High Quality Content by WIKIPEDIA articles! Aquatic respiration is the process whereby an aquatic animal obtains oxygen from water. Earth's natural bodies of water have a low oxygen concentration much lower than the level of oxygen in air at the Earth's surface. Smaller organisms can obtain sufficient oxygen through the skin , but larger organisms require special structures to collect enough oxygen to sustain life. This oxygen comes from molecules of oxygen gas (O2) dissolved in the water. The oxygen atom present in the water molecule (H2O) is not suitable for respiration. Fish have developed gills for respiration which have large surface area which is needed for more oxygen to get in high blood flow, small/short diffusion distance, contain 4 gill arches (Bony fishes), two gill arches or 7 gill baskets. In fish, the long bony cover for the gill can be used for pushing water. Some fish pump water using the operculum. Without an operculum, other methods, such as ventilation, are required. Some species of sharks use this system. When they swim, water flows into the mouth and across the gills. Because these sharks rely on this technique, they must keep swimming in order to respire.