fish??

Question
Why can't saltwater fish live in fresh water?  

Answer
Hi Jessica,

   Beacause of the way they process the saltwater in their environment they have adapted over millions of years to live in the ocean. The freshwater fish process freshwater the same way. There are also some species of fish that live in brackish water. Fresh and saltwater combined in an ecosystem where a river meets the sea or in certain marsh enviornments. These fish can tolerate high levels of salt and very little salt because they have adapted to those parameters over millions of years. Some creatures can live in high methane gas environments that would be lethal to us. So, I guess it all depends how a creature develops in special ways  to adapt to its environment. Gerry.