Capable of both sexual and asexual reproduction, coral communicate in order to coordinate behavior. Coral live in colonies of tiny identical individuals called polyps, and hermatypic corals obtain most of their nutrition from a symbiotic relationship with a type of photosynthetic algae called zooxanthellae. Hermatypic coral build reefs by secreting tough calcium carbonate skeletons. Ahermatypic coral can live in cooler and deeper waters than their reef-building counterparts (See Reference 1).
Hermatypic corals are the hard corals and reef builders. These include stony corals, fire corals, tubipora or "pipe" corals and the helipora corals, which are popular in marine aquariums because they leave behind a striking blue skeleton when they die; when alive, they're brown, the color of their zooxanthellae (See Reference 2). These coral form associations with zooxanthellae, photosynthetic algae that use energy from the sun to provide the coral with carbohydrates. This is important because the coral require a lot of energy to secrete the hard carbonate skeletons that make reefs.
In return for providing the coral with carbohydrate energy, the zooxanthellae receive shelter and protection inside the hard body of the coral.
These coral are soft-bodied and tend to resemble vegetation rather than the rocky formations of hermatypic corals. Because ahermatypic coral do not form associations with zooxanthellae, these coral do not form skeletons and build reefs. Common ahermatypic corals include sea whips, sea feathers and sea pens.
Hermatypic corals are found throughout the world's warm and shallow tropical oceans. Australia's Great Barrier Reef is the largest coral reef system in the world, followed by Belize's Barrier Reef that stretches from southern Quintana Roo, Mexico, to the Bay Islands in Honduras.
Because ahermatypic coral do not associate with photosynthetic algae, they're not restricted by access to sunlight in the same way hermatypic coral are. That's why ahermatypic coral are found in cooler and deeper waters such as off the West Coast of the United States and even in Norway's cool waters.
According to the report "The Status of Coral Reefs Around the World: 2004," 20 percent of the world's coral reefs have been destroyed, and another 50 percent are at risk for collapse. Pressures include over-fishing, coral mining, pollutants and coral disease. Global warming threatens coral reefs through increasing the acidification of the ocean and causing coral bleaching (See Reference 3). Careless tourism is also a threat, as coral are killed when divers, swimmers and fisherman grab, kick or walk on coral.