Coral Disease

Rapid Tissue Necrosis

Coral disease is of increasing concern to reef scientists, with disease incidence increasing, and new diseases discovered regularly. Although it is generally accepted that coral disease is now a significant cause of mortality in Caribbean corals, very little information is available for the Great Barrier Reef. Most basic aspects of coral disease are still not known, including causes, modes of infection/ transmission, environmental correlations, methods of prevention or management, and, most importantly, the long-term effect on reef communities.

Progressive tissue sloughing (tissue detaching from the coral skeleton) is described as a symptom for many coral disease conditions, including White Band, White Plague, Shut-Down Reaction, Rapid Tissue Necrosis, Stress-Related Necrosis, and White Syndrome. The only obvious distinctions between these described disease conditions are the rate of progression, species affected, and regional separation; however, it is not known how or if these diseases are related. At this time, no pathogens have been found to cause these conditions, but several studies support the hypothesis that external stress is involved in triggering the occurrence of tissue sloughing.

Rapid Tissue Necrosis

Rapid Tissue Necrosis (RTN) is a term used specifically for the occurrence of tissue sloughing in captive corals (i.e. laboratory and aquarium corals). Neither a cause nor the mechanisms involved in tissue loss has been determined, but two hypotheses have been proposed by Borneman (2002). In the first, a pathogenic agent causes RTN. In the second, RTN is solely a response to an external stress, such as physical damage, nutritional deficiencies, or high temperatures, that results in autolysis or a breakdown in the immune system functioning. A third hypothesis should also be considered, in which a potentially pathogenic agent normally present in coral mucus or tissue becomes infectious when a coral’s immune system is weakened by stress.

The Coral Reef Exhibit (CRE) at Reef HQ Aquarium in Townsville, Australia, is a 2.5-million litre living mesocosm designed to replicate an inshore reef (Eager and Peterson, 1988). Although the CRE has many normally-functioning components, the long-term survival of some hard corals is relatively low. Preliminary research indicates that the main cause of mortality in the CRE is RTN (primarily for acroporids and pocilloporids). This provides a unique opportunity to study RTN on a larger scale with the additional advantages of constant monitoring of environmental parameters and easy access to RTN-affected corals for sampling and surveys. Understanding tissue sloughing in captive CRE corals will contribute significantly toward understanding the cause of tissue sloughing for other coral diseases found on natural reefs.

To investigate the underlying cause of RTN and the mechanisms involved in triggering tissue sloughing (TS), a three-pronged approach is proposed, combining ecological, microbiological, and histological studies, and utilising both the CRE and natural inshore reefs. Specific objectives are to:

1) Follow coral health, survival, and the occurrence of RTN/TS in the CRE and/or natural reefs over time, and relate these factors to various environmental factors;

2) Understand the role of coral microbial communities in RTN/TS occurrence and determine whether a pathogen is involved;

3) Identify histological, intracellular, and/or other physical or physiological changes in corals prior to and during the progression of RTN/TS; and

4) Better understand the factors involved in triggering tissue sloughing in order to prevent or minimise the impact of RTN.