Coral reefs are second only to rainforests in biodiversity of species. Despite how important coral reefs are to life in the ocean, all of them in the world add up to less than one percent of the sea floor – an area about the size of France. About 10 million bacteria can live on one square centimeter of coral.
Coral reefs are found all around the world in tropical and subtropical oceans. They are usually found in shallow areas at a depth of less than 150 feet. However, some coral reefs extend even deeper, up to about 450 feet deep.
Coral polyps eat in two different ways according to the species. Many coral polyps are nourished by tiny algae called zooxanthellae (pronounced zo-zan-THEL-ee). The algae live within the coral polyps, using sunlight to make sugar for energy just like plants. Zooxanthellae process the waste of the polyp to retain important nutrients and in turn provide the polyp with oxygen. Meanwhile, the coral polyps provide the algae with carbon dioxide and a safe, protected home. Zooxanthellae living within the tissue of hard corals can supply them with up to 98 percent of their nutritional needs.
Another way that corals eat is by catching tiny floating animals known as zooplankton. At night , coral polyps come out of their skeletons to feed, making the reef look like a wall of hungry mouths. The polyps stretch out their long, stinging tentacles to capture the zooplankton that are floating by. The captured plankton are then put into the polyps’ mouths and digested in their stomachs.
While corals may appear to be little more than stone or plants, these are in fact live animals related to jellyfish and sea anemones. Plants do play a very important role in the lives of corals though, most notably the symbiotic algae called zooxanthellae (ZOH-oh-ZAN-thell-ee) that live in their tissues. These single celled plants convert sunlight into energy that the corals use to grow, hence insuring a healthy coral polyp and colony overall. Most coral polyps have clear bodies and their skeletons are white, like human bones. Generally, their brilliant color comes from the zooxanthellae.
Coral reproductive methods vary according to the species. Some species, such as Brain and Star corals, are hermaphrodites, meaning they produce both sperm and eggs at the same time. Other corals, such as Elkhorn and Boulder corals, are gonochoric, meaning that they produce single-sex colonies. In these species, all of the polyps in one colony produce only sperm, and all of the polyps in another colony produce only eggs.
Coral larvae are formed in two different ways. The larvae are either (1) fertilized within the body of a polyp or (2) fertilized outside of the polyp’s body in the water. Fertilization of an egg within the body of a coral polyp is achieved from sperm that is released through the mouth of another polyp. The sperm and egg merge and matures inside the body of its mother. When the larva is ready, it gets spit out into the water through the mouth of its mother.
Other species of coral reproduce by ejecting large quantities of eggs and sperm into the surrounding water. When this happens, the eggs and sperm fertilize in the water. This process is called coral spawning. In some areas, mass coral spawning events occur on one particular night per year and scientists can predict exactly when this will happen.
Once in the sea, larvae are naturally attracted to the light. They swim to the surface of the ocean, where they remain for days or even weeks. If predators do not eat the larvae during this time, they fall back to the ocean floor and attach themselves to a hard surface. Once attached the larvae changes into a coral polyp and begins to grow. It divides itself in half and making exact genetic copies of itself. As more and more polyps are added, a coral colony develops. Eventually the coral colony becomes mature, begins reproducing, and the cycle of life continues.
Despite the importance of coral reefs, these wildlife habitats are in danger throughout the world. A recent report estimated that 75 percent of remaining coral reefs are currently threatened, and many have already been lost. Even some of the most remote and pristine reefs are losing species. The main threats to corals are ocean acidification, ocean warming and coral bleaching, water pollution, destructive fishing practices, coral mining, and careless tourism.