Fish need at least a 30% saturation of oxygen in the water, otherwise the water is termed "anoxic". A healthy area has 80% oxygen saturation. Pollution and eutrophication depletes oxygen. Fish kills can then occur as well as mollusks and different types of worms. Some adaptations animals have to obtain oxygen in the water is subcutaneous respiration(skin breather) or the use of gills. For example, if something is a skin breather, diffusion through their thin moist skin occurs and oxygen enters the body in that way. For fish which utilize gills, their gills must be kept moist. The operculum or gill cover opens and closes and water covers the gills. Diffusion across the thin, feathery gill filaments occurs and oxygen is absorbed into blood vessels which are located inside the gills. Gills provide tremendous surface area for the exchange of respiratory gases. Marine mammals must come to the surface to breathe. Many can stay underwater for up to an hour because they have efficient lungs that can exchange 90% of their volume with each breath. Also, some mammals can store high amounts of oxygen in their blood and muscles when diving. Marine reptiles like turtles, must eventually come to the surface to breath air into their lungs.