abstract

Black Band Disease (BBD) is a widespread coral disease that is considered an important factor in the recent, worldwide decline in coral reefs.^1-4 Although BBD has been described in the literature since mid-1970, controversies regarding the identification of a primary pathogen or a mechanistic description of the disease process are currently unresolved. BBD is grossly characterized by a circumferential, dark-colored band that is between several mm to several cm wide and can advance very rapidly (several mm/day) across a coral colony, killing coral polyps in its wake³. Most investigators believe that BBD is caused by a polymicrobial infection with several species of Cyanobacteria emerging as a key component in the process. Mechanistic descriptions of the disease process affecting the coral polyps range from the release of Cyanobacterial toxins (e.g., Microcystins) to an anoxic, sulfide-rich microenvironment produced by the advancing polymicrobial mat (e.g., the black band).^5-10

In the present study we used a surgical method to isolate individual polyps from both healthy and diseased areas of a coral colony with BBD (Figure 1). We propose that this new approach is superior to existing fragmentation procedures because it minimizes disruption of the coral colony and allows precise sampling of individual polyps relative to the advancing polymicrobial mat. The histopathology of individual coral polyps within the black band region revealed a massive invasion of the polyp tissue by Cyanobacteria (Figure 3). The morphology of the Cyanobacterial-polyp interaction suggested that the tissue invasion occurred over a relatively short timeframe. This finding correlates with the rapid progress of the black band migration and suggests that Cyanobacterial tissue invasion contributes to the pathogenic mechanism of BBD.

Click on an image to see a larger view.

	Figure 1. Individual coral polyps isolated from a healthy region (left panel) of a coral colony of Montastrea cavernosa and a coral polyp recovered directly under the advancing black band (right panel).
	Figure 2. Histological sections of the healthy polyp shown in the left panel of Figure 1. Left panel: low magnification of the entire polyp in cross section. Right panel: higher magnification of apical tentacle area showing intact epidermis, gastrodermis (with zooxanthalle) and nematocysts.
	Figure 3. Histological sections of the diseased polyp shown in the right panel of Figure 1. Left panel: low magnification of polyp showing less affected polyp tissue on left and massive invasion of Cyanobacteria on right (arrows). Right panel: higher magnification of Cyanobacteria invasion of coral tissue. Note the complete replacement of the original tissue form by Cyanobacteria.

Acknowledgements

This presentation and research were supported through a basic research fund from St. George's University and a research grant from the Office of Naval Research (N00014-00-1-0609). We thank Dr. Milton McAllister, Ms. Sharon Meachum, Mr. A. Piggot and Dr. R. Johnson for assistance in laboratory and field work.

References

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