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Solutions today for reefs tomorrow
Dark spot disease (DSD) is a non-lethal coral disease that is characterized by lesions of purple or brown discoloration. Dark spot disease is known to affect 21 scleractinian coral species and is most prevalent on the reef-building species Siderastrea siderea. Since it was first reported in Colombia in the 1990s, DSD has been observed throughout the wider Caribbean, Brazil, and in the Indo-Pacific.
Dark spot disease lesions appear as multifocal to coalescing areas of purple or brown discoloration, ranging in size from 1-5cm, that can be located diffusely over the entire colony. The discolored areas can be round, oblong, or irregular spots with smooth or undulating margins.
Dark spot disease lesions can progress over time, though the progression rate typically remains low at <2cm/ month. In some cases, the center of lesions can exhibit subacute tissue loss with algal overgrowth (indicating the characteristically slow lesion progression rate) or can have a depressed relief. It is currently unknown whether this feature is caused by slower growth or active decalcification in affected areas.
Dark spot disease lesions are not lethal and can persist on a colony for years, with a notable exception being a mortality event of Agaricia agaracites in Colombia in 2004.
The precise cause of DSD is not currently known. Some microbiological studies have found that DSD-affected tissues host a different bacterial consortium than that found on healthy tissues, including a notably high abundance of the bacterium Vibrio carchariae, but inoculating a healthy coral with the bacteria isolated on a diseased coral does not produce a DSD lesion. This, coupled with the ineffective treatment with antibiotics, suggests that a bacterial pathogen is unlikely to be the cause of DSD.
Histopathological studies on DSD from the Indo-pacific and Florida have identified communities of fungus within the skeletons underlying DSD lesions. The fungal mats, known as “endolithic fungi,” because of their residence within the skeleton of a coral, are dense within the skeleton and appear to affect the architecture of adjacent coral tissues. This, coupled with the characteristically slow and chronic lesion behavior, suggest that a fungal pathogen may be the cause of DSD, though this needs to be confirmed with culturing and inoculation studies.
The overall impact of DSD on reef coral cover and biodiversity is low. Dark spot disease lesions are non-lethal, and while they can be chronic and persist for multiple years, there is also some evidence of healing and tissue regeneration.
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