Coral Diseases

How does the ocean's changing chemistry and temperature relate to coral disease?

Link to Climate Change

Though diseases have affected corals like any other animal, climate change has significantly increased disease persistence over the years. Epidemiological studies have shown the changes in the environment increases stress levels, which in turn make species more susceptible to disease. Rising temperatures provide pathogens the optimum conditions in which to reproduce and progress quickly. For corals, the link with rising temperatures is further enhanced during the summer when seasonal increases generally yield increases in disease. Likewise, acidfication causes bleaching, leaving coral colonies vulnerable.



Link between Bleaching and Coral Disease

Case Study Brandt et McManus 2009: Disease incidence is related to bleaching extent in reef-building corals

Brandt and Mcmanus tried to determine the links between disease incidence and bleaching events in reef building corals.They did the study in the Florida Keys regions of Cheeca Rocks and Coral Gardens.


Their methodology was to do a 2 year field study without manipulations by observing coral colonies within 16m^2 quadrants. Observations were made at random 5 locations every other week, weather permitting.


Results: The found positive correlation with bleaching extent and disease incidence. However, the correlation varied depending on the coral species and type of disease. Thus, further studies which include greater sample size, manipulations, and lab locations might be helpful.

Fig.20:  Coral observing quadrants (Lusebrink, 2009)

Fig.21: Coral observing quadrants (Farlow, 2007)

It is important to look at specific diseases now that we have established that increased bleaching and temperature (thus climate change) are linked with disease. Though there are more than thirty known coral diseases, there are a few summarized below to show the effects of climate change.

                                                                                        Black Band Disease

Fig. 22: Infected brain coral (Garrison, 2011)

Diseased coral show a dichotomy between “living skeleton” and exposed white skeleton with a black line serving as the demarcation.  Exposed white skeleton is left as the coral tissue is destroyed  when the pathogen moves up the colony. It is proposed that the pathogens of this disease infect the colony and create an anoxic micro-environment that mitigates coral tissue.

The time length needed for this disease to kill numerous colonies is just a few months. Colonies might not undergo total coral tissue loss but instead be partial. In addition, some colonies have been known to recover after infection disappears, but this is dependent on the extent of tissue damage.

The pathogen that is thought to cause this disease is a bacteria called P.Corallyticum. This pathogen exposes coral tissue to H2S and decreases oxygen levels, causing tissue death.

 Dark Spot Disease

Fig. 23: DS diseased coral (Bruckner, 2012)

This disease is characterized by discolored spots on coral colony. It was first reported in Colombia in the mid 1990s but now has become rampant in nations around the Caribbean. It is mostly found in the genera Stephanocoenia, Siderstria, and, Monostraea but has also been seen in others too. Tissue that is infected appears in patches of gray, black, brown, and purple. The patches can come in circular or irregular shapes and increase in size and radius as infection spreads across colony. The radius of the infection increases as pathogen moves upward. Polyps in infected areas tend to reduce in size and infected tissue usually caves in. The pathogen that causes this disease has not been clearly determined but many hypothesize it to be a fungus. 

Fig.24: WP infected coral (Sanchez, 2012) 

White Plague Disease
For this disease, infected colonies show contrast between infected areas, usually white, and healthy areas, retain natural color.  It was first observed in the 1970s off the coast of Florida. Nowadays, it is commonly found in other areas of the Caribbean. However, is has not been reported in any of the regions of the Indo-Pacific. Its disparity between these locations shows that there is something in the Caribbean that makes its pathogenesis effective. Unlike black band disease, there is no visible pathogen band in between. The pathogen causing this disease is known to be a bacteria called A.coralicida. Each bacterium is rod-shaped and a whole colony of the bacteria is formed by branching chains with their polar flagella.  

How Prevalent are Diseases?

Figure 25 allows us to understand just how prevalent diseases have gotten over the decades. Over the years, in the Indo-Pacific,  it is not just the amount of disease that has increased but also the types of disease. As we can see, between the 1998 and 2000 there was a dramatic increase in number of disease, that is only a two year span! Thus, it makes one think that though disease is natural consequence of life but that the sheer unabated prevalence of it is quite unatural and most likely linked to human induced climate change.

Fig.25: Disease in Indo-Pacific over the years (Bruckner, 2001)

Next, we will look at case studies revolving around how disease is affecting coral populations, how disease is affecting other organisms in the reef system, and whether there is room for recovery/adaptations.

Disease background information compiled from: (NOAA, 2012)