Released February 4, 1999

Diseases of corals and other organisms are having significant, negative impacts on the structure and appearance of coral reefs. On some reefs, the effects of disease have been of a similar magnitude to more familiar disturbances, such as outbreaks of the crown-of-thorns starfish in the Indo-Pacific and coral bleaching associated with elevated sea temperatures. A new scientific awareness of diseases on coral reefs leads to a host of questions about the novelty of recently discovered syndromes, the importance of observed trends toward increasing infection rates, and the extent to which human activities are responsible. This statement, issued by the International Society for Reef Studies (ISRS), summarizes current knowledge on the subject. It was compiled by an ad hoc group of scientists in ISRS, composed of individuals who are directly or indirectly considering disease as part of their research programs.

Disease is a natural process that has been poorly studied in the oceans because of its ephemeral nature. Epidemics in animal populations, called epizootics, are a serious threat to the health of coral reefs worldwide. Recent observations of epizootics affecting sea urchins and scleractinian corals show that diseases on reefs can devastate their target populations and act as agents of rapid and dramatic community change. Marine pathologists and microbiologists are attempting to identify the causes of infection, but the pathogens responsible for most diseases affecting reef organisms remain elusive. These difficulties are complicating efforts by scientists and managers to study outbreaks and decide if control measures are warranted. It is becoming clear, however, that human activity is at least partially responsible for disease outbreaks on coral reefs over the past decade.

Corals are colonial invertebrates related to sea anemones. They lay down the limestone foundations of coral reefs, protecting tropical shorelines and providing habitat for the many fish and invertebrate species that feed a substantial proportion of the world's population. Like all living organisms, corals are prone to diseases of various sorts. The incidence of disease on coral reefs may be on the rise, but without historical, baseline data it is difficult to determine if the observed increase is real or simply a reflection of increased research activity. Recent scientific reviews list four to six confirmed coral diseases in the Caribbean region alone; other estimates, based only on observed symptoms, run as high as fifteen. Bacteria, fungi, and cyanobacteria ("blue-green algae") are known to cause diseases in corals. Sick and dying corals are cause for concern, because coral death slows the rate of reef construction. Reefs devastated by disease (or by other causes of coral mortality) may not be able to keep up with sea-level rise, which is naturally slow but may be accelerating due to global warming. And as reefs degrade, fish and other seafood resources decline as well.

Three coral diseases, "white-band," "black-band," and "plague," were first reported in the Caribbean in the 1970s. The first documented, regional-scale epizootic, however, affected the black-spined sea urchin, Diadema antillarum. In 1983-84, a disease carried by ocean currents killed more than 95 percent of the Diadema throughout the Caribbean. This epizootic clearly demonstrated that diseases can have major impacts on reef ecology. Before its mass mortality, Diadema was an important herbivore: it ate fast-growing fleshy algae (seaweeds), keeping space free for corals to survive and grow. After the urchins died, algae increased dramatically on many Caribbean reefs. They colonized corals that had been killed by hurricanes and by white-band disease.

Although the infective agent of white-band disease remains unknown, there is some evidence that it is bacterial. White-band disease infected populations of staghorn and elkhorn coral (Acropora cervicornis and Acropora palmata) throughout the Caribbean region in the 1980s and 1990s, inflicting enormous losses. Because Diadema also disappeared, seaweeds rapidly colonized the dead coral skeletons, and as a result large areas of Caribbean reefs have been covered with fleshy algae for over a decade. Paleontologists working in Belize recently uncovered evidence that the epizootic of white-band disease is without historical precedent: staghorn coral has not died off before on a regional basis in at least several thousand years. Staghorn and elkhorn corals are major constructors of reef framework, and their loss could slow the rate of reef growth in the Caribbean.

Many marine scientists suspect that human activities, such as pollution and changing patterns of land use, promoted the spread of white-band disease in Florida and the Caribbean. There is little evidence for a human connection, however, other than the historical novelty of the outbreak. Eutrophication, the enhanced input of nutrients by humans, may be an important source of stress to reef organisms. Eutrophication may compromise disease resistance, allowing opportunistic infections to take hold and new diseases to emerge. A fungal infection of sea fans appears to provide a link to human activity. The fungus, Aspergillus sydowii, has infected large populations of sea fans in the Florida Keys and throughout the Caribbean. Aspergillus sydowii is thought to be a land-based fungus that has invaded the marine environment via the sediment in terrestrial runoff.

Reliable information exists for two other diseases: black-band disease and "plague type II." Black-band disease, caused by a consortium of bacteria (including cyanobacteria) attacks and kills massive, head-forming corals. Black-band disease could pose a serious threat to populations of brain corals and star corals, which, like the Acropora species, are important components of reef framework in the Caribbean. Plague type II attacks head corals in Florida, but it has also been observed elsewhere in the Caribbean. In this case, rigorous microbiological work showed that the disease is caused by a single bacterium, a new species of Sphingomonas.

Other epizootics are killing corals and many other important species on reefs of the Pacific, Indian and Atlantic Oceans. Black-band disease and white-band disease have now been identified on reefs throughout the tropical Indo-Pacific, including the Red Sea, Mauritius, the Philippines, Papua New Guinea, and the Great Barrier Reef of Australia. In the Arabian Gulf, the newly discovered yellow-band disease is affecting up to 75 percent of the coral colonies in local populations. In addition, diseases of algae, sponges, and fish have been and continue to be identified.

Reefs throughout the world were stressed by unusually high sea temperatures in 1997-98, and the worldwide episode of coral bleaching that resulted may render corals more susceptible to disease. In the Mediterranean, bacterial infections are associated with bleaching, and disease outbreaks have been linked to predation by coral-eating snails in the Red Sea. The causal connections among bleaching, predation, and disease remain obscure, however.

The role of disease on coral reefs and possible interactions with environmental influences should be a research priority over the next several years. Despite the frustrating inability to identify pathogens in most cases, reef scientists have detected symptoms that could represent over a dozen new diseases. Diseases are now recorded as part of standardized reef monitoring programs throughout the world, including the Caribbean Coastal Marine Productivity (CARICOMP) Program, the worldwide Reef Check, the Atlantic and Gulf Reef Assessment (AGRA) Program, and a variety of government and private programs in Australia.

Because corals grow slowly, live for decades to centuries, and reproduce sporadically, today's epizootics will probably have consequences that reach far into the future. Multidisciplinary efforts, combining microbiology, coral physiology and pathology, ecological monitoring, and paleontology, will be necessary if we are to understand what is happening and devise management strategies in response. The International Society of Reef Studies endorses existing government and private funding of multidisciplinary programs to promote research on the changing nature of coral reefs. The Society recognizes the need for an increased level of support if the many threats to reefs worldwide are to be understood and mitigated.