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The dragons of my world are serpentine fish-like animals with the ability to manipulate gravity

They have gills like bony fish, which they can keep in a state of apparent freefall to prevent collapse. The gills can also retain moisture. The dragons are warm-blooded and around 5m long

Would these dragons need lungs to provide oxygen, or would the gills be enough?

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  • $\begingroup$ Why would lungs be necessary in magical, gravity-manipulating dragons? Why wouldn't they? Who's to say? $\endgroup$
    – BMF
    Nov 21, 2021 at 19:00

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No.

Fish can obtain oxygen in a number of ways. Obviously gills, but oral membranes and skin are also part of some fishes' respiratory machinery.

Woolly sculpins can survive quite a while outside of water without lungs. According to the article, breathing in air is actually more energy efficient for these fish. They would have to remain moist for gas transport to occur either within the gills or on the skin.

You dragons will do the same. They might have a water bladder rather than lungs. This bladder would transport water to the gill arches where it is secreted in order to keep the mouth and gills moist.

Water could also be transported to the skin. I'd suggest that their scales can be vented slightly to allow air & water to mix next to the skin. When leaving the water, the dragons quickly flare then shut their scales while filling their water bladders. As they fly through the air, the high oxygen concentration enters perhaps through the nose where it enters the oral & gill chambers. They vent their gills very slightly. Just enough to create a flow of air through the chamber and out.

Water from the bladders is secreted into the nasal, oral and gill tissues which keeps the surfaces moist. Water is also secreted onto the skin under the scales. The scales allow moisture to be retained on the skin they cover, but also allow air to enter the space. This preservation of moisture is what allows dragons to exist in air for quite a long while before having to dive into a source of water.

I suspect that, eventually, they might very well evolve into a kind of creature that can properly drink water, rather than have to dive into it, in order to refill their water bladders. Once they do that, you've got a species of dragonfish that quite literally never have to enter water!

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