So, now we know that water sorts itself much like oil does on other layers of water allowing fish to survive in pockets of less salty and warmer/cooler water.
What about the built-in way a fish’s body works to allow them to survive in a changed environment?
All fish have the same amount of salt in their bodies, about 0.9%. Because of osmosis, freshwater fish (where the water s 0.1% or less salt) don’t ever have to drink, the water gets sucked in through their skin, but they do have to make urine to keep all that water from popping them!
Saltwater fish have the opposite problem because ocean water is usually 3.5% salt. These fish drink all the time and their kidneys collect the salt for them to excrete (go to the bathroom). A large salmon drinks as much water as a grown-up human in a day (several quarts/liters) to keep the salt from shriveling up its organs.
We know there are a number of fish that move from salt to fresh waters and back again. How do they do it? Salmon are born in freshwater streams usually far from the ocean. When they are strong enough, most salmon swim downstream until they get to the mouth of the river. Then they spend some time getting used to the saltier water before heading out into the open ocean.
This adjustment time can last from a few days up to several weeks. When it is their time to return to their home streams to spawn (lay eggs), they again spend up to a few weeks at the mouth of their river to readjust to the lack of salt. Built in directions in their DNA (tiny books God wrote in every cell) help them switch from pushing salt out to pushing water back out.
American and European Eels spend their lives in the exact opposite way. They are born in the Sargasso Sea off the north-eastern coast of Cuba and then swim back to their proper homelands (that they have never been to). Scientists can’t explain why the American Eels don’t end up in Europe and the other way round. I couldn’t find out if they have to hang out in the Brackish (half fresh/half salt) water along the coasts or not before swimming up river, but they do change from see-through to brown colored. After quite a few years of living in freshwater rivers(up to 20 or even longer), the grown fish head back to the ocean to the place they were spawned to lay their own eggs and never return.
There are also a number of fish that like to live in the Brackish water where rivers meet the ocean. Sturgeon, Mollys, Cichlids [sick-lids], Gobys, Flounders, Puffers and many more thrive in these sort-of salty-water environments.
We know that freshwater fish and saltwater fish are of similar types to each other, with there actually being a lot more surviving freshwater species than saltwater (even with all that extra room to live in!). It seems that whether a fish does well in one type of water or another has to do with which genes in their DNA are turned on to tell it what to do to survive.
As for all the other fresh water organisms, it’s harder to find info on them. I did find an interesting study on the increasing saltiness of Australia’s freshwater systems and what happens to the clams and things in them. The adults die off if it gets too salty for them (just think of all the fossil clams, etc. in the rocks today), but their eggs survive and wait for the salt levels to drop before hatching. The same would be true for all the plants that can’t handle salt. They would die, but their seeds would survive. Some of the water plants could have even survived by floating in those low-salt pools until dropping into newly formed freshwater systems.
For more check out Dr. Walt Brown’s page on freshwater fish survival
Or speak to the earth, and it shall teach thee: and the fishes of the sea shall declare unto thee. Who knoweth not in all these that the hand of the LORD hath wrought this? Job 12:8,9
Sites I used to build this post:
Salt Water in Freshwater Aquariums (it mentions Cichlids specifically near the end)
PDF Study of increasing Salinity of Australian Freshwater Ecosystems (this is the one with clams, etc on page 6)