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The creatures in question have roughly similar biochemistry to earth tetrapods. The respiration mechanism should be able to supply enough oxygen for a 1 kg animal to move slowly through the water and shuffle about on land. It would be ideal if the respiratory system could be easily optimized for air-breathing through minimal alteration, although this is not a must.

Edit: two respiratory systems which work separately to bring oxygen into the body is also fine.

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  • $\begingroup$ For large life ( ones that actually need a breathing mechanism) there simply isn't one. All the lifeforms that transitioned from aquatic to territorial changed their breathing mechanism. The environments are just too different. $\endgroup$
    – John
    Oct 20, 2020 at 3:10
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    $\begingroup$ Lungfish are quite big. @John: Surprisingly, having lungs is the ancestral condition. It is the fish who modified their lungs into swim bladders, not the tetrapods who developed lungs. $\endgroup$
    – AlexP
    Oct 20, 2020 at 6:55
  • $\begingroup$ en.wikipedia.org/wiki/Amphibian The young generally undergo metamorphosis from larva with gills to an adult air-breathing form with lungs. Amphibians use their skin as a secondary respiratory surface and some small terrestrial salamanders and frogs lack lungs and rely entirely on their skin. $\endgroup$
    – Sol
    Oct 20, 2020 at 7:02
  • $\begingroup$ @alex the lungs still evolved to extract oxygen from air. $\endgroup$
    – John
    Oct 20, 2020 at 13:10
  • $\begingroup$ @John: Yes they did. The point is that lungs are basal in the line of bony fishes (Osteichtyes); the ancestors of tetrapods already had lungs when they crawled onto the shore. Then the line split, and tetrapods lost their gills while the remaining fish repurposed their lungs as flotation devices. $\endgroup$
    – AlexP
    Oct 20, 2020 at 13:13

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Water holds much less oxygen than air

This is why gills are such fragile delicate things. The amount of oxygen absorbed scales roughly linearly with two variables. One, the concentration of oxygen in the medium it wants to absorb from. And two, the area of the exchange surface (as the rate is approximately constant per unit area). So gills need a much larger surface area to transfer an equivalent amount of oxygen - to make up for the much more oxygen-poor medium it's trying to extract the gas from.

In turn this means gills have a much more complex internal structure. This structure is also designed to work while soaked in water. When removed from water, the delicate folds will tend to stick to one another and the structure tends to collapse. This is why fish suffocate in air even though air is much more oxygen rich.

Air is about 21% oxygen by volume. It holds approximately 210 milligrams per liter whereas water holds maybe 8. If it's oxygen rich. Air is more than 25x richer:

Well-oxygenated surface water may only contain around 8 mg O2/l, while the air contains 210 mg O2/l.

So, you can't have a single organ be both lung and gill. What do real-world amphibians do? Generally they use their whole skin to "breathe" (and die if they dry out).

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It would depend HUGELY on the atmosphere of your world.

But assuming Earth sealevel normal: Simple damp skin, well-supplied with blood circulation, is enough to survive and will work in air or submerged. It would only support very minimal exertion though.

This is exactly what a normal lung is, just with a very convoluted geometry to fit more surface area into the same volume, and to keep it moist.

It is also what gills are, except that gills tend to be optimized for use in water.

Both lungs and gills are merely damp thin skin exposed to the environment, with structures to make as much surface area as practical.

If your creature needs a constantly active lifestyle, it will need lungs or gills. If it only requires a burst of activity interspersed with long periods of inactivity, consider storing oxygen in internal storage organs.

Wild idea: creature has bare-survival-level lungs/gills/whatever. It also has oxygen storage ability. The Nest is a construct or symbiote that is sessile, and gathers a store of the oxygenated juice. Effectively, you could have a mobile creature without the encumbrance of complex lungs, but capable of high metabolism by ingesting both fuel and oxygen from external sources.

P.S. A creature with a build-in oxygen reserve like this would be a very scary thing in a fire. Containing both fuel and oxidizer, it would be somewhere between flammable and downright explosive!

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There are some fairly large crab-like arthropods (coconut crabs, which mostly live in trees -- not true crabs, they're a variety of hermit crab) that depend on gills while on land -- but they have a means of keeping their gills wet, as well as a method of reoxygenating the water they carry around for this purpose.

Some other species of hermit crab can also survive for rather a long time on land -- when kept as pets, they're usually in a terrarium (they must have a ready supply of water, but they don't live in it), though there are also hermit crab species that live in water full time (my partner, who keeps a reef aquarium, calls them "snail murderers" because of their lust for ever-larger stolen shells).

As far as I know, however, the coconut crab is the largest of these, at around 1 kg body mass for a large example.

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