A view of our boat at Gaulin Reef in the Bahamas
Coral Reefs are the backbone of life under water, and are often dubbed as a keystone species—a species that many other species rely on in an ecosystem. Being such an integral part to marine life, the fate of corals can also result in the fate of many other aquatic species. Because of coral’s significance, it is important to note the various diseases effecting coral.
Disease and Organisms
Diseases can affect organisms and populations in a variety of ways. It can shorten the life span of an organism, alter metabolism and nutritional needs, reduce growth, halt or alter healthy reproduction, and diminish tolerance to stress. These factors lead to altered populations by reducing the abundance (possibly contributing to the threat of extinction), and it can skew the distribution, possibly separating one population into two separate populations. In addition, this variability in population alters competition with other species significantly.
Coral Reef Fragility
An overwhelming amount of symbiosis takes place on a reef, for example, between fish and coral, and coral and algae. A specific type of algae called zooxanthellae live inside each coral polyp and photosynthesize light into energy for the coral, while the coral provides shelter. Similarly, herbivorous fish also display symbiosis with coral—they have been named the reef’s “immune system” in recognition of “their role in limiting algae growth, and allowing corals to flourish” (Davidson 96). If algae were allowed to flourish, the coral would smother and die. When coral dies, it can no longer sustain a viable habitat for the many creatures that call it home. Not only does the coral suffer, but so do the many species that rely on coral for food and shelter. Since coral reefs are so impertinent to marine habitats, a coral disease can offset an entire community, and even neighboring communities.
When considering the following criteria for classifying disease, some coral illnesses become questionable of whether they are truly diseases:
“Disease are usually characterized (1) as an identifiable group of signs (observed anomalies indicative of disease), and / or (2) a recognized etiologic or causal agent, and / or (3) consistent structural alterations (e.g. developmental disorders, changes in cellular composition or morphology, and tumors)” (Dr.Arnfried Antonius).
It is interesting to ponder coral bleaching under this definition. Coral bleaching is the “death of coral caused by the loss of zooxanthellae” (Stafford-Deitsch 192). This loss of “zoox” is often caused by rising sea temperatures which makes global warming a huge threat to corals. Warmer waters cause zoox to produce too much oxygen creating “toxic by-products…that are damaging to the tissues of the coral polyp,” and the zoox are sometimes lost through the damaged polyp wall (Stafford-Deitsch 58). The remaining coral skeleton could possibly “recover by obtaining new zooxanthellae from the surrounding water but it is more likely, however, that it will die” (Stafford-Deitsch 58). This loss of zoox, or bleaching, can be characterized as a disease by the above definition in that the change in color from zoox color to pure white, which is a “consistent structural alteration.”
Other coral diseases result in a similar consequence of the loss of zoox, but they are caused by factors other than increasing water temperature. Pathogens are also a threat to coral and a cause for coral disease. Many of these pathogens are opportunistic, in that they wait until the coral is weakened to attack and they leave the coral with much less resistance to stress. Some corals are said to be resistant to disease for unclear reasons, but “if diseases are indeed triggered by stress, they could well become more common” (Wells 65). This leads to a curious idea that maybe those corals somehow fit enough to resist stress may also be able to resist disease.
This idea could be seen as a natural Darwin-like house cleaning in that many “unfit” or stress prone corals are dying out, which could be caused by natural disturbance stress such as hurricanes and predation (Wells 65). A more realistic view of coral depletion is the idea that humans are causing stress. Corals can recover from small amounts of stress but “the ability of reefs to recover from natural disturbances is easily upset by the addition of human impacts” (Wells 65). Tourism (scuba diving/snorkeling), global warming, pollution, damaging fishing mechanisms (blast and cyanide fishing) and over fishing are all ways that humans are contributing stress to corals, and possibly disease.
Types of Coral Diseases Affecting the Caribbean
When classifying diseases it is important to note the difference between abiotic and biotic induced diseases. Abiotic diseases result from abiotic environmental stresses such as “changes in physical conditions optimal for growth (salinity, temperature, light intensity or wavelength, sedimentation, oxygen concentrations, currents) or exposure to toxic chemicals (such as heavy metals and organics like crude oil, its derivatives and even pesticides)” and causes abiotic tissue bleaching (TBL) (Madl). On the other hand, biotically induced diseases results when the “etiolic agent is a living organism such as opportunistic pathogens or parasites that take advantage of a weakened host organism. Infectious agents, those that are spread from host to host, include viruses, bacteria, fungi, and protozoans (also known as microparasites), and metazoans such as helminths and arthropods (macroparasites)” which causes biotic tissue bleaching (TBL) (Madl). Humans are mostly responsible for the abiotically induced diseases, but both are harmful to coral and their surroundings.
Black Band Disease (BBD) is usually seen as a “dark pigmented microbial mat” which is “usually about 1 to 30mm wide and ca. 1mm thick and separates living tissue from the recently denuded skeleton” (Madl). It is “found mainly in massive brain corals and gorgonians” and it “appears to kill coral tissue with poison, and seems to be more frequent on reefs under stress from other impacts such as bleaching” (Wells 65). Interestingly enough, the “BBD mat is dominated by an unidentified cyanobacterium” in the Caribbean (Madl).
Dark Spot Disease / Syndrome (DSD/S) affects 11 scleractinian species in the Caribbean and is widespread (Madl). It is distinguished by “irregularly shaped dark spots of purple, maroon, or brown coloration on normal tissue” (Madl). DSD can be associated with tissue necrosis, in which “a spot expands into a characteristic dark ring separating dead skeleton from living tissue” (Madl). Eventually over long periods of time, DSD/S causes typical depressions on the coral surface and the dark colored areas expand and stain the skeleton. In the Caribbean DSD/S is most seen at depths above 10 meters and temperatures higher than 28 degrees Celsius (Madl).
Red Band Syndrome (RBS) was documented around the same time BBD was, and was affecting soft corals and hard corals in a similar manner, except the coloration of the band separating living tissue from dead skeleton was more red-brown or brown-black rather than black. Although very similar to BBD, RBS was eventually considered to be separate because the “dominant component of the BBD consortium, was not present within the red band. Instead, it was populated by other cyanobacterial species” (Madl).
Tumors (TUM) usually refer to abnormal tissue proliferation but in corals tumors usually refer to abnormal skeletal growth. Neoplasia is a common cause for tumors in corals, which can be characterized by “thinning of coral tissue…increased porosity of coral skeleton…loss of mucous secretory cells…loss of endosymbionts…loss, reduction, or degeneration of normal polyp structures” (Madl). Not only is the physical structure of the coral degraded in the presence of tumors but important endosymbionts, such as zoox are dying or leaving. Without their presence the coral cannot survive. In other words, tumors devastate corals from the inside out. Tumors also compromise defense mechanisms and “impaired mucus secretion inhibits removal of foreign material from the coral surface [which] contributes to cell death and increases the coral’s susceptibility to invasion by filamentous algae…In addition, porous skeletons are more susceptible to storm-related damage” (Madl).
Neoplasia refers to abnormal lumps or tumors which may be cancerous or lead to cancer. It spreads quickly because the “affected area is more porous and connects the polyps by tiny canals” which “facilitates the exchange of nutrients throughout the colony and in particular of the diseased region” (Madl). Diseased neoplasia regions appear smooth because of the altered skeleton, and “lack symbiotic algae as well as secretory mucus cells” (Madl). This loss not only creates sediment accumulation which damages the underlying tissue, but with no “polyps present in the affected area the coral produces reduced numbers of eggs and sperm” ultimately reducing reproduction rate (Madl).
Hyperplasia can be compared with gigantism, in that growth is very rapid in some areas but the new cells still obtain their normal structure. More specifically, “patterns of ridges and valleys (in brain corals) or circular polyps (star corals) in affected regions are pronounced” and noticeably stick out from the rest of the coral body (Madl). Hyperplasia can be caused by “an obligate endoparasite of corals, invades a normal coral polyp as a larva and matures within the polyp” (Madl). This It results in the formation of an enlarged (hypertrophied) corallite with abnormal septae” (Madl). The abnormalities in structure may hinder the coral.
White Band Disease (WBD) is most often seen in branching corals, as it “kills and bleaches the tissue in parts of the colony, leaving behind a white skeleton” (Wells 65). Although bacteria have been found in affected areas, the cause of WBD is still unknown (Wells 65). According to Gladfelter, WBD destroys the reef structure because the dead coral skeleton brought about by the algae is colonized by algae, invertebrates, gastropods, and boring cloinid sponges that work to weaken the coral skeletons making them more susceptible to breakage during storms (1982). Similar to other band diseases, WBD also has a band separating living tissue from dead skeleton.
White Pox Disease (WPX) “was first documented in 1996 on reefs off Key West, Florida and is now observed throughout the Caribbean” (Madl). Oddly enough, it is caused by a common fecal enterobacterium. This bacteria can be found in “feces of humans and other animals, in water and soil and has been found in the marine environment of sewage-polluted estuaries” (Madl). This disease demonstrates the toll sewage pollution can have on coral. Corals with this disease have “irregularly shaped distinct white patches of recently exposed skeleton, surrounded by a necrotic front of normally pigmented living tissue” (Madl). Multiple randomly distributed abrasions on the coral body can develop at the same time, intensifying the coral’s fate.
Yellow Blotch Disease (YBL), previously termed Yellow Band Disease, has been reported throughout the Caribbean since 1994 effecting nine different species, mostly near the upper surfaces of the coral bodies (Madl). YBL is distinguished by “circular to irregularly shaped patches or bands of discolored coral tissue,” but there is no “band” separating living and dead tissue (Madl). Although, as the disease advances, “the tissue in the center of the patch dies and the area fills with sediment and filamentous algae, resulting in a band of yellow tissue around the enlarging sediment patch” (Madl). Corals with YBL have less endosymbionts (zoox) than healthy corals which makes survival much more difficult.
How coral diseases spread
The ocean’s currents and migrators are two examples of how the seven seas do not have distinct boundary lines. connect the seven seas. “The interconnectedness of the oceans means that disease can spread widely through populations of marine animals” (Wells 65). An interesting debate arises from this reality, whether coral diseases can spread among different communities. According to Edmunds regarding coral, “the exact method by which the diseases are transmitted are unknown” yet he ascertains that “even though healthy corals may get BBD through contact with an infected coral, diseased corals are not aggregated naturally on the reef and can be separated by great distances” (1991). He also suggests that “BBD can be spread by currents through trichomes that come off infected colonies and land on other colonies” (Edmunds 1991). These trichomes, or “plant hairs,” show how marine life can in fact help the spread of coral diseases. If plants can spread diseases, it is likely fish wondering among coral can spread diseases as well.
Attempts to stop disease spread
Recent effort to stop the spread of coral disease includes the aspirator method and the cement/clay method. The aspirator method includes an aspirator sucking off the diseased coral tissue and then this tissue is disposed of on land (Wells 65). The second method is comprised of coating the infected area with cement or clay in hopes this covering will contain the disease (Wells 65). Unfortunately both methods are extremely time consuming and expensive. As important as coral reefs are to life under water and on land, it is unrealistic to cover or suck off diseased coral when an entire community has been infected. Prevention methods would be a better use of time and resources, such as global warming and pollution prevention.
Davidson, Osha G. The Enchanted Braid. New York: John Wiley & Sons, Inc., 1998.
Edmunds, P.J. 1991. Extent and Effect of Black Band Disease on a Caribbean Reef. Coral Reefs,
Gladfelter, W.B. 1982. White Band Disease in Acropora palmata: Implications for the Structure and
Gorwth of Shallow Reefs. Bulletin of Marine Science, 32(2):639- 643.
Madl, P. (2005, February 10). The Silent Sentinels...The Demise of Tropical Coral Reefs.
Retrieved May 10, 2007, from University of Salzburg, Dept. of Ecology Web site:
Sprung, J., & Delbeek, J. C. (1997). The Reef Aquarium . Coconut Grove, Florida :
Stafford-Deitsch, J. (1991). Reef: A Safari Through the Coral World . San
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Vevers, G. (1971). The Underwater World . New York: St. Martin's Press.
Wells, S., & Hanna, N. (1992). The Greenpeace book of Coral Reefs. New
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Publications, Inc., Ltd.
For Further Info on this Topic, Check out this WWW Site: http://www.sbg.ac.at/ipk/avstudio/pierofun/reefs/ch4.htm.
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