Biologists refer to coral reefs as the rain forests of the sea. Coral reefs are the habitat of 33 of the 38 identified animal phyla, whereas tropical rain forests give shelter to only eight phyla. Troubles of coral reefs are well known and there are various causes on injury taking place, including population expansion and change in climate. Estimate for the area coral reefs occupy range from 284.000 to 512.000 Km2, however, they are the habitat of more than 25% of the ocean’s biodiversity giving them a characteristic of no otherworld ecosystem. Corals are invertebrate animals belonging to the phylum Cnidaria (the same as jellyfish). The phylum name refers to the hurtful cutting hairs of the corals (stinging cells) called cnidocytes. The coral body looks like a vas sac (polyp) that has an opening at one end (mouth) surrounded by stinging long flexible limbs (tentacles) to capture food. Although corals belong to the animal kingdom, yet they behave in many respects like a plant, as building reefs near the water surface where they can easily reach sunlight. Symbiosis (close association with plants and animals) influences the colors of corals (Staub, 1-3).
Either coral diseases occur because of a biological insult as infection with bacteria and fungi or non-biological causes as increased water temperature or water pollution. Biologists consider these diseases a major threat to coral reefs health and reproduction. The Western Atlantic reefs (especially Caribbean) are the most affected by disease. Correlation between coral disease prevalence and environmental factors is a reason for increasing the severity of infection (Raymundo, 1-3). The first case of cold-water infective disease occurred in 2002 at South West England coast affecting Eunicella verrucosa, the disease caused by bacteria with no fungi isolated. The disease characterized by tissue sloughing and exposure of skeletal gorgonin to putrefactive bacteria causing foul smell (Hall-Spencer and others, 87).
A report of the National Oceanic and Atmospheric Administration on the main diseases affecting corals described eight infective diseases (<http://www.coris.noaa.gov>).
1- Black Band Disease: The characteristic to this disease is a black concentric or crescent-shaped band of 1-30 mm wide and nearly 2 m long. The band erodes coral tissue as it extends over the colony surface, leaving bare skeleton. Cyanobacteria associated with sulfide-oxidizing and reducing bacteria are the main causative organisms. Opportunistic protozoa and fungi (nematodes, ciliate protozoan, flatworms and fungal filaments) are adding to the damage. Photosynthetic pigments of dominant cyanobacteria produce the black color of the band; however, the lesion usually shows a white dusting produced by sulfur-oxidizing bacteria. 2- Coral Bleaching: The normal color of most stony corals is yellow to light brown because of photosynthesis pigments of coexisting zooxanthellae. On unrestrained stress, corals expel their zooxanthellae, or concentrates of photosynthetic pigments declines (bleaching event). What produces bleaching; is when coral’s color either disappears or becomes pale, the coral’s skeleton white color appears through the semi-transparent coral tissue. 3- Dark-Spots Disease: The disease affects the tissue of pink starlet coral (Siderastrea siderea) and the blush (cherry red) star coral (Stephanocoenia intersepta) and to a lesser extent in Montastraea annularis. What characterizes this disease is appearance of dark purple, gray or brown patches of discolored tissue. The spots are circular or irregular in shape and spread either on colony surface, or at its margin. The dark tissue extends from inside outward as the area first affected dies. Affected coral polyps are pressed down and they become less in size than normal polyps. 4- Red-band Disease: The disease appears as a narrow band of wool like threads of cyanobacteria gradually extending over the surface of a coral, destroying living corals as it progresses. Two types of red-band disease exist, first, red-band disease-1 that looks like black-band disease, with two differences; it is red to reddish-purple in color and the cyanobacterial wool like threads are more limply arranged. The second type is red-band disease-2, which is characterized by spreading out of the cyanobacterial wool like threads as if it is a mesh over the surface of the colony. 5- White-band Disease: Identification of this disease for the first time was in 1977 on reefs surrounding St. Croix. It affects Staghorn and Elkhorn corals. This disease characteristic is necrosis (sloughing) of the coral frame in a consistently regular band pattern, starting, usually, at the colony’s bottom and spreading up to reach branches ends. The width of the band varies from a few millimeters to 10 cm wide, and the rate of tissue is loss is nearly 5 mm a day. 6- White plague: It looks like white-band disease except that it affects many coral species and it starts at the colony base and extends rapidly in an upward and outward direction. 7- White Pox Disease: The disease affects Elkhorn coral in Florida bay and in most of the Caribbean. The disease characteristic is white rounded lesions on the surface of infected colonies. It rapidly destroys coral tissue giving the chance for algae to grow rapidly within days. Bacteria are the initial organism causing the disease followed by secondary algae infection. 8- Yellow Blotch Disease: It affects only star corals (genus Montastraea) and the brain coral (Colpophyllia natans). The disease may begin as light pastel, rounded stains (blotches) of clear tissue or as a narrow band of pale tissue at the edge of coral colony. Normal, fully pigmented tissue surround affected areas. As the disease advances, the tissue in the center of the patch dies, and algae colonize exposed skeleton. The area of affected tissue continuously spread from the center outward, slowly destroying the coral (US Department of Commerce, The National Oceanic and Atmospheric Administration report 2004).
Biologists recognize coral diseases as significant phenomena that can alter the structure and composition of coral reefs. Compelled by ecological needs, (Bruckner 1-2) suggested the following management strategies to control these diseases. There should be a notification system against disease outbreaks; to record information of geographical distribution and chronological changes of coral diseases. Bruckner suggested also that dealing with environmental changes that enhance infection progress should induce a change. Other strategies included development of consistent terms for diseases and directing more attention to study their pathology and causative factors to help identify possible new types of diseases. Studying the epidemiology of coral diseases is required to find out what causes spread and transmission disease-producing agents. More research is needed to know the effect of coral diseases on the building frame of coral specie, and the ecosystem of coral reefs; finally there is a need to find a way to reduce the effect of disease on coral population or in simple term is there a treatment for coral disease.
Coral diseases are important reasons for degrading reef ecosystems as well as seriously influencing economies that depend upon them. The knowledge of emerging diseases, their geographic distribution the affected species is growing however, the scientific understanding of these phenomena remains limited. Biological Research provided progress in characterizing the ecology and causes of common coral diseases, yet most diseases are still poorly understood. Future research should focus on better understanding of causes of coral diseases, acquire proper management approaches that tackle the causes of these disease outbreaks and finally provide site-specific conservation approaches to reduce disease occurrence.
Staub, F. International Year of the Reef. Coral Reefs: Delicate, Diverse, Degraded and Fast Disappearing. 2008. 16/05/2008 <http://www.thew2o.net/images/folder/pdf>.
Raymundo L J. “Coral Disease as an Emerging Management Issue”. ICRI GM. International Coral Reef Initiative (ICRI). Japan, Koror, Palau. 31 October-2 November 2005.
Hall-Spencer, J M., Pikes, J and Munn, C B. Diseases affect cold-water corals too: Eunicella verrucosa (Cnidaria: Gorgonacea) necrosis in SW England. Diseases of Aquatic Organisms. Vol 76 2007. p. 87-98.
United States Department of Commerce. National Oceanic and Atmospheric Administration. Major Reef-building Coral Diseases. 2004. 16/05/2008 <www.coris.noaa.gov.htm>.
US Department of Commerce. National Oceanic and Atmospheric Administration. National Marine Fisheries Service. Priorities for Effective Management of Coral Diseases. By Bruckner, A W. 2002. 16/05/2008 <www.nmfs.noaa.gov/habitat/ead/ecosysdocs/ManagementPrioritiesforCoralDisease>.
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