Our bodies aim to maintain homeostasis: a constant balance of all minerals and ions. This is why it is tiring when we move towards high elevations where our bodies adapt by creating more red blood cells. Similarly, as fish move between salinities, they must adapt to the change in ions which is harder than it sounds. Euryhaline fish are the fish that can achieve this feat.
Why is this special?
You might be thinking that, if all humans can adapt to higher/lower elevations, why can’t all fish adapt to different salinities? The reason for this is that adapting to various salinities requires various mechanisms.
Saltwater is hypertonic to marine fish, meaning that water is continually being lost through the skin. This is due to a higher concentration of water inside the fish. To combat this, marine fish intake more water and they filter out excess salt in their kidneys and gills.
Similarly, freshwater is hypotonic to freshwater fish, meaning that water is being pushed into the skin. This is due to a higher concentration of water in the surrounding water. Fish combat this by urinating more frequently.
Because of these differences between freshwater and saltwater fish, fish can’t easily switch from different salinities. Different salinities require different mechanisms.
Oddly enough, you will find most euryhaline species in brackish waters and estuaries. This makes sense because salinity can drastically vary in these two regions as freshwater meets with the sea. An example of a euryhaline species found in brackish water is the flounder.
The mechanics behind Eurahyline Species
The ability to handle different salinities depends on how well a fish can osmoregulate: maintain a balance between freshwater and saltwater. To do this, euryhaline species change the function of their organs to maintain homeostasis.
One example is salmon which spend their lives in the ocean and in rivers. When salmon enter saltwater for the first time, they produce concentrated NaCl urine through their kidneys and produce an enzyme that pushes salt out of the skin. When the salmon start to head back upstream, they change their body functions again. They produce more dilute urine and an enzyme that pushes salt back into the body. These transformations require the fish to slowly enter the new environment.
I find it interesting how only certain fish can adapt to varying salt concentrations. This is because it seems like an important skill that natural selection would have made more frequent.