Coral Diseases

Four coral conditions have been identified as diseases: white band disease (WBD), black band disease (BBD), bacterial infection, and shut down reaction (Richmond 1993). They are also susceptible to tumors and parasitic worms. These maladies are all stress related, and anthropogenic stresses can increase a coral's susceptibility to these diseases.

Diseases such as BBD and WBD actually kill coral tissue while advancing in a band around the coral and leaving the white coral skeleton behind. Edmunds (1991) stated that BBD, caused by cyanophyte Phormidium corallyticum, may have a role in maintaining coral diversity because it is most prevalent in coral species that form large colonies and provide a structural framework for the reef. When BBD kills part of these colonies, the skeleton is available to be colonized by other coral species recruits. However, after 25 months, there were not coral recruits among corals infected by BBD (Edmunds 1991).

WBD, which is believed to be caused by a bacteria pathogen yet unknown, has much of the same effect on corals, leaving behind a white, lifeless coral skeleton. Gladfelter (1982) does not see WBD as being beneficial to reefs. He feels that 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. This situation is also probable for BBD.

The exact method by which the diseases are transmitted are unknown. 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. Thus, it is also suggested that BBD can be spread by currents through trichomes that come off infected colonies and land on other colonies (Edmunds 1991).

Corals under stress are more likely to be infected, and BBD has a higher rate of infection in warmer water. Thus seasonal temperatures affect the spread of BBD, and also any anthropogenic warming of water temperatures may increase BBD. BBD was also found to be more abundant near anthropogenic disturbances. However, WBD has not been found to be related to anthropogenic disturbances (Gladfelter 1982).

Corals may also be affected by diseases indirectly. Diseases that infect other reef inhabitants may affect ecology of corals. In 1993, a newly encountered pathogen was found to affect coralline algae in the Pacific Ocean (Littler and Littler 1995). This pathogen also leaves the coralline algae skeleton white as it progresses in a orange band, destroying the algae. Coralline algae help the coral reef community by cementing together sand, coral fragments, and other debris into a suitable hard substrate for the establishment of coral colonies and by absorbing wave energy in the outer reef rim that would otherwise erode the shoreline (Littler and Littler 1995).

Another disease which caused a Caribbean-wide mortality in the reef dwelling sea urchin Diadema antillarum was also shown to indirectly affect coral reefs (Lessions 1988). The sea urchin functions as both a grazer on algae that can otherwise smother coral and as a bioeroder of corals as it feeds on them (Lessions 1988).