Coral reef diseases usually occur in response to stresses such as bacteria, viruses, increased water temperatures, Ultra-violet (UV) radiation, and pollutants; one type of stress may weaken the coral, leaving it vulnerable to another. The frequency of coral diseases has increased recently, due in much part to global warming and deteriorating water quality, but exact causes have yet to be determined.
Black-band disease (BBD) is caused by cyanobacteria combined with sulfur-oxidizing and sulfur-reducing bacterias. It is characterized by a maroon-to-black-colored band, one to 30 millimeters wide and up to two meters long, that eats at living coral tissue as it passes over the colony, leaving a bare skeleton; the skeleton is rapidly attacked by algae and other organisms. BBD is most often present at depths below 100 feet.
Coral bleaching occurs when corals are stressed and expel zooxanthellae, which in photosynthesis gives corals their color. As a coral's discoloration disappears, the coral skeleton shows through the translucent coral tissue. Some species are more suitable candidates for bleaching, and it is rarely uniform over colonies, as corals may be affected in different areas or in different patterns. Localized bleaching may be attributed to high light levels, high water temperatures, or bacterial infections.
Dark-spot disease (DSD) has been noted for many years, but only recently studied (late 1990s). Affected areas are dark purple, gray, or brown, circular or irregularly shaped, and are scattered on a colony; the discolored tissue increases in size as the affected area dies.
Red-band disease (RBD) is a narrow band of cyanobacteria that advances across the coral surface, killing living tissue. RBD-1 resembles BBD, except the band is reddish-maroon. RBD-2 spreads like a net over the colony.
White-band disease (WBD) is characterized by tissue that peels off in a uniform band, ranging from a few millimeters to 10 centimeters. Type II WBD is sometimes mistaken for bleaching, because tissue attached to exposed skeleton bleaches before if dies. Type I WBD does not have the bleaching phenomenon. There is no known cause for either type.
White plague is similar to WBD, with its white band of exposed coral skeleton separating living tissue from the algal-colonized skeleton. There are three types: Type I causes tissue mortality at a rate of three millimeters per day and affects ten species of coral. Type II has a tissue mortality rate of two centimeters per day, and affects 32 species. Type III affects large reef-building corals, with a much greater tissue mortality rate.
White Pox Disease has a rapid rate of tissue damage which allows for quick algal colonization. It is characterized by white circular lesions, and there is no sure cause.
Yellow Blotch Disease (YBD) affects only certain star corals and brain corals. It begins as pale blotches or bands surrounded by healthy tissues. As the disease progresses, it radiates outward, first killing the tissue at the center of the blotch or band. The tissue mortality rate is five to 11 centimeters per year.