Fish being an aquatic animal, has a respiratory system that is different from that of animals that live on land. It is capable of breathing under water without making frequent trips to the surface to breathe air like marine mammals such as the Whale. This is due to the fact that fishes have a breathing organ known as gills. This is made up of thin feathery sheets of tissue membrane containing many blood vessels through which oxygen passes allowing fishes to breathe.The process begins by water in the fish’s surrounding entering its mouth‘s. Water enters the mouth by a very effective pumping system that involves the mouth and the outer flexible bony flap that cover the gills called the operculum. This is achieved by the opening and closing of the fish's mouth as it pumps water through the gills. When the fish’s mouth is open, the operculum closes and water is drawn into the fish’s mouth. Conversely, when the fish closes its mouth, the operculum opens and fresh water is then allowed to flow across the gills.