Coral Reefs are aragonite structures produced by living organisms, found in marine waters with little to no nutrients in the water. High nutrient levels such as those found in runoff from agricultural areas can harm the reef by encouraging the growth of algae. In most reefs, the predominant organisms are stony corals, colonial cnidarians that secrete an exoskeleton of calcium carbonate. The accumulation of skeletal material, broken and piled up by wave action and bioeroders, produces a massive calcareous formation that supports the living corals and a great variety of other animal and plant life. Although corals are found both in temperate and tropical waters, shallow-water reefs are formed only in a zone extending at most from 30°N to 30°S of the equator. This zone is very important to whales because many types of plankton live there. Tropical corals do not grow at depths of over 50 m (165 ft). Temperature has less of an effect on the distribution of tropical coral, but it is generally accepted that they do not exist in waters below 18°C., and that the optimum temperature is 26-27° Celsius for most coral reefs. The reefs in the Persian gulf however have coral adapted to changing temperatures of 13°C in winter and 38°C in summer, thus having significantly colder and hotter ambient environments respectively than most coral reefs. This photograph is part of the Imagine Images Collection, hosted by dreamstime.