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Picky Dwellers
For delicate coral reef communities to survive, water conditions must be optimal. Reef-building coral thrives in the tropics where waters are warm--usually 70 to 85 degrees Fahrenheit (21 to 30 degrees Celsius)--and relatively shallow to allow life-supporting algae to collect sunlight for photosynthesis. The water cannot be saltier than the open ocean, which precludes many continental coastline waters, so reefs tend to collect around tropical islands. Also, waters must be relatively clear. Excess sediment clogs the feeding mechanisms of polyps, the individual organisms in a colony of coral.
Living on Skeletons
Only the very outside of reefs are comprised of living matter. Coral reefs are formed as colonies of living organisms grow and secrete hard exoskeletons made of calcium carbonate, or limestone. These creatures bud off new polyps then die, leaving their exoskeletons behind. As the new polyps secrete their own exoskeletons, they build upon the remains of their predecessors, thereby extending the mass of the reef, which grows larger with each generation of polyps. In addition, algae help cement the dead bodies of fish and plants to the reef, stabilizing the structure.
Symbiotic Relationships
Coral polyps and different kinds of coralline algae form a symbiotic relationship, with each producing food for the other and supporting the other's existence. Single-celled algae plants living within each coral polyp create food for the polyps. As plants, algae use sunlight to break carbon dioxide down into edible parts. Meanwhile, the polyps, who also use their tentacles to capture and eat any plankton nearby, produce waste that the algae consume as food. Algae also help to build and cement the reef, often comprising almost half of the reef's mass.
Darwin's Sinking Islands
Scientist Charles Darwin, renowned for his evolution theories, described the three main types of coral reefs as stages of an island's evolution. The first stage, a fringing reef, refers to reefs that appear attached to the islands they ring. The second stage, barrier reef, looks from above like a halo around the island, with significant depths of water between the ring and the island itself. The third stage, atoll reef, looks like a ring whose central island has already sunk into oceanic depths. In the 1950s, Darwin's theories were verified as accurate in many cases, but not all.