An Introduction to Diseases of
Coral Reef Organisms
The Offline Version

The impetus for this page began with an article in Geotimes(Vol. 41, No. 4, April 1996) that discussed diseases that affect corals. These diseases have in common the ability to destroy the plants and animals that build the structural framework known as a coral reef.

A shallow reef in good condition. Photo by E.C. Peters

Some people think of a coral reef as a single entity. However, a coral reef is actually much more complex. What most people identify as a coral reef is actually a bioherm - a solid structure primarily composed of calcium carbonate that is the product of a relatively thin layer of living cells. Over thousands of years, coralline algae and scleractinian corals have built the framework that we recognize as a coral reef and that provide food and shelter for thousands of species of algae, invertebrates, fishes, and sea turtles.

Destruction of the reef framework is occurring in many places, with declines in the numbers of plants and animals present on the reefs compared to only a few years ago. Some of these changes are related to water quality. The organisms that build coral reefs generally need: clear water, warm temperatures, constant salinities, and adequate light levels. Increases in human populations in coastal regions near reefs mean increased construction and industrialization, resulting in siltation and turbidity from runoff and dredging activities, high levels of nutrients from sewage discharges, and the release of potentially toxic contaminants to nearshore waters.

A reef damaged by turbidity and siltation. Photo by E.C. Peters

Disease is defined as:

Any impairment of an organism's vital functions or systems, including interruption, cessation, proliferation, or other malfunction.

Although many people think that diseases are caused only by microorganisms, diseases can have many causes.

Infectious diseases are caused by pathogens

Noninfectious diseases can be caused by:

• Poor nutrition (lack of sufficient or the right kind of proteins, carbohydrates, fats, vitamins, minerals, and water). For example, in humans, scurvy results from a lack of vitamin C.

• Genetic mutations that cause abnormalities in cell metabolism or structure. For example, sickle cell anemia results from a gene mutation which produces a form of hemoglobin that causes red blood cells to develop elongated shapes when blood oxygen levels are reduced.

• Genetic mutations that alter the cell growth cycle and result in proliferations of abnormal cells, for example, cancer.

• Exposure to conditions outside the range normally tolerated by an organism that damages its body functions. For example, prolonged exposure to higher than normal temperatures can disrupt the structures of key enzymes and result in the malfunction of metabolic processes, potentially leading to death of the individual.

Most diseases are the result of more than one factor and determining the specific cause of a disease can be difficult. Exposure to adverse environmental conditions or changes in age or reproductive status can alter an organism's defense mechanisms and immune system, decreasing its ability to resist invasion by pathogens. Changes in environmental conditions can help pathogens multiply more rapidly and increase the possibility that they can infect a susceptible host. Introductions of species into habitats where they do not normally live can bring microorganisms into contact with other hosts, which might be more susceptible to infection and disease produced by the microorganism than the host that it normally infects.

Disease can affect not only an individual organism, but also the community in which it lives. Diseases can alter the reproductive potential of a population, alter interactions among populations, and cause mortalities, leading to changes in ecosystem composition, structure, processes, function. Mass mortalities can lead to the extinction of species.

By the 1970s, scientists realized that coral reef organisms were susceptible to damage from natural and human-induced physical and chemical changes in water conditions. Sediment could abrade or smother sedentary plants and animals, and changes in temperature and salinity or increased exposure to ultraviolet radiation, nutrients, and toxic chemicals could make them sick and cause them to die. In other cases, coral reef organisms appeared to be sick or dying as the result of interactions with microorganisms.

What's next?

  At this point, you can go to the disease key

Or, look for information on a specific disease.