Beneath the surface of both the oceans and the rivers of the world, there is extraordinary world. That world is filled with fish. All fish look the same, they have the same body design ( Bony skeleton, gills, fins, and scales.) However, there are a bunch of difference between different species of fish. Most of them behave differently, most of them have different feeding and hunting habits, most of them reproduce and grow up differently. But the biggest difference between most fish is where they live, either in the saltwater of the sea or the freshwater of the river. 

Evolution of Fish