Coral reefs are composed of the limestone skeletons of coral polyps. A coral polyp is an invertebrate, or an animal without a backbone. Coral polyps extract calcium carbonate from seawater to build protective skeletons around their fragile bodies. When the coral polyps die, their skeletons are left behind and used by other polyps as a foundation to build their skeletons. As a coral colony grows, it eventually forms branches or mounds, which are large groups of coral polyps. As more and more mounds grow within proximity, eventually a coral reef forms.
The Coral Reef Alliance says that, "although there are hundreds of different species of coral, they are generally classified as hard coral or soft coral." Hard corals are the species of coral responsible for coral reefs because they build limestone skeletons. Soft corals do not create protective skeletons for themselves and therefore do not help build coral reefs. Although soft corals do not add to the physical structure of a reef, they do contribute to the ecosystem and biodiversity of coral reefs.
Coral reefs can be found in warm, clear waters throughout in the world in more than 100 different countries. Waters with temperatures between 70 degrees and 85 degrees Fahrenheit provide optimal growth conditions for corals. In addition, coral reefs need clear water that gets plenty of sunlight because they depend on photosynthetic algae.
Coral has formed a symbiotic, or mutually beneficial, relationship with a tiny species of algae called zooxanthellae. This algae live inside the coral polyp and provide it with oxygen and nutrients. In return, the coral provides carbon dioxide and protection for the algae. In addition to the nutrition provided by zooxanthellae, some species of coral polyps will extend their tentacles out of their skeletons to catch and eat zooplankton.
Within the past 10 years instances of coral reef diseases has increased and been compounded by destructive human activities. The National Oceanic and Atmospheric Administration ( NOAA) believes this increase in disease "is related to deteriorating water quality and anthropogenic [human caused] pollutants and increased sea surface temperatures." These three factors may be contributing to the "proliferation and colonization of disease-causing microbes."
Coral reefs are not only susceptible to natural disasters such as hurricanes, cyclones and typhoons, but also human pressures. Coastal development, sewage treatment plants and overfishing have all placed additional stress on coral reef ecosystems. Blast fishing, in which dynamite is used to catch fish, and cyanide fishing, which uses cyanide to catch fish, are two forms of fishing that actually damage and kill coral in the process.