Coral reef disease has increased significantly over the past fifteen years resulting in extensive damage to coral reef colonies. Approximately thirty percent of the world's coral reef population has been degraded as a result of both human action and natural forces. In addition, over ten percent of the current coral reef population has been destroyed beyond repair.
Coral reef disease usually begins with the invasion of a particular bacteria, fungi or virus into the soft tissue of a coral polyp. Various causes for the invasion and ultimate degradation of the coral polyps include both biological and external stresses placed on the coral due to increased water temperature, excessive exposure to ultraviolet radiation and pollution.
Here are some of the most common and detrimental diseases affecting coral reefs:
Black-band disease is caused by the invasion of cyanobacteria into the soft tissue of the coral polyps. The disease is characterized by a dark black band that envelops the surface of the coral. The band can be between one to thirty millimeters wide and up to 2 millimeters in length. The bacteria degrade the coral's soft tissue smothering and killing it as it passes over the surface. The photosynthetic pigment of the cyanobacteria gives the resulting band a dark maroon or black appearance. The band of bacteria is easily dislodged by water motion and swiftly moves across the coral surface a several millimeters each day
Coral Bleaching occurs as a result of coral polyp colonies expelling the zooxanthellae algae residing within the soft tissue of the polyps. Coral polyps usually expel excess algae from the tissue to reduce excessive photosynthetic processes conducted by symbiotic algae. However, when coral polyps become inordinately stressed they have the tendency to over expel the algae from the soft tissues resulting in the discoloration or whiting of the skeletal structure of the reef. Without the symbiotic relationship with zooxanthellae algae the coral reef colony will eventually die. Death of the reef colony remains dependant upon the severity and duration of the coral bleaching process. Many affected colonies have been known to recover. Coral are sensitive to varying water temperatures, salinity, UV radiation, transparency and nutrient density. Bleaching usually occurs as a direct result of excessive variations taking place within the reef environment
Dark-Spots Disease is characterized by various discolored markings occurring within the tissue of coral reef colonies. The spots appear as purple, gray or brown irregular markings and spots that invade the soft tissue of the polyps. Their appearance is usually restricted to the surface or the outer margin of the reef colony. As the disease spreads the coral is degraded and dies.
Red-band disease appears as a thin band of filamentous cyanobacteria that travels slowly and methodically over the surface area of reef colonies. As the bacteria moves along the coral the tissue is destroyed and the coral ultimately dies.
White-band disease causes the tissue of the coral to peel or fall off the skeletal structure of the coral. The disease works in a uniform and strategic manner starting at the base of the reef colony and then moving up along the branches to the tip of the coral.
Scientists continue to study the devastating effects of coral disease in order to determine not only the cause of these diseases but also the affect they have on the unique and delicate ecosystem in which the coral resides.