Disease and immunity in Caribbean and Indo-Pacific zooxanthellate corals

Since the mid-1990s, coral diseases have increased in number, species affected, and geographic extent. To date, 18 coral diseases, affecting at least 150 scleractinian, gorgonian, and hydrozoan zooxanthellate species, have been described from the Caribbean and the Indo-Pacific, These diseases are associated with pathogens including bacteria, cyanobacteria, fungi, and protists and with abiotic stressors including elevated seawater temperature, sedimentation, eutrophication, and pollution. Etiologies of only 5 of the 18 coral diseases have been determined through fulfillment of Koch's postulates. Corals and other invertebrates utilize innate immune mechanisms including physiochemical barriers and cellular and humoral defenses against pathogens. Here we review the described coral diseases, known etiologies, and efforts to determine unknown etiologies. We define disease terms, discuss the limitations of Koch's postulates, describe alternative techniques for identifying disease-causing organisms, and review coral immunology.