The digestive, circulatory, and nervous systems are very similar to those of other vertebrates. However, what really makes a fish different from other animals is its respiratory system. A fish s respiratory system is determined primarily by the fact that it spends its entire life in water. Unlike the marine mammals such as whales, a fish has evolved in such a way as to not require frequent trips to the surface to breathe air. Fish have developed gills, on which they rely for the oxygen necessary for a fish s limited metabolism.
Fish use their gills to extract oxygen from their watery environment.
The process starts with the fish s mouth, which is how the fish takes in water. When a fish opens and closes its mouth, it is actually pumping water back through the gills and is thus breathing. Most fish have an effective pumping system that involves the mouth and the outer cover of the gills, called the operculum. When the fish s mouth opens, the operculum closes, drawing water into the fish s mouth. When the fish closes its mouth, the operculum opens, allowing fresh water to cross the gills.