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I cannot understand this answer. I don't know much about biology or have a lot of trouble recalling stuff from a university biology book I bought for a course I never took because it was so long ago and it was confusing even then. I think I once learned that salmon can live in salt water or fresh water. According to https://fishcostarica.org/this-fish-lives-over-60-years/, tarpon can also live in fresh water or salt water. It seems to me that for tiny fish, they deed a different type of body with more solutes inside the cells in order to live in salt water and the tiny fish that can live in fresh water have a different type of body with fewer solutes inside the cells. And advantageous traits come with costly traits so a small fish that could do both would have more of a costly trait and therefore be outcompeted to extinction. There are plenty of fish in fresh water already eating the resources in the fresh water and plenty of fish eating the resources in salt water. And being able to move back and forth between them doesn't really enable a small fish to get more food. I think a big fish on the other hand would have a much slower rate of osmosis so it can easily pump water our as fast as it is getting absorbed into the cells or in as fast as it is getting absorbed out of the cells. Salmon also have some reason to go back and forth between fresh water and salt water.

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  • $\begingroup$ Timothy, you should word the title better: bigger than the smallest of fish is about 8 millimeters: what size is bigger than a critical size that is bgger than 8 millimeters, that doesn't seem very big?, i am confused already. $\endgroup$ – aliential Feb 23 at 21:26
  • $\begingroup$ @aliential I guess I'm asking whether it's the case that there is some critical size bigger than the smallest of fish where it tends to be evolutionarily harder to live in both salt water and fresh water. $\endgroup$ – Timothy Feb 23 at 21:38
  • $\begingroup$ Theres small species known to be Euryhaline. The comment you just wrote is a lot easier to read wording than the original question text, you should edit the question with just those words. $\endgroup$ – aliential Feb 24 at 11:04
  • $\begingroup$ "It seems to me that for tiny fish, they deed a different type of body with more solutes inside the cells in order to live in salt water and the tiny fish that can live in fresh water have a different type of body with fewer solutes inside the cells. And advantageous traits come with costly traits so a small fish that could do both would have more of a costly trait and therefore be outcompeted to extinction." Do you have any evidence for that claim? And what does body size have to do with anything? I'm struggling to understand what the purpose of this question is. $\endgroup$ – MattDMo Feb 24 at 17:41
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    $\begingroup$ Also, please note that the answer you linked to was to a question about anadromous fish - those that migrate up streams to spawn. There's a big difference between that and euryhaline fish, which can survive in salt and fresh water. I would imagine that anadromous fish would tend to be bigger, given the strenuousness of their migrations, while euryhaline fish could be any size at all. $\endgroup$ – MattDMo Feb 24 at 17:45
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Mollies , there are a few kinds , many are 2 to 3 " , live in fresh or salt . They seem to prefer fresh but often live in sea coast areas where tides ,etc. may cause intermittent changes between fresh and salt ( and brackish of course) . I used black mollies to start a salt water tank, I had a couple in the salt for months because it was not easy to catch them in a large tank. I expect there are other small fish such as puffers. The mollies adapt very quickly , just drop them in whichever water and they behave normally.

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  • $\begingroup$ You just give a single example. That doesn't tell me whether they're a tendency that way. $\endgroup$ – Timothy Feb 23 at 21:13
  • $\begingroup$ There are also some really small fish. I guess I defined big to mean big enough to be able to live in salt water or fresh water. There are some really small fish. If an answer confirming that none of the fish that size can live in either salt water or fresh water will do. $\endgroup$ – Timothy Feb 23 at 21:15
  • $\begingroup$ I fixed my the titled of my question. $\endgroup$ – Timothy Feb 23 at 21:18
  • $\begingroup$ Wikipedia has a listing for brackish water aquarium fish ; most can be in fresh or salt water. $\endgroup$ – blacksmith37 Feb 24 at 18:37
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I didnt read beyond the paragraph about your study books situation, Euryhaline on wiki reveals tiny euryhaline fish... 4-5 cm Fundulus heteroclitus which presumably can withstand salt changes as juveniles, 2-3cm. Probably there are species a lot smaller because individual cells perform the osmotic rebalance, and cells are much bigger thans water and salt, so a smaller fish can just develop more of apparatus for osmotic balancing.

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