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Merfolk gills, when not set into the chest, can be placed within the cheeks

Specifically, they are found above the angle of the mandible, near the back of the jaw-region. They are structured like in chondrichthyes, though with fewer slits

Regarding the anatomy and development of this structure and the surrounding, could the system realistically exist?

Assume a human jaw-set, and complete elasmobranch-type gills. The gills wouldn't be the only gas exchange organ

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  • $\begingroup$ Is there an operculum? Is water flow controlled by jaw, tongue and facial muscles, like taking in a mouthful of air while keeping a closed glottis? $\endgroup$
    – Mike Serfas
    Jan 7, 2022 at 18:13
  • $\begingroup$ @MikeSerfas There is no operculum, as in elasmobranches $\endgroup$
    – Ichthys King
    Jan 7, 2022 at 20:02
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    $\begingroup$ can you give more info on what you expect as an answer? I do not see a/the problem $\endgroup$
    – bukwyrm
    Jan 7, 2022 at 23:29
  • $\begingroup$ @IchthysKing Elasmobranchs need to swim constantly to keep water moving over the gills. Your mermaids aren't sharks, so they could constantly force water through with their jaws/tongue/cheeks, but I'm thinking it would look very strange, and I don't know if it could move enough water that way. Do they only use these organs while swimming rapidly? What do they use while not moving underwater? $\endgroup$
    – Mike Serfas
    Jan 8, 2022 at 16:22

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Yes this system is plausible, but:

It's much more likely to evolve in natively aquatic species with singular circulatory system not divided in systemic and pulmonary circulation (maybe branchiary circulation here), because in divided circulatory systems main gas exchange organs are always relatively close to a heart, and having them in the neck would make it an even more vulnerable area with 2 arteries (at least) and a pair of gills with huge amount of blood flowing through them, ready to be ripped apart by predators or other merfolk (yes i know that gills in the neck are vulnerable enough without arteries, but i would really prefer to have my jugular cut OR lung punctured and not both at the same time)

I don't think it is as plausible for terrestrial species adapted to aquatic environment, because every stage of evolving a new organ must be beneficial to an organism, and creating completely new gas exchange organs seems much more demanding than adapting existing ones.

Personally i would use one set of retraced/retractable gills supported by breathing through skin, like a mudskipper. But since you want separate organs for breathing on land, i would suggest a second set of gills set into the chest, maybe even completely retracted inside a body, without slits, but with a set of orifices, limiting their drying. This would work fine with undivided circulatory system and doesn't look very out of place, just two sets adapted to different environments. Land gills could still support water gills while submerged, maybe even pumping water in and out themselves by muscular contractions.

I hope i helped.

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  • $\begingroup$ Welcom3 Adam. Nice answer, not sure that it could be achieved in human-like species anyhow without a radical increase in tissue-mass for gills, just in terms of the size - it'd be like having a beachball for a head. $\endgroup$
    – Escaped dental patient.
    Mar 9, 2022 at 21:20
  • $\begingroup$ Hello. Yes, I'm aware of that, realistically gills would at least bulge out neck a lot, but to be honest I'm not a hard science purist and can tolerate a little bit of hand-waving to make a concept "cooler" if it's realistic enough. $\endgroup$
    – 4dam
    Mar 9, 2022 at 21:32
  • $\begingroup$ That's fine, the question thankfully didn't ask for hard science and is more interesting for being without it. $\endgroup$
    – Escaped dental patient.
    Mar 9, 2022 at 21:40

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