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Dark spot syndrome(DS), also known as dark spots disease, is characterized as being the degeneration or necrosis of scleractinian coral tissue. Dark spot syndrome appears as multiple clusters of discolored, dark tissue spread throughout a colony of coral. The degree of persistence of the disease and the duration of exposure determines the severity of coral damage. The most commonly affected species are Siderastrea siderea, Montastraea annularis and Stephanocoenia intersepta.

Appearance
Dark spot syndrome was first observed on reef systems in Colombia in the early 1990’s. The disease may be identified by purple, brown or black lesions occurring in circular, oval or ring shaped formations. The blemishes may manifest on the coral in several different ways: covering the entire coral surface, spread apart in different sized clusters, bordering the separation between coral skeleton and live tissue. Lesions may gradually widen with prolonged persistence, and may create a slight depression in the coral at the location of the blemish. Dark spot syndrome is the most frequently observed disease in shallow depths ranging between 0-10m. Though documented in deeper waters, dark spot syndrome decreases in occurrence below depths of 12m.

Research and survey methods have characterized four types of dark spot syndrome. The first grouping encompasses small, round lesions possessing a diameter less than 1cm. The second grouping contains the larger, round lesions with a diameter greater than 1cm that are typically observed at the edge of the colony. The third type includes crescent shaped lesions that are created by boring polychaetes such as Spirobranchus giganteus. The final group consists of large, round lesions with filamentous algae growing in the center.

Etiology
The origin of dark spot syndrome is currently unknown. Two leading hypotheses are that the lesions result from a pathogen infection or as a behavioral stress response. Recent studies have indicated that the degradation of coral may be triggered by an invasive endolithic fungus. However, other research suspects that the discolored lesions are caused by increased zooxanthellae pigmentation in response to environmental stressors. Such stressors include, but are not limited to, sedimentation, hurricanes, coral bleaching, UV radiation and increased water temperatures. The observed thermal stress is attributed to the increase in frequency and intensity of El Niño events and global warming. As a coral colony experiences environmental stress, the immune response and defenses are lowered, thus making the coral more susceptible to the disease.

Because the direct cause of dark spot syndrome is unknown, there is no definitive course of action to treat infected coral colonies. Further research is necessary to discover the initial source of the disease, as well as possible treatment methods. In addition, it would be beneficial to examine possible resistant coral species to try to determine a remedy.

Effects
Coral species affected with dark spot syndrome follow one of two paths: recovery or mortality. The outcome is dependent on disease persistence; the longer a location is exposed, the greater the degree of damage. The coral may experience death of an individual polyp or death of the entire colony. However, if the length of exposure is minimal, and the size of the infected area is small, the coral can experience recovery. A recovered coral will continue to grow and reproduce, but at a reduced capacity. There is no exact time line or progression of the disease; dark spot syndrome has been observed in several coral species, over varied time lengths and to different degrees of intensity.

The disease is capable of altering the symbiotic environment of algae and coral. Possible side effects are decreased photosynthetic light penetration, altered chemical balance and decreased nutrient transfer (e.g. nitrogen). Each alteration is capable of interfering with the delicate exchange of nutrients between the algae and coral; decreased zooxanthellae production of organic material restricts food for the coral, which in turn reduces the amount of nutrients produced by the coral and required for survival of the algae. Therefore, any significant change in the mutual relationship can induce coral mortality. The observed rate of tissue loss in a coral affected by dark spot syndrome is 3.99 cm per month.

The known long-term effects are degradation of coral holobiont, large-scale mortality of scleractinian corals and loss of habitat of coral-dwelling fish and invertebrate species. A loss of habitat can result in a decrease in the abundance and distribution of prominent reef species. Therefore, a loss of species richness and biomass among inhabitant plants and animals can also be observed. The coral eventually becomes a macroalgae dominated reef system. The calcium carbonate skeleton will be broken down by erosion from wave action and sedimentation until there is a complete loss of coral structure. Currently, seventy-five percent of the total reef systems are characterized as simple, flat reefs as opposed to complex reefs.