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aquatic respirationa process in which freshwater and marine organisms carry out gas exchange with the water that surrounds them. Fully saturated water contains only about 0.02% as much oxygen per unit volume as air. However, since the tissues are in direct contact with the gas-carrying medium flowing past, the system is very efficient and often a relatively small respiratory surface is required.
Many aquatic organisms (particularly the less advanced groups, e.g. protozoa, algae, nematodes) have no special respiratory organs, relying on a large surface area for gas exchange. Others have developed specialized structures which are either external (e.g. gills of the ragworm Nereis), or internal (e.g. gills of fish). GILLS are richly supplied with blood vessels for transport to and from the body tissues, and many organisms with gills expend large quantities of energy creating a regular flow of water over the respiratory surfaces.