useful for a saltwater amphibian. It will also allow your mermaids to go into brackish or overly briny water

These features should make rather nice merfolk


If you don't really care about ensuring a humanoid framework, there are many more adaptations you can add:

Respiratory

  • Thick neck. If you give your merfolk gills, you could also try bulking up the neck to provide extra space for said gills. This couldd limit the flexibility of the neck, though there is alway an option of simply making it longer

  • Operculum. An operculum covering the gills would be quite useful, especially on a thicker neck. This will not only protect the gills, but aid in pumping water over the gills

  • Book gills. Book gills on the stomach, as in that of the horseshoe crab, can be used for extra gas exchange in the water. They could also be angled, for extra spanwise flow as they swim

Sensory/Communication

  • Bulbous eyes. Large round eyes like a fish will let your mermaids see much more of their surroundings than human eyes would, which is useful in a 3D world like the sea

  • Eyestalks. Just as said above, a wide field of view is useful, and moveable eyestalks can maximise this. However, bulbous eyes are still useful with eyestalks as they increase the solid angle visible at once. Furthermore, the eyestalks could quite simply be made retractible to allow for more streamline

  • Antennae. Long antennae with scent and taste receptors will be useful in sensing scents and tastes in the water, as well as triangulating their position

Digestive

  • Strong jaws. A powerful bite force seems rather useful to aquatic creatures. This will require a tough jaw system and large muscles, as in crocodiles and such creatures

  • Crop. A crop for storing food would be useful, as it is hard to carry food under the surface. A crop could be used as in ants, as a communal stomach into which food can be shared

Locomotive/Musculoskeletal

  • Streamlined head. A long, pointed head like a shark or whale will be remarkably useful for a swimming animal

  • Cephalopod arms. Having the manipulators off of the limbs would be rather useful for a swimming animal. Cephalopod limbs can also be much better at manipulation. These limbs should be placed around the mouth, as the lips. This can still be kept streamlined, as seen in creatures like cuttlefish

  • Catch connective tissue. Catch connective tissue is a type of connective tissue which can quickly change its stiffness. This could be useful in cephalopod arms, to allow the limbs to become stiff or flexible for swimming or manipulation

  • Fusiform body. If the neck is thickened up and the face streamlined, it makes sense to go all the way and give these merfolk a spindle-shaped body to cut through the water

Defensive

  • Spines. Spines over the body should be useful in protecting the body. This could be combined with the false hair, which would also protect, and perhaps support, the hairy gill-surface

  • Stinger. A stinger, like spines, could be used to poke at predators. Stingers could also inject painful venom into predators. A good solution here would be something like in the platypus, with the stinger on the feet/tail-fin

Other

  • Blubber. Blubber will keep the body warm, which is important underwater, due to the higher thermal conductivity of water. This is doubly useful in the case of mesothermy where there's not as much heat being produced

If a strict adherence to homology is not required, there are several more additions you could make:

Respiratory

  • Spiracles. Normally, the first gill-tube is occupied by the ear. If this structure is duplicated, however, then these merfolk could have spiracles on their head, thus allowing them to breathe water without involving the mouth

  • Gills as lungs. Replacing the lungs entirely with a set of gills will massively increase the oxygen uptake of the merfolk. However, it will likely entirely use up the oxygen of the water, hence the outflow of the gills shouldn't flow back into the coelom if other respiratory organs are placed there. The lack of lungs shouldn't be much of an issue: Gills can adapt to use air, and with this (and earlier adaptations) they likely won't need to come up for air anyway

Digestive

  • Belly proboscis. A proboscis like that of the tricladids could serve as a useful backup to the mouth, due to the true mouth's involvement in the gas exchange systems

Locomotive/Musculoskeletal

  • Tentacle arms. Using squid-like tentacles to replace the arms could be an interesting adaptation for your merfolk: As they lack bones, it should be plausible enough to make these arms entirely retractile, which allows them to entirely eliminate the arm's drag when swimming

  • True tail legs. Using a pair of actual tails as legs, rather than using a lungfish/human hybrid structure, would be a rather useful feature. It would allow for a much more sensible anatomy regarding the tail fins, and would make the strength and fish-like appearance much more sensible

Reproductive

  • Transposed uterus. Moving the uterus away from the anus and mantle cavity would allow for viviparous reproduction to coexist with the mantle, which would at the very least make them more compatible with humans.
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    $\begingroup$ King you just outed yourself as a genetically engineered merperson. Or maybe one of the emgineers. $\endgroup$
    – Willk
    Dec 3, 2021 at 3:45
  • $\begingroup$ What does not having a penis or clitoris have to do with choking??? $\endgroup$
    – Daron
    Mar 17, 2022 at 21:55
  • $\begingroup$ @Daron If they have a mantle, this would put the reproductive orifices in the same region as the gills, which could cause choking if these orifices are still erogenous $\endgroup$ Mar 17, 2022 at 22:53
  • $\begingroup$ How effective would a two-tails/Melusine style arrangement be for swimming? $\endgroup$ Dec 29, 2023 at 22:21
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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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