Coral reefs are found in the shallow coastal waters of over 100 countries. They grow profusely in warm, well-circulating calm waters and cover an estimated 231,600 square miles (600,000 sq km) worldwide. Home to a vast number of different species, coral reefs are second only to rainforests in species richness.
Coral consists of thousands of invertebrate marine animals - known as "polyps" - with a hollow, cylindrical structure and a skeleton containing calcium carbonate. The lower end of the coral is attached to a rock or another polyp. At the free end is a mouth, surrounded by tentacles that can be extended to paralyze prey. While corals living in deep water (see pages 32-33) rely on this method, corals in shallow water obtain most of their food from photosynthesis by algae living inside them.
Sick coral provides an early warning that entire ecosystems are in danger. Since the 1980s, dozens of new infections, including white band and yellow pox, have attacked corals. Few of these ailments have a known cause, but human development of coastal zones is a likely factor.
Increases in sea temperature and level can cause the coral polyps to shed the algae on which they depend, causing them to lose their color and die ("bleaching"). Although some reefs do recover over time, if climate change makes such episodes more frequent, this will no longer be possible.
The rising concentration of carbon dioxide in the atmosphere means that more is being absorbed by the world's oceans. This is causing the water to become more acidic, which will eventually make it impossible for organisms, including corals, to form shells. Scientists are now warning that this might occur in surface waters as early as 2050.
Illegal fishing methods, such as the use of cyanide and explosives to stun fish, damages coral. Over-fishing also affects the ecological balance of a reef, which can become overgrown with algae if grazing fish are removed.
The loss of coral reefs is likely to reduce the fish catch of many tropical developing countries, around 25 percent of which comes from reef environments. Where reefs may have acted as barriers against erosion, their destruction may also allow the sea to encroach on coastal regions.
Over 350 protected areas include coral reefs, but these are often in countries without adequate resources to enforce the necessary controls. Tourism can both harm coral reefs, and supply the incentive and finances to protect reefs, but it has to be carefully managed.
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