Habitat

Fish can be divided into two primary groups: freshwater and saltwater (or marine) fish. Our oceans hold 48 percent of the world’s fish population, whereas 41 percent inhabit lakes, rivers, and streams. One percent of fish inhabit both and are referred to as anadromous fish.

Fish breathe oxygen by pushing water through their gills, by either gulping the water or by moving through the water with their mouths open. The functioning of the gills, kidneys, and air bladder differs between marine fish and freshwater fish based on their response to the amount of salt in their environment. Interestingly enough, both fishes and all vertebrates for that matter have the same salinity content in their bodies. Marine fish are thus in an environment where they are surrounded by more salt than their bodies can support. The process of osmosis, going from low salinity to high salinity, forces the fish to expel the excess salt in their system and, ironically enough, stay hydrated.