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inspire by xenomorph acid blood from alien movies.

so the water either rivers, lakes, and oceans in my world or some of the regions is highly acidic because some of the dirt or stone minerals content mix with the water make it into corrosive acid that can corrode or melt normal skin tissue upon contact, and during prolong contact it can melt the normal flesh and damage or corrode metals especially iron/steel, and it also can rapidly damage normal wood especially wooden plank not native from this environment, but its not in the point to boil the water or increase/decrease the water temperature by itself nor creating dangerous gas, and rain water is safe though not acidic or not that high or corrosive.

the water also have color (not decide the color yet or will ends up add it in or not later) because some of the dirt or stone and water plant contain some natural dye minerals but its pretty safe for the creature, also the soil outside of the water is in permafrost state (not decide is it mix with the same acid or just normal ice yet, but eitherway even if it contain acid it quite small and not in liquid state so i dont think it will be dangerous or harmful), i add this in case its considered an important information to make better/clearer answer.

so far i assume the water creature can protect their skin or flesh by covering themselves with carapace and other type of shell like crustaceans and Bothriolepis fish, but i dont know how to protect their gills and eyes.

another is base of human stomach using mucus, but i dont know is it effective outside the body or for external organs (at least i think it can protect some of the internal organs or the flesh parts inside of the mouth) but i assumed the mucus can also block the gills making the creature unable to breath or can even still make the creature can see at all because of the mucus covering the eyes.

the aquatic creature not necessary need eyes but i add it just to see the visibility to protect that organs.

also the aquatic creature is form from natural evolution, not from genetic modification by inteligent lifeforms.

and so i want to know what water creature like fish and crustaceans like crab need in order to survive in this environment especially to their gills and eyes, and would there be need a dfferent biological solution or evolution between plain water and salt water.

also curious to see a better solution outside of exoskeleton or shell type ( not exclude it for answer just want to see is there a better possible solution outside of that.)

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The most generic answer? Lots of food, and proton pumps.

There are Earthling extremophile organisms that can thrive in highly acidic environments. Generally, they do it by carefully excluding the acids from their intracellular spaces--how acidic the external environment is then doesn't matter. Some of this can be accomplished with purely chemical means, by synthesizing as part of their metabolism basic and pH buffer compounds to counteract the effects of any acids that may diffuse through the cell membrane, but the primary defense is the proton pump. All Earthling cells have these mechanisms already; they are, for example, critical components of mitochondria, which set up proton gradients to power ATP synthase. It is, therefore, a relatively simple matter to repurpose that machinery to just continually pump stray protons out of the cell entirely to keep the pH down.

The catch is that this is a very energy intensive process. A complex multicellular organism with a large surface area exposed to the acidic environment, especially one that is expressly designed for exchanging dissolved materials with said environment (like, say, gills, or roots and leaves), will be expending a significant fraction of its total energy budget on Not Dissolving.

There is thus a significant advantage to simply altering ones biochemistry to tolerate the acidic conditions from the get-go. And seeing as how life is likely to start in the seas, or at least in groundwater (unless this is originally-alien life that has adapted itself to this world), that seems likely to have happened. Exactly how such adaptation would occur, however, depends on specifically which acids are present; different acids do not attack all other chemical substances equally. Perhaps, for example, your world's surface waters have a high concentration of sulfuric acid; in that case, you don't need to worry about dissolving so much as having all of your organic molecules forcibly dehydrated, reducing you to a hot pile of activated charcoal and nitrogen gas. Silicones with small organic side-chains, however, are not subject to the same kind of chemical attack by sulfuric acid--so maybe life on this world is just based on silicones in sulfuric acid solution, and actually has the opposite problem of keeping excess fresh water out of its cells! (Which would cause rapid heating from the heat of solvation and burst cell membranes from osmotic pressure.) A heavy acid like sulfuric acid would work especially well for your scenario, since rainwater, while still acidic from our point of view, would be naturally distilled and end up quite pure in comparison with sea water (just like our seas are filled with salt, but our rain isn't--and we Earthling organisms whose Latest Universal Common Ancestor evolved in sea water, do indeed have to worry about maintaining osmotic balance and not letting too much fresh water into our cells all at once, so that isn't even all that exotic of a concern in basic principles).

If, for example, your world is rich in nitric or hydrochloric acid instead, the necessary biochemical adaptations would be rather different.

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    $\begingroup$ @Chickensarenotcows Quite the opposite--you'd only get a lot of sulfuric acid if there's spare oxygen to go around. I mean, you could get a world where there's enough oxygen to make sulfur trioxide, but not a lot left over, but it's hardly certain. And if you did, the oxygen in that sulfuric acid would be just as, if not more, important as a metabolic oxidizer as the sulfur--i.e., you're looking at sulfate reduction, producing water and waste sulfur, not sulfur reduction, producing hydrogen sulfide. $\endgroup$ – Logan R. Kearsley Aug 29 at 4:09
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    $\begingroup$ On further reflection, while microorganisms (and plants, which can recycle the material on-site) may reduce sulfate all the way to elemental sulfur grains, if the environment is anoxic and complex heterotrophic organisms rely on sulfate reduction for respiration, I would expect them to only reduce sulfuric acid down to water and sulfur dioxide, since that can be more easily excreted in gas or solvated liquid phase. $\endgroup$ – Logan R. Kearsley Aug 29 at 4:23
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    $\begingroup$ I be fascinated how a metabolic system might pan-out in such a world, or rather a set of metabolic systems forming an ecosystem. Energy intensive, as you say, somewhat speculative though. That's for another question. $\endgroup$ – 011358 smell Aug 29 at 4:26
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    $\begingroup$ @Chickensarenotcows Should you wish to ask such a question, I'd be happy to pull out my copy of Stephen Gillett's Worldbuilding and expound on the section about sulfur worlds.... $\endgroup$ – Logan R. Kearsley Aug 29 at 14:03
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    $\begingroup$ @LiJun Yes, it is possible to save energy by only using proton pumps for pH maintenance on exposed tissues; you needn't even necessarily use it on eyes, if you have, e.g., compound eyes with silica corneas. Silicone-based organisms would not be edible for us; it would be massive good luck if they merely turned out to not be incredibly toxic. You'd get as much nutrition from them as from eating a silicone oven mitt. The rainwater would probably not be drinkable as-is, but it would be relatively simple to make it safe. And yes, all the same mechanisms would work for plants. $\endgroup$ – Logan R. Kearsley Aug 30 at 2:45

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