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Sperm whales can go 90 minutes at a time without needing to surface.

I am interested in an idea for a creature that is air-breathing, but spends a lot of time at depths of (100m to 300m) and sometimes briefly on land. An adult is about the size of a human, and is warm-blooded, although not necessarily mammal.

An air-breathing version of a mermaid/man, that spends weeks under water without surfacing.

I'm flexible on shape and form, but I'm after the biological modifications I have to make to such an air-breathing animal who spends its weeks-long migration periods submerged under the ocean surface.

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    $\begingroup$ Fun Fact: Breathing isn't necessary. Getting air into the blood is all that's important. In fact, we can do it now. $\endgroup$ – Frostfyre Nov 29 '15 at 0:31
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    $\begingroup$ @Frostfyre - Think of SpecOps personnel using it to oxygenate their blood without scuba or rebreathers ... I don't know how long it would keep things oxygenated when the body is being utilized (like in swimming) but this sounds like something someone should throw into a book or something ;-) $\endgroup$ – Pᴀᴜʟsᴛᴇʀ2 Nov 29 '15 at 0:52
  • $\begingroup$ see thaddus answer, but with modification. The creatures have lungs and can live indefinitely in water, but they are better at breathing. When they exert themselves they burn through calories, and thus oxygen, faster and so need to return to the surface for air. For low exertion times they can last quite awhile using just gills, but returning for air allows them to fuel more extensive work. $\endgroup$ – dsollen Nov 30 '15 at 18:29
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The word you are looking for is amphibious. If your creature is amphibious, it can live on land and in water in equal measures.

Being amphibious means your creature could indeed have gills, perhaps they are poorly formed, vestigial or are fading away as the creature approaches a life which can be spent almost completely on land.

  • Frogs and toads are both amphibians which start their lives in water and slowly transform until they are mostly land-dwelling, air breathing creatures.)

  • If you insist your creature be warm-blooded, you are insisting your creature be able to maintain a constant flow of oxygen. Maintaining that body temperature requires both fuel and oxygen. Warm blooded animals burn more energy and require more air to breath, hence the sperm whales (indeed most whales) need for regular oxygen.

  • If you were to turn your creatures into cold-blooded creatures you could reduce that oxygen intake significantly, assuming you reduced the movement activity required by the species to survive.

  • Streamlining the creature, making its ability to navigate water environments low in energy, perhaps it lives in environments with strong currents for example and it specializes in recognizing and utilizing deep water currents for high levels of mobility. This might be why it dives so deep and holds its breath (or uses gills) for extended periods. It is utilizing the underwater currents for fast travel across wide bodies of water.

  • Perhaps this is a migration technique, the creature consumes many calories, fills up and then dives into the water swims out to where it knows deep water currents flow, then slows its metabolism, activates its vestigial gills and sinks for a low-energy swim through deep water currents where it can move three or four times as far for the same amount of energy. It only surfaces briefly where it oxygenates its blood and then returns to the rip currents.

  • At the end of the migration, it reorganizes itself and becomes air-breathing again.

If the idea of vestigial gills doesn't work for you:

  • though it should because most creatures who live on land don't just decide to return to water without there being a biological imperative, you will have to consider more efficient ways of binding oxygen to your creature's life cycle.

  • Perhaps through more complex chemical means, stored in glands designed to liberate oxygen slowly. But if they creature is warm-blooded, it may need quite a bit of this material which means it may need to be many times larger than a Human. Perhaps you could give it a diaphanous skin layer which expands like a dorsal fin for example or extraneous limb finds where such fluids could be found and they could store oxygen.

  • You could, if such expanded fins were large and diaphanous enough, perhaps use them as an oxygen exchange system, using an osmosis or chemical process to capture oxygen from the water, thus reducing, but not eliminating the need to surface every few weeks.

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  • $\begingroup$ This is so close to my answer that I don't think I will add my own. but one key update, I would suggest making the creature theoretically able to survive indefinately underwater, if low activity. The gills can sustain a certain level of activity, but high activity burns through oxygen faster. Thus when exerting oneself heavily, fleeing creatures, fighting, whatever, they have to fall back on their lungs and reserve of oxygen to keep up with the high activity. They go back for breaths when their oxygen reserve is depleted. $\endgroup$ – dsollen Nov 30 '15 at 18:27
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How active is your creature during that submerged migration?

If it's safe during that time, it might hibernate while floating with the currents, only waking up a little when its feels its time to find the next current to continue its journey. Weeks without breathing still seems improbable, but a very low metabolism combined with some oxygen absorption through the skin and some oxygen storage like sperm whales have should work.

If the creature needs to hunt food and/or escape predators or otherwise be active, it will need to store oxygen in its tissue and slowly release it. The time spent on the land will let the creature recharge its reserves of oxygen.

Humans have hemoglobin to bind oxygen, but unless your creature has giant blood bladders throughout its body, it will need some more efficient chemical process to store the oxygen safely and release it as needed.

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  • $\begingroup$ The only answer here that addresses the question... $\endgroup$ – DraxDomax Dec 1 '15 at 14:33
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How about a totally different metabolism?

Terrestrial life is based on the oxygen metabolism being in the cells. Lets try something totally different: The lungs are a much more complex system, the oxygen metabolizing system is in the lungs--instead of the bloodstream delivering oxygen it delivers the energized molecule that the creature runs on. The creature has a two-part bladder, one half holds the energized form and the other side holds the depleted form. It's a far more compact storage system than our bodies use, thus the creature can store much more energy.

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I want to address the question, not propose an entirely different idea and not mention gills, as you are clearly avoiding them.
Here's some sci-fi:
What if it had an organ that would pressurize oxygen into a highly dense crystal?
Metallic oxygen can be produced in lab, either very low temperatures or very high pressures.
Impossible in real terms.
But let's be creative!

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In that case, simply give the creature extremely large surface area of his lung surfaces, so that it can draw much more oxygen from the air than we can.
Then give to glands, controlled by the automatic nervous system, that secrete two chemicals, one binds to oxygen and the other releases that stored oxygen.

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