Difference Between Freshwater and Saltwater Fish

The most obvious difference between freshwater and saltwater fish is there habitat. While freshwater fish live in fresh water salt water fish live in very salty water. Freshwater fish are able to concentrate salts within their bodies in a salt-deficient environment. That allows them to survive in fresh water. But saltwater fish do not have that capability they excrete excess salts which makes them have to live in salt water. One of the main differences is the ionic composition of bodies of water utilized as habitat. Chloride cells in the gills of freshwater fish fish make enzymes that lets them get rid of excess salt in their plasma, which builds up when they drink seawater. They use the enzyme to pump sodium out of their gills at the cost of energy. The physiological mechanisms of freshwater fish allow them to concentrate salts to compensate for their low salinity environment. They produce extremely dilute, copious urine to rid themselves of excess water,while performing an active uptake of ions at the gill.

 
I was always curious what the difference between saltwater and freshwater fish were. I didn't know if there was a very large biologic reason or if it was just because of where they evolved.  I think that it is very cool that they need to live in the different conditions to maintain there salt levels. I think it is kind of like cold blooded and warm blooded animals. They live in different conditions to maintain there body heat. I think that the difference between freshwater and saltwater fish is very interesting.

M B

http://www.aquaworldaquarium.com/GeneralCare/general_care_for_saltwater_fish.htm

http://www.scientificamerican.com/article.cfm?id=why-do-some-fish-normally