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Diseases of scleractinian corals are thought to have increased significantly over the last decade, and they may play a significant role in the widespread mortality of important reef-building species reported in the last few years (Antonius 1988, Williams and Bunkley-Williams 1990, Edmunds 1991, Bythell and Sheppard 1993, Kuta and Richardson 1996, Peters 1996, Brucker and Bruckner 1997, Richardson et al. 1998). Recent studies suggest that coral diseases are affecting a greater number of species, are increasing in frequency and distribution, and are spreading to new regions faster than ever recorded in the past. Although coral diseases are receiving greater attention from the scientific community, most diseases have not been fully characterized, and their etiologies, including the causative agents, remain largely unknown. Identification of the causative agents, quantification of the distribution, abundance and impact of coral diseases, and the possible associations of coral disease with anthropogenic stressors are high priority research areas needed for effective coral reef management. The term disease denotes a demonstrable, negative deviation from the normal state or condition, and can include a functional and/or structural impairment. Coral diseases occur in response to biotic and abiotic stressors. Biotic diseases are caused by microrganisms such as bacteria, cyanobacteria, fungi, protozoans, and possibly viruses, although they may be exacerbated by abiotic factors. Some biotic diseases are highly contagious, and they probably spread through direct contact, through passive dispersal of pathogenic agents via water currents, or via biological vectors. To date, microorganisms have been identified for only two diseases of stony corals, black-band disease (BBD) and white plague (plague type II); a bacterium is suggested to be the cause of white-band disease, although studies are not conclusive. Abiotic diseases, such as bleaching occur in response to physical or chemical stress, and are non-infectious. The recent emergence of coral diseases may be a consequence of deteriorating water quality that favors the proliferation of microorganisms, or of a reduced ability of a coral to defend itself against pathogenic organisms or abiotic stresses. In the 1990s there has been an emergence of newly described coral diseases with new types of tissue conditions not observed in the past. In some cases, similar disease signs have been given different names, and signs of predation have been misidentified as new diseases. In order to standardize the reporting of coral diseases, and to differentiate signs of disease from predation, underwater identification cards containing photographs of the most common coral syndromes are available to assist AGRRA and other reef monitoring groups in accurately assessing coral condition (Bruckner and Bruckner 1998). Each set of AGRRA Disease and Predation Identification Cards includes 4 cards: one card focusing on Montastraea spp. (star corals), one card for Acropora spp. (elkhorn and staghorn corals), one card containing other coral species, and one on disease, predation and tissue anomalies of stony and soft corals. Each card is a compilation of color photographs illustrating the various disease signs, with corresponding descriptions on the reverse side in English and Spanish. With the assistance of the AGRRA Disease and Predation Identification Cards, researchers can try to attribute signs of recent mortality to coral disease, predation by fish or invertebrates, bleaching, or other condition (algal or invertebrate overgrowth, sedimentation, aggression). Areas of recent mortality and adjacent coral tissues should be examined closely to determine if calices are disrupted or damaged (fish bites), if corallivores are present (e.g. gastropods or polychaetes), if polipary structures are abnormally large or tumorous (hyperplasm or neoplasm), or if a disease condition exists. Diseases are recorded based on color, including black (black-band), white (white-band or other white-type diseases in acroporids; white plague in massive corals), yellow (yellow-blotch) or red (red-band). White areas must be examined closely to distinguish areas without living tissue (e.g. exposed skeleton caused by disease or predation) from bleached tissue. Dark-spot disease also has a mottled appearance, however it is manifested as irregular patches that are darker than the normal pigmentation and these areas may be slightly depressed. Predation can often be differentiated from disease (and bleaching) by examining tissues at the interface of white, exposed skeleton - diseased tissue will appear irregular and may be sloughing off the skeleton. Elevated “chimneys” on Acropora spp. and “ridge mortality” on brain corals are induced by repeated three spot damselfish (Stegastes planifrons) bites. DISEASES are characterized using the following color categories:
For more information about coral diseases visit McCarthy and Peter’s web site: (http://ourworld.compuserve.com/homepages/mccarty_and_peters/coraldis.htm). Stoplight parrotfish (Sparisoma viride) predation includes (1) focused biting, where the fish repeatedly bite at one location on a colony, removing large areas of tissue and the underlying skeleton, and (2) spot biting, which appears as multiple, small excavations (generally <1 cm diameter) scattered over the colony surface, sometimes with clearly defined upper and lower jaw marks. Several other parrotfish species, damselfish and other fish also bite live coral; these fish bites appear as small circular or oblong lesions. Bite marks with characteristic grooves are generally visible only on corals with dense skeletons, such as Siderastrea and Acropora, however the removal of tissue combined with the excavation of skeleton is characteristic of fish bites, as described and photographed in the early 1970s in Barbados.
photos by Ken Marks, copyright 1998. Active black band on Montastraea sp. colony (left). Diploria labyrinthiformis with recent mortality caused by white plague (right) Note light green color over recently dead area is thin layer of turf algae (right).
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