Abstract

Climate change will exact adverse effects on coral reefs through a variety of mechanisms. Warming temperatures will exacerbate bleaching events where corals lose their symbiotic algae leading to loss of coloration and death. Ocean acidification is already impairing the ability of many marine organisms to deposit calcium in the shells. The effects of acidification on corals that deposit calcium carbonate skeletons and form the foundations of reefs will be particularly dire. In face of all these stressors, disease in corals is likely to play an increasingly important role in decline of coral reefs. Diseases threaten corals globally, but 40 years on, their causes remain mostly unknown. In the Western Atlantic, cover of corals has been reduced by 80%, mainly because of losses resulting from disease. Whereas we once attributed malaria to swamp gas, molecular and immunological tools are now used to assign causes of coral disease in absence of objective data on cell pathology. Application of pathology at the cellular level in conjunction with molecular and immunological tools should focus on what, at the cellular level, is killing corals. A process exists that has historically been successfully used to find cause of animal diseases including presentation (detection of wildlife mortalities), pathology (systematic description of lesions at the gross and cellular level), corroboration (confirming presence of suspect causative agent), laboratory investigations (confirming causation by controlled experiments) and field investigations (how agent, host, and environment interacts to identify points in the disease cycle where intervention can be effected to prevent or reduce spread of disease. Pathology at the cellular level plays a central role throughout, because it provides a logical framework to guide subsequent laboratory confirmations, gives concrete evidence at the cellular level of potential causes of disease and host response, and serves as a reference point to relate experimental studies to disease in the wild. Unfortunately, this approach is not routinely used in corals. A review 415 papers on coral diseases published between 1965 and 2011 reveals that pathology comprised 6% of diagnostic effort, this percentage has remained steady, 60% of papers incriminating an infectious agent do not provide supporting evidence at the cellular level, and 40% of "known" causes of coral disease are now refuted or cannot be confirmed. Over 90% of coral reef resources under US jurisdiction are in the Pacific, and efforts are ongoing there to apply biomedical approaches to disease investigations. There is a clear need for more biomedical experts with knowledge of animal diseases to get involved in this issue because if you lose corals, you essentially lose the environment. Pathology and pathogenesis of disease of mucosal surfaces could serve as a useful model for diseases of corals thereby easing biomedical experts with knowledge of animal diseases into a field where they are sorely needed.