What Causes Coral Diseases?

At first glance, a coral head may appear more like a colorful rock than a living organism. Upon closer observation, a diver will discover that a piece of coral is made of a hard limestone skeleton inhabited by hundreds, even thousands, of tiny animals called polyps. Coral diseases can spread through these tightly packed colonies of coral polyps in a matter of days.

Although coral diseases are not a new occurrence, they have increased in both incidence and severity over the last decades.

Pathogens such as bacteria and fungi cause a variety of coral diseases. These microorganisms have extremely short reproductive cycles, allowing them to evolve quickly. This makes the many coral diseases difficult to combat as they are constantly changing. For example, a coral disease named white plague disease type I was first seen in the 1970s. Type II was reported in 1995 and type III followed shortly after in 1999.

People tend to be most susceptible to illness when physically or mentally stressed, and the same is true of corals. Corals experience two main types of stress:

• Abiotic stress is caused by non-living factors, such as sedimentation, pollution, or an increase in seawater temperatures.

• Biotic stress is caused by living factors, such as bacteria, fungi, or viruses.

When both types of stress are present, one may intensify the other. This is becoming increasingly common. Coral has little chance of escaping disease infection while dealing with multiple, varied sources of stress.

Human-related pollutants are known to deteriorate water quality, which may contribute to disease proliferation. Our daily activities, like car driving, also add to the problem by causing ocean acidification. Humans may not be directly responsible for causing coral diseases, but their actions are in a large part responsible for the inability of corals to recover from disease infection.

Corals can be incredibly resilient; they have survived the evolution of the oceans for hundreds of thousands of years. Reefs are capable of recovering from water temperature changes and hurricanes. They were once able to recuperate from an incidence of disease infection, but this is no longer the case.

An increasing number of human activities threaten the survivability of reefs: overfishing, climate change, pollution, and habitat degradation (due to dynamite fishing and trawling, for example). These multiple stressors make it nearly impossible for coral to recover from a disease.

Coral death caused by diseases results in dramatic changes to the composition of a reef. When a coral polyp dies, the empty space may be occupied algae or other invertebrate creatures such as sponges. In only a few years what was once a thriving reef full of abstract colors and shapes, can be quickly overgrown by algae. Fish and other reef creatures can not survive in such an environment, and rapidly die off. Unless the situation changes, the future does not look bright for coral reefs. Keep reading

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