Only until very recently have coral pathogens been closely studied and identified by researchers. In the 1970ís, the first reports of coral disease patterns were characterized by a dark band that separated healthy, living coral with dead coral. This band moved across coral colonies while completely destroying coral tissue (Richardson 1998). Microscopic observation of diseased tissue revealed the presence of various bacteria. However, many of the researchers in the 1970ís made little effort to isolate the causative agents using Kochís postulates. By the early 1980ís scientists had begun to make much more detailed descriptions of the various coral diseases, and had determined that there were four distinct types. ìOf the four, the identity of only one primary causative pathogen had been proposed- the cyanobacterium found in Black Band Disease (Richardson 1998).î
Black Band Disease, (BBD), was first described infecting scleratinian corals on the reefs of Belize, but has since been found throughout the Caribbean and Indo-Pacific (Edmunds 1991). Species most susceptible include Montastrea annularis, M. cavernosa and Diploria strigosa. Even though it is estimated that BBD has infected only 2% of corals worldwide, it is significant because of the massive size of these corals and their importance in building the framework of the reefs (Edmunds 1991).
The disease consists of a distinct dark band (about 1mm to 1 cm wide) that separates the bare coral skeleton and living coral tissue. As this band migrates across the coral, the living coral is destroyed, leaving the calcium carbonate coral skeleton exposed. The reason that BBD has such damaging effects is due to the fact that it spreads across the coral surface much faster than the slow growth rate of the scleractinian corals that it infects (Richardson 1996).
The actual band that causes BBD is composed of a complex microbial community which appears dark because of the red cyanobacterium pigment phycoerythrin (Kuta 1996). The dominant species that makes up this microbial community and gives it this dark color is the gliding, filimentous cyanobacterium Phormidium corallyticum. Also included are numerous heterotrophic bacteria, marine fungi, and the sulfide-oxidizing bacterium Beggiatoa (Richardson 1996). Together, these microorganisms are able to spread across the coral at a rate of about 1 cm per day. It is important to understand that the corals that the band infects only grows at a rate of 1 cm in height and 2 cm in diameter per year. Therefore, it is impossible for the coral to outgrow the migrating band (Richardson 1996).
Studies that have been done on BBD have focused on the factors associated with the spread of the disease, or motility responses. Some of the key factors that have been identified include temperature, light and pollution. Infection rates of BBD increase in the warmer summer months and almost disappear in the cooler winter months (Edmunds 1991). It was also determined that forward migration of the band occurred the fastest during the day (Richardson 1996). Some other factors that could lead to BBD is the fact that higher instances of BBD have been seen near industrialized areas. This anthropomorphic stress is thought to contribute to the advancement of the disease (Edmunds 1991).
What is still not known about BBD is the, ìmechanism of infection and the conditions that predispose a colony to the disease (Edmunds 1991).î Healthy corals can become infected with BBD by direct contact. However, infections are not seen on healthy coral specimens when placed a mere 2mm away from diseased colonies (Edmunds 1991). This data suggests that BBD is not highly contagious. Further examples of this are demonstrated by the lack of clumped distribution of BBD across the reef. The only explanation made by the study is that BBD is likely spread when injured colonies encounter trichomes of P. corallytician by chance. Injured colonies have been documented becoming infected when located 15 mm from a diseased colony or 1 meter downstream from a BBD infected colony in the field (Edmunds 1991).
Black Band Disease has been thoroughly studied and researched partly because it was the first coral disease whose pathogen was identified. It is important to realize that to identify the cause of a coral disease, the causative agent must be isolated, grown, and then used to infect additional specimens that exhibit identical symptoms (Towle 1986). Using Kochís postulates is the only method to ensure positive identification, which can ultimately lead to understanding why outbreaks are occurring. Without following these guidelines, much confusion is created when collaborating with other scientists and researching the diseases.
One major example of this confusion deals with reports of the so called ëred band diseaseí. In 1983 red band disease was reported as a variation of black band disease that was found on gorgonians (Richardson 1998). However, no migration of the band or coral degradation was seen. In addition, no attempts to culture the disease were made and when the researchers went back to the same site the next year, no disease activity could be seen (Richardson 1998). To make matters worse, this undocumented information was published as a nonpeer ñreviewed paper and displayed as a representative image on a coral disease web site. It turns out that the picture of ëred band diseaseí that was published on the web site was actually a stage of aspergillosis disease of sea fans where the fan tissue turns purple. The picture of ëred band diseaseí on stony coral was also clearly identified as black band disease (Richardson 1998).
Another distressing occurrence was seen at an international workshop on coral pathology. Several scientists that were familiar with coral diseases were asked to observe an area of the same reef at the same time. While looking at the same specimens, one research termed the affliction ëyellow blotch diseaseí and ëyellow band diseaseí. Another investigator termed the same colonies as having ëdark spot diseaseí. A third said it was ërapid wasting diseaseí. And finally a fourth group of investigators claimed that ërapid wasting diseaseí is caused by parrot fish bites and therefore, termed the disease as ëparrot fish syndromeí (Richardson 1998).
While the enthusiasm for identifying and researching coral reef pathogens is encouraging, it is not without problems. Clearly, as stated in the examples above, there is much organization that needs to be done regarding coral pathogens. In order to effectively document the characteristics of these diseases, care must be taken to set up uniform methods of collaboration. Hopefully, scientists can use the information gained by studying Black Band Disease as a blueprint. A blueprint that can add to and help obtain an enriched understanding of reef ecology.
Edmunds, Peter J. (1991) Extent and Effect of Black Band Disease on a Caribbean Reef. Coral Reefs. 10:161-165
Kuta, K.G. and L.L. Richardson (1996) Abundance and distribution of black band disease on coral reefs in the northern Florida Keys. Coral Reefs. 15:219-223
McCarty, H.B. and E.C. Peters (1998) The coral disease page. Available at: http://ourworld.compuserve.com/homepages/mccarty_and_peters/plague.htm
Richardson, L.L. (1996) Horizontal and Vertical Migration Patterns of Phormidium corallyticum and Beggiatoa spp. Associated with Black Band Disease of Corals. Microbial Ecology. 32:323-335
Richardson, L.L (1998) Coral Diseases:What is really Known? TREE vol. 13 no. 11:438-443
Towle, Albert. Modern Biology. New York; Holt, Rinehardt and Winston, Inc. 1989
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