Environmental effects

Corals are highly sensitive to environmental changes. Scientists have predicted that over 50% of the coral reefs in the world may be destroyed by the year 2030, as a result they are generally protected through environmental laws. A coral reef can easily be swamped in algae if there are too many nutrients in the water. Coral will also die if the water temperature changes by more than a degree or two beyond its normal range or if the salinity of the water drops. In an early symptom of environmental stress, corals expel their zooxanthellae; without their symbiotic unicellular algae, coral tissues become colorless as they reveal the white of their calcium carbonate skeletons, an event known as coral bleaching.

Many governments now prohibit removal of coral from reefs to reduce damage by divers. However, damage is still caused by anchors dropped by dive boats or fishermen. In places where local fishing causes reef damage, education schemes have been run to inform the population about reef protection and ecology.

The narrow niche that coral occupies, and the stony corals' reliance on calcium carbonate deposition, means they are very susceptible to changes in water pH. Ocean acidification, caused by dissolution of carbon dioxide in the water that lowers pH, is currently occurring in the surface waters of the world's oceans due to increasing atmospheric carbon dioxide. Lowered pH reduces the ability of corals to produce calcium carbonate skeletons, and at the extreme, results in the dissolution of those skeletons entirely. Without deep and early cuts in anthropogenic CO2, scientists fear that ocean acidification may inevitably result in the severe degradation or destruction of coral species and ecosystems.

A combination of temperature changes, pollution, and overuse by divers and jewelry producers has led to the destruction of many coral reefs around the world. This has increased the importance of coral biology as a discipline. Climatic variations can cause temperature changes that destroy corals. For example, during the 1997-98 warming event all the hydrozoan Millepora boschmai colonies near Panamá were bleached and died within six years - this species is now thought to be extinct.

Live corals

Local economies near major coral reefs benefit from an abundance of fish and octopus as a food source. Reefs also provide recreational scuba diving and snorkeling tourism. Unfortunately all these activities can also have deleterious effects, such as removal or accidental destruction of coral. Besides the recreational use, coral is also useful as a protection against hurricanes and other extreme weather.

Live coral is also highly sought after in the aquarium trade. Although difficult to maintain in some or most cases, they add a striking beauty. Soft corals are considered easier to maintain in captivity than hard corals Provided the proper ecosystem, live coral makes a stunning addition to any salt water aquarium.

Deep sea live bamboo corals (Isididae) may be among the first organisms to display the effects of changes in ocean acidification caused by excess carbon dioxide. Also, they produce growth rings similar to those of tree and can provide a view of changes in the condition in the deep sea over time. Other research has presented the possibility that Isididae corals, because of their potential to mimic biological properties, may potentially be used as living bone implants as well as in aquatic cultivation

Coral as a gemstone

Intensely red coral is sometimes known as fire coral (but this is not at all the same thing as fire coral). This extremely red coral is very rare now because of overharvesting due to the great demand for perfect red coral in jewelry-making.

Ancient corals

Ancient coral reefs on land are often mined for lime or use as building blocks ("coral rag"). Coral rag is an important local building material in places such as the East African coast.

Some coral species exhibit banding in their skeletons resulting from annual variations in their growth rate. In fossil and modern corals these bands allow geologists to construct year-by-year chronologies, a form of incremental dating, which can provide high-resolution records of past climatic and environmental changes when combined with geochemical analysis of each band.
Certain species of corals form communities called microatolls. The vertical growth of microatolls is limited by average tidal height. By analyzing the various growth morphologies, microatolls can be used as a low resolution record of patterns of sea level change. Fossilized microatolls can also be dated using radioactive carbon dating to obtain a chronology of patterns of sea level change. Such methods have been used to used to reconstruct Holocene sea levels.