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I'm creating a humanoid aquatic race (not merfolk) with freshwater and saltwater variants. I have a general idea of what the race as a whole looks like, but I'm struggling with figuring out how the different environments would result in different appearances. E.g. colouring, patterns, build. The main thing I'm looking for is differences that will be visible, so that other races can tell which variant of the aquatic race they're looking at.

More specifically, I'd like to know what physical differences between real-life freshwater and saltwater creatures I could apply to my aquatic race.

If it helps, the aquatic race has the general body shape of a human, with slightly larger and webbed hands and feet. There are tentacle-like growths from the back of their heads instead of hair, similar to Kit Fisto from Star Wars. They also have ear fins. I do not want the realism or biology of this general appearance to be subject to discussion, only what the differences between the appearances of the variants could be.

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  • $\begingroup$ I believe you should be more specific with your question. Remember, this species is fictional, so you could theoretically make the changes whatever you want between the species. I think you should narrow your question down to "What is one universal physical difference between real-world freshwater and saltwater fish that can be applied to my fictional species?" That way you're asking a more technical question. $\endgroup$ Mar 14, 2022 at 3:22
  • $\begingroup$ Please clarify your specific problem or provide additional details to highlight exactly what you need. As it's currently written, it's hard to tell exactly what you're asking. $\endgroup$
    – Community Bot
    Mar 14, 2022 at 3:56
  • $\begingroup$ Appearance wise I don't know but your marine being must consume large quantities of saltwater to counteract osmosis then maybe cry a river to flush out all the salt later ;D $\endgroup$
    – user6760
    Mar 14, 2022 at 7:34

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There are no significant visible differences between freshwater fish and saltwater fish. All the significant differences are internal, and are not visible. Freshwater fish don't drink, as water is constantly entering their bodies through their skin, gut and gills, and the excess must be excreted. To contrast, saltwater fish drink frequently, as they lose water through their gills and skin, and they produce very concentrated urine.

Since there are no significant visible differences, any difference that the OP wants to implement would be unlikely to be significant in affecting the survivability of the race in their preferred fresh or salty environments as long as the difference wouldn't affect survivability in a single environment.

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  • $\begingroup$ And in addition there are many fishes who live part of the lives in salt water and part in fresh water. $\endgroup$
    – AlexP
    Mar 14, 2022 at 9:22

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