Coral reefs are found almost exclusively in the seas and oceans between the Tropic of Capricorn and the Tropic of Cancer. In this region, water temperatures are warm and stable year-round (64 – 86 degrees Fahrenheit, 18-30 degrees Celsius), and longer days bathe the waters with sunlight.
Though thriving coral reefs are a collection of many different plant and animal communities, the members of individual coral colonies (polyps) actually build the reef’s limestone, or calcium carbonate, structure. Polyps consist of a tube and an oral disc, or mouth, surrounded by tentacles, which the polyps use to capture food. The tube and oral disc sit inside a calcium carbonate cup. As polyps grow, they produce small buds.
Each type of polyp buds in a different way, leading to a large variety of shapes and sizes of coral colonies ranging from the rippled ball of brain coral to elegant fans, flat discs, graceful branches and columns.
As a single polyp dies, its soft tissue decays, but the calcium carbonate cup remains. Other polyps build on top of the cup, and when they die, other polyps will build on their cups. Over time, this process creates larger and larger coral reefs. Several kinds of algae help hold the reef together by growing between the colonies of coral polyps and keeping the sand that accumulates there from washing away.
Living within each coral polyp is a small plant, a single-celled algae called zooxanthellae. The algae provide food to the polyps by photosynthesis, which means the algae use sunlight to break carbon dioxide down into oxygen and carbohydrate. In turn, the polyp provides food to the algae with its waste products. The algae store the waste as ammonia and break it down into nitrogen and phosphorus, which the algae use for energy. This beneficial relationship is a type of symbiosis called mutualism. Both the polyps and the algae are helped and neither is harmed by their relationship. Polyps can also draw food directly from the water, using their tentacles to catch drifting plankton.
Coral reefs flourish in shallow areas (less than 120ft, or 37m) in tropical latitudes, or where warm ocean currents flow into more temperate areas. In deeper waters, not enough light penetrates the depths, which means the reef’s main food producers, algae and plankton, cannot photosynthesize. Large reef-building areas include the Caribbean Sea, the western Indian Ocean and the western reaches of the South Pacific. Non-tropical coral reef zones include the Red Sea, where lots of heat from the sun caused by the surrounding desert climate provides the needed warmth, and Australia’s Great Barrier Reef, which is kept warm by a tropical Pacific Ocean current.
Use the coral reef map by clicking on the names of the reefs. Link to use the map
Different zones of a reef are better suited to different living things. For example, some types of algae thrive in the rough seaward edge of the reef, where waves are constantly slamming into the coral, and delicate sea-grasses prefer the calm protected water of the sandy reef flat behind the main reef.In addition to the variety of coral polyps whose calcium carbonate actually form the backbone of the coral reef, large numbers of fish, crabs, shrimp, sponges and seagrasses make their home in coral reefs.
Amazing colors and shapes and thousands of fishes make coral reefs one of the most spectacular underwater places for people to visit. Each reef is completely unique in its shape and the kinds of animals that call it home. A great number of plants and animals that live in coral reefs have not been studied, or even named, yet. Some of the ones we are studying may give us new medicines to treat cancer, protection from too much sunlight and antibiotics to fight infections.
Erosion does not affect coral directly, but silt, mud, fertilizers and herbicides that wash off the surrounding land damage delicate coral reefs. Silt and mud cover the corals with a cloud of haze, and herbicides kill not only the algae that live within coral polyps, but plant life all over a reef. Fertilizers and untreated sewage provide an excess of nutrients where certain types of algae will grow faster than they normally would.
The overgrown algae choke the coral reefs and attract predators, such as sea urchins and crown-of-thorn starfish, which destroy living coral. A major problem in coral reefs around the world is coral bleaching. algae that live inside the polyps and gives them their brilliant colors is killed or ejected. When this happens, an entire coral reef can turn white. Scientists are still not sure exactly what causes this to happen. One theory suggests that coral bleaching is the result of higher than normal water temperatures.
Humans have done a great deal of damage to coral reefs around the world. Oil spills and pollution suffocate living corals and poison the waters. It can take many years for a reef community to recover from the damage a single oil spill can do. Tourism is another area where human impact has been harmful to reefs. To attract tourists, some islanders have blasted boat channels through reefs. Divers and snorklers damage or break fragile coral branches by simply touching them or grazing them with a careless hand or flipper. Dive boat operators and fishermen destroy reefs with anchors and propeller blades. Souvenir-seeking tourists snag pieces of coral, which took the reef years to build.