The coral creatures are soft-bodied and are retracted inside the coral skeletons, typically emerging during hours of darkness. This type of coral is fast-growing, as much as an inch or more a year. Over a period of many months or sometimes years, the colony recovers to its original delicate form.
Coral colonies require clear, nutrient-free, sunlit water to thrive.
Many varieties are hermatypic , possessing algae in their branches that process sunlight into food for the coral colony. If the water is polluted, the sunlight is blocked, preventing the algae from providing nutrition for the coral. If the water has excess nutrients, algae suspended in the water near the surface blooms excessively, in turn blocking the sunlight from reaching the reef corals. When a coral colony dies, the entire ecosystem of the reef is affected, since most of the reef creatures depend on the coral structures for shelter. Probably foremost is damage from soil and pesticide runoff from land.