For my future world, I've decided that coastal cities need fresh water and there isn't enough of it to go around.

I don't know how jellyfish work. Is it feasible or completely impossible for scientists to bio-engineer giant jellyfish to convert salt water into fresh water? Do they store water in their bodies, or are they just made of water?

I would imagine that the jellyfish would convert the salt water into fresh water and it would be stored in its body for humans to harvest later. Could that work or do jellyfish not absorb water into their bodies and can't do this?

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    $\begingroup$ "I've decided that coastal cities need fresh water" Honestly, that's not a particularly revolutionary decision. $\endgroup$
    – RonJohn
    Commented Nov 7, 2017 at 19:58
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    $\begingroup$ If they do, wouldn't they just spew it back out into the ocean? Or would you hook up hoses to them, to reclaim the purified water? Also, why jellyfish instead of any other animal? $\endgroup$ Commented Nov 7, 2017 at 20:18
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    $\begingroup$ @BrettFromLA - Isn't the idea to harvest and process the jellyfish, themselves? $\endgroup$ Commented Nov 7, 2017 at 22:14
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    $\begingroup$ I think you have to ask... is bio-engineering jellyfish and then harvesting the fresh water from millions of them more cost effective than other forms of desalination? $\endgroup$ Commented Nov 7, 2017 at 23:18
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    $\begingroup$ "I don't know how jellyfish work." How about you open a new tab in your browser, and enter en.wikipedia.org/wiki/Jellyfish in the address line? $\endgroup$
    – Karl
    Commented Nov 8, 2017 at 6:38

9 Answers 9


No. Jellyfish are osmoconformers

Osmoconformers are marine organisms that maintain an internal environment that is osmotic to their external environment.[1] This means that the osmotic pressure, or osmolarity, of the organism’s cells is equal to the osmotic pressure of their surrounding environment.... Most osmoconformers are marine invertebrates such as echinoderms (such as starfish), mussels, marine crabs, lobsters, jellyfish, ascidians (sea squirts - primitive chordates), and scallops.

Vertebrates maintain an internal environment that is slightly hypoosmolar - less salty - than seawater. Blood is not as salty as the ocean. I have read that the salinity of blood is the same as the salinity of the ocean in the ancient period when our ancestors evolved closed systems.

All that said I like the idea of giant bioengineered jellyfish. I can definitely imagine an anime with loads of giant jellyfish all around. Or was that Ponyo?

  • $\begingroup$ damn, I wanted to disagree but as my answer progressed it became apparent nothing would resemble a jellyfish. $\endgroup$
    – anon
    Commented Nov 7, 2017 at 20:38

Is it possible? yes but...

There are biological constructs capable of filtering salt. Could a creature theoretically convert salt water to freshwater, yes but the big question is what does it do with the salt or water?

The law of conservation of matter still applies.

I would think this creature could be kind of like a stromatellite building a big salt rock on its inside. Though as soon as it punctured it would be chaos.

Maybe it could store the Na and Cl ions as a means of metabolic energy.

It would need energy to do this.

Could it look like a jellyfish? Maybe, depending on what it does with the salt and how it acquires energy will greatly affect its appearance.

If it photosynthesizes, it wont be that clear color. If it makes a big salt rock it likely wont float or if it does it sure wont look like a jellyfish.

Im also not convinced it would be capable of completely desalinizing water but im not sure it couldn't either.

But, on the salt rock train, I could see a thermally powered anemone concept that looks like a jellyfish stuck on the floor.

There is also the inverse of storing salt and instead store the water. In this case the jellyfish would inevitably float and be stuck on the surface which is useful because it makes harvesting easier. They would be like organically grown pre-filled water bottles. But no amount of tinkering could overcome the buoyancy problem because the stored water would be less dense than the ever increasing saline water, that is in addition to retaining the jellyfish structural appearance.

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    $\begingroup$ I think the bigger question isn't what you do with the salt. Organisms have many ways of dealing with waste products. The real question is how you collect the fresh water... even if you manage to engineer an organism that filter out the salt, how do you make it practical to collect the resulting fresh water? $\endgroup$ Commented Nov 7, 2017 at 22:26
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    $\begingroup$ So you took the inverse of my dilemma which was instead of storing the salt you store the purified water which is really the same problem. HOWEVER the benefit of your approach is you have solved the collection and purification problem. A: harvest the jellyfish and sell them as organically grown water bottles. $\endgroup$
    – anon
    Commented Nov 7, 2017 at 22:37
  • $\begingroup$ But the problem still remains because it would still sink $\endgroup$
    – anon
    Commented Nov 8, 2017 at 13:04
  • $\begingroup$ What if the stromatellite was coated in something water-insoluble? $\endgroup$ Commented Nov 8, 2017 at 15:19
  • $\begingroup$ The problem with a fresh water containing jelly fish there would be constant osmotic pressure pulling the water out. A jelly fish like thing would not work well. $\endgroup$
    – P Chapman
    Commented Nov 8, 2017 at 15:44

Sorta maybe.

Jellyfish don't do much, but some do generate pressure to swim. If you put a valved membrane across the ring muscle reverse osmosis could give you fresh water.

However fresh water is bad for jellyfish, and moving it from the mouth to somewhere you want it is work. So you need to give the the jelly some non-jelly plating, and a second exit for the water to be pushed into. With those changes you are halfway to a squid.

Really though I'm just using it as a living pump, so most creatures would work fine. Filter feeders like mollusks might be a better choice since they already are capable of pumping, filtering, surviving hostile environments and don't have a lot of unnecessary propulsion systems. And as a bonus make pearls and shells.

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    $\begingroup$ The osmotic pressure will compress your jellyfish into sth. very small and wrinkly. Sea- to freshwater has 30 bars. $\endgroup$
    – Karl
    Commented Nov 8, 2017 at 6:43

Seawater fishes do have the same problem.

Their skin (and scales) aren't exactly watertight - meaning, next to swallowing up seawater with their food they take up seawater through their skin as well which would raise the salinity of their blood and tissue.

So, to counter that their kidneys do actively transport the excess salt into their urine to be expelled whch gives their urine a higher concentration of dissolved salt than the surrounding seawater.

Meaning, a natural desalination mechanism already exists, though only in higher order organisms which need to rely on more complex mechanisms to expel waste material, but it could be feasible to engineer a bladder-like structure with a membrane with active transport to move a highly salinated solution out of it, thus reducing the salinity inside the bladder. Or, on the other way round, these organisms could be engineered to "pull out" the salt out of the solution they swim in and sequester it in their bodies, much like kidney stones growing.


Well, not in the way you're probably thinking of. Jellyfish aren't filters, and they don't extract salt from the ocean as they pass through it- so you can't use them directly to extract the salt from ocean water. As Will points out, they're osmoconformers and could care less about the salinity of the water.

However, I can think of a few ways to use jellyfish as part of the water purification process.

1) Use them for transport

This idea requires a bit of technology on the scientist's part, but well within reality. The key tech here is a semipermeable membrane that allows water to pass through, but not salts. These are commonly used in modern science and are part of the reverse osmosis desalination process. With a semipermeable membrane, all that's required is a pressure differential.

Jellyfish are bad at a lot of things, but moving vertically in the water column is not one of them. Some jellies, such as the beautiful and massive Lion's Mane jellyfish undergo daily migrations between the surface and the depths, known as diel vertical migration. If properly trained, I can easily imagine our massive Lion's Mane jellies making daily trips to the seafloor with a semipermeably sealed container on their bells, which would fill with fresh water, rejecting salts, as the jellies sink to the seafloor. Each night, they would return to the surface with containers filled with fresh water.

2) Engineer them as substrates

Some jellyfish would be excellent candidates for the harvesting of biological, semipermeable membranes. In particular, the Portuguese man o' war, while not a true jellyfish, has a large float made of a single cell. This float could be repurposed or engineered by scientists to produce biological semi-permeable membranes that could be harvested for the larger desalination plants, solving the problem of constant, expensive filter replacement. Additionally, semi-permeable membranes made from jellyfish would likely be less vulnerable to dissolved organic carbon, which is actively problematic for current systems.


This is really not what you asked, but your question got me thinking. In the real world, the favorite solution for coastal cities without enough water is to channel it from other areas. When that's impossible, the solution is physical desalinization in a process called reverse osmosis. You're looking at biological desalinization.

Jellyfish are a bad choice. As Will said, they maintain the same salinity as the water around them. Changing this would involve so many changes to their physiology that the result would not be jellyfish. They'd need a circulation system like you see in fish to isolate the salt from the surrounding water.

Usually, single celled organisms will be easier to breed into a specific task like this than complex organisms like jellyfish. So what you can do is breed bacteria that maintain internal salt content higher than the water around them. I'm handwaving how you would do this, but a microbiologist might have some ideas. Then you filter the salty bacteria out of the water, leaving behind less salty water, or even fresh water if the bacteria are efficient enough.

If you really want jellyfish, you could make them host the engineered bacteria. But at that point, you'll have a huge drop in efficiency because instead of just feeding bacteria, you have to feed the jellyfish plus the bacteria, and will probably have much less bacteria.

  • $\begingroup$ I read this as: the jelly-fish consume food and as byproduct of digesting the food the jellyfish produces fresh water, which is then harvested. Is that correct? $\endgroup$
    – Pieter B
    Commented Nov 9, 2017 at 10:34
  • $\begingroup$ @PieterB, No. It's that bacteria, as part of some unexplained metabolic process, sequester salt internally apart from contact with the water. Once the bacteria are removed from the system, the water remains fresh. Jellyfish are merely an awkward, unnecessary complication to my answer. $\endgroup$
    – Karen
    Commented Nov 9, 2017 at 17:40

Since you're free to explain this any (plausible) way you want, you could have bio-engineered jellyfish that are captive. They are tethered offshore and are connected to a pipe system that continually transports the fresh water back to shore. The jellyfish are filter-feeders, taking-in nutrients and sea-water, excreting digestive waste and salt back into the ocean, and having the fresh water tapped from their bodies in some way.

You could even make them "solar powered" like a lichen by having a captive plant contained within the body of the jellyfish to provide extra energy. Since the jelly fish have clear bodies, they could be tethered right at the water surface and sunlight will easily penetrate within the body to provide the power.


At the very least those jellyfish will need to resist the osmotic pressure, so they will need a hard carapace and / or thick rubber-like skin able to withstand that pressure. So the answer is "no", unless you're ready to call such creatures jellyfish.

I suppose it would be easier to bio-engineer a regular fish into a slow-moving bloated organism consisting primarily of water with low salinity. Still, it would never achieve the body water content that jellyfish have.


As someone said, what do you do with the salt? It would eventually make the ocean too salty as you remove the fresh water. Better invent a machine to separate H20 and use sodium and chloride, both very unstable elements to generate the heat(energy) to do it. BTW fresh water is available aplenty in the colder waters of the artic within the old ice, and in deep water. Since hot salt water floats to the top, perhaps you can find cold fresh water deep down in pits on the ocean floor. Or perhaps you can make a plant that drinks salt water, survives and yet produces a freshwater fruit like Watermelon.

  • $\begingroup$ '...It would eventually make the ocean too salty ? There's so much oceanic volume that a tidbit of extra salt won't make any difference at all. $\endgroup$
    – AliceD
    Commented Nov 8, 2017 at 8:57
  • $\begingroup$ "use sodium and chloride, both very unstable elements to generate the heat(energy) to do it." - Err, if you turn salt into sodium and chlorine, you have to put a huge amount of energy in to the process. You can then get (most) of that energy back by reversing it, but this is only an energy storage mechanism (and not a very good one), not an energy source. $\endgroup$ Commented Nov 8, 2017 at 10:10
  • $\begingroup$ Also, salt water is denser than fresh water of the same temperature, so the pits on the ocean floor are likely to be saltier than the bulk ocean. $\endgroup$ Commented Nov 8, 2017 at 10:11
  • $\begingroup$ The last suggestion is the closest to plausibility - coconut palms for instance are tolerant of at least brackish water, and one can drink the coconut milk. $\endgroup$ Commented Nov 8, 2017 at 10:14

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