Corals grow best in clear water with a temperature between 20 and 30 oC (70 and 85 oF), with the optimum temperature being about 24 oC (75 oF.) These environmental factors limit where coral reefs can be found, but in the Indian Ocean, Southeast Asia, Central Pacific, Southwest Pacific and Caribbean conditions are perfect and coral reefs can be found in abundance. Cold currents on the west coast of African and American continents limit coral in these waters to a few locations close to the equator. Silt in the water cuts out sunlight and hence constrains the rate at which corals can grow. Therefore on the east coast of South America, murky water from the Amazon estuary limits coral growth in an area otherwise ideal. There are exceptions where coral can grow below the temperature limits e.g. the northern Red sea where in winter the sea temperature can drop to below 18 oC (64 oF). In some locations corals survive in water as warm as 33oC (90 oF), the Persian Gulf and northern part of the Australian Great Barrier Reef being examples.
This map shows the distribution of coral reefs all over the world. Reef-building corals are throughout the tropical and subtropical Western Atlantic and Indo-Pacific oceans.
Reef building corals are generally found at depths of less than 46m, i.e. where sunlight can penetrate, though some corals have been found up to 6000m below the surface. Reef development is also increased by strong wave action as waves carry food, nutrients, and oxygen to the reef, distribute coral larvae, and prevent sediment from settling on the coral reef.
Precipitation of calcium from the sea is necessary for the build up of the limestone coral skeleton. Precipitation is at itís greatest when the water temperature and salinity is high and carbon dioxide levels are low, thus shallow tropical waters are favoured.