Reef-building corals are restricted in their geographic distribution by their physiology. For instance, reef-building corals cannot tolerate water temperatures below 18° Celsius (C). Many grow optimally in water temperatures between 23° and 29° C, but some can tolerate temperatures as high as 40° C for short periods. Most also require very saline (salty) water ranging from 32 to 42 parts per thousand, which must also be clear so that a maximum amount of light penetrates it. The corals’ requirement for high light also explains why most reef-building species are restricted to the euphotic zone, the region in the ocean where light penetrates to a depth of approximately 70 meters.