Mass mortalities due to disease outbreaks have recently affected major taxa in the oceans. For closely monitored groups like corals and marine mammals, reports of the frequency of epidemics and the number of new diseases have increased recently. A dramatic global increase in the severity of coral bleaching in 1997-98 is coincident with high El Niño temperatures. Such climate-mediated, physiological stresses may compromise host resistance and increase frequency of opportunistic diseases. Where documented, new diseases typically have emerged through host or range shifts of known pathogens. Both climate and human activities may have also accelerated global transport of species, bringing together pathogens and previously unexposed host populations. The oceans harbor enormous biodiversity by terrestrial terms (1), much of which is still poorly described taxonomically. Even less well known are the dynamics of intermittent, ephemeral, threshold phenomena such as disease outbreaks. Despite decades of intense study of the biological agents structuring natural communities, the ecological and evolutionary impact of diseases in the ocean remains unknown, even when these diseases affect economically and ecologically important species. The paucity of baseline and epidemiological information on normal disease levels in the ocean challenges our ability to assess the novelty of a recent spate of disease outbreaks and to determine the relative importance of increased pathogen transmission versus decreased host resistance in facilitating the outbreaks. Our objectives here are to review the prevalence of diseases of marine taxa to evaluate whether it can be concluded that there has been a recent increase. We also assess the contributing roles of human activity and global climate, and evaluate the role of the oceans as incubators and conveyors of human disease agents.

Populations of the shallow-water Caribbean elkhorn coral, Acropora palmata, are being decimated by white pox disease, with losses of living cover in the Florida Keys typically in excess of 70%. The rate of tissue loss is rapid, averaging 2.5 cm 2 ⅐day ؊1 , and is greatest during periods of seasonally elevated temperature. In Florida, the spread of white pox fits the contagion model, with nearest neighbors most susceptible to infection. In this report, we identify a common fecal enterobacterium, Serratia marcescens, as the causal agent of white pox. This is the first time, to our knowledge, that a bacterial species associated with the human gut has been shown to be a marine invertebrate pathogen.

Oceanography march 2007 a c r o s s t h e g l o b e , we are witnessing the decline of coral reef ecosystems. One relatively new factor contributing to this decline is the outbreak of destructive infectious diseases, especially on Caribbean reefs. As the Coral Disease Working Group of the Coral Reef Targeted Research Program, our research focuses on four priorities: (1) assessing the global prevalence of coral disease, (2) investigating the environmental drivers of disease, (3) identifying the pathogens that cause disease, and (4) evaluating the coral's ability to resist disease. Monitoring has revealed new coral-disease syndromes at each of four Global Environmental Fund Centers of Excellence: the Caribbean, the Philippines, Australia, and East Africa. Over the last 20 years, drastic (> 50 percent) loss of coral cover has occurred on the Yucatán Peninsula, even in pristine areas. Global surveys have revealed signifi cant levels of disease and disease outbreaks occurring not only in the Caribbean "hotspots," but also in sites throughout the Pacifi c and Indian Oceans. By monitoring coral disease, we will create a baseline and long-term data set that can be used to test specifi c hypotheses about how climate and anthropogenic drivers, such as decreasing water quality, threaten coral reef sustainability. One such hypothesis is that high-temperature anomalies drive outbreaks of disease by hindering the coral's ability to fi ght infection and by increasing the pathogens' virulence. We observed recurrent outbreaks following the warm summer months of two of the most damaging diseases in the Caribbean. In addition, we found that coral disease in the Great Barrier Reef correlated with warm temperature anomalies. In the Caribbean and Mediterranean Seas, virulence of known coral pathogens and the normal coral fl ora changed during high-temperature periods. Other stresses such as high nutrients and sedimentation may similarly alter the balance between the coral and its resident microbial fl ora.