Background Scenario

There are many species of coral that grow throughout the world’s oceans, but most all corals have common structural characteristics. Corals have been in existence for nearly 200 million years but have reached their current level of diversity only 50 million years ago.

There are two main types of corals; hard corals composed of stony calcium carbonate, and soft corals made up of a protein/calcium carbonate material. Scleractinians, or hard corals such as brain, star, staghorn, elkhorn and pillar corals have rigid exoskeletons, or corallites, that protect their soft delicate bodies. Gorgonians, or soft corals, such as sea fans, sea whips, and sea rods, sway with the currents and lack an exoskeleton.

A coral contains a very thin outer layer of living coral polyps. Although most corals contain hundreds or thousands of polyps, some contain only one. The polyp is a hollow, cylindrical animal with a mouth that is surrounded by tentacles armed with stinging cells for capturing food. Underneath these coral polyps is the dead coral skeleton composed of calcium carbonate. Each coral polyp excretes a calcareous exo-skeleton that lives in a symbiotic relationship with zooxanthellae, a host algae located in the tissue of the polyp, that gives the coral its green, blue, purple of brown color.

When corals become stressed, which occurs in most cases when sea surface temperatures are too hot or too cold, many reef polyps expel their zooxanthallae. The polyps of the coral then lose their pigmentation and appear nearly transparent on the animal's white skeleton. This phenomenon is referred to as coral bleaching.

Prior to the 1980’s, most bleaching events reported attributed to localized phenomena such as major storm events, severe tidal exposures, sedimentation, rapid salinity changes, pollution, or thermal shock. Since 1980, however, bleaching events have not been so easily explained. Research has shown a direct relationship between bleaching and water temperature stress. Extreme water temperatures have been implicated in the majority of the major bleaching events of the 1980s and 1990s.