The significance of emerging diseases in the tropical coral reef ecosystem

Novel pathologies of coral reef organisms, especially reef frame building s cleractinian corals, have escalated during the decade between 1987 and 1997 . These emerging diseases have appeared with progressively greater frequenc y and over wider distribution, and have revealed more diversified character istics than ever before. The causes of most of these infections are not yet confirmed, but they evidence a gradual decline in the vital status of the coral reef ecosystem. As specific causes are identified for these afflictio ns, terminology will shift from non-specific descriptions,such as "white ba nd", "white plague", "white pox", "yellow band" and "black band" diseases, to etiological and pathognomonic characterizations (e.g. aspergillosis and cyanobacteriosis). Stony corals are vulnerable to sedimentation, nutrient o verloading, and chemical pollution from agricultural, urban, and domestic s ources. They are incapable of relocation to other sites or of self-protecti on from cumulative effects of exposure to nitrates, phosphates, herbicides, pesticides, and raw sewage. In contrast to stresses attributed to warm wat er seasonal anomalies (e.g. coral reef bleaching), stresses imparted by pat hogenic micro-organisms occur throughout the calendar year, fluctuate with changing temperature, and invariably result in tissue mortality. The coral has several mechanisms for defense. The epidermis, especially in tentacles of the coral polyp, contains nematocysts which are released in response to predators. The epidermal cells also possess cilia and a flagellary apparatu s which are responsible for generating microcurrents in boundary water adja cent to the organism. These currents facilitate the entry of food into the coelenteron for digestion. Mesenterial filaments extend through the epiderm is, sweep the surface of the colony, initiate digestion of food particles, and eventually return to the coelenteron. Both the epidermis and the gastro dermis contain mucocytes (or "immunocytes") which release a mucous secretio n. That mucous blanket physically insulates the tissue from particulates or soluble toxins, and may also be bacteriostatic because of immunoglobulin ( IgA). The recent emergence of diseases in corals may be interpreted as the consequence of (1) changing coastal ocean water quality favoring the prolif eration, attachment and colonization of microbes, and (2) reduced efficienc y of the coral's normal defenses. In order to appreciate these changes, res earch efforts to evaluate the microbial content of reef waters and to analy ze the respective roles of mucus, cilia and flagella, and nematocysts of th e corals are necessary. In this study, we have begun to detail the structur al, physiological, chemical, and immunological attributes of the coral. Our analysis suggests that at least some of the emerging coral diseases may be explained by a decline in the capacity of coral colonies to mount effectiv e protection against the increasing prevalence and varied invasive strategi es of marine pathogens.