Coral Tissue Necrosis – The Mystery of RTN and STN

Slow Tissue Necrosis (STN) and Rapid Tissue Necrosis (RTN) are two problems no reefer wants to experience with their coral. STN and RTN are generalized terms based on the clear visual symptom of loss of tissue in live coral. Aquarists can see a distinct delineation of living tissue and bare skeleton as the disease progresses. The loss of coral tissue can be quite fast or drag on for weeks where it may be partially completely stripped of living tissue, the latter case resulting in death. Leading us to the obvious questions: What causes it? How to stop it? and Can it be prevented?

Rapid tissue necrosis affecting a coral frag
Tissue necrosis in a coral frag – Photo provided by Coral Rx

These questions have been discussed and debated for over 20 years. Back in 2002 at the first Coral Disease and Health Consortium (CDHC) it was determined that there needed to be a set of conditions and definitions to identify the various coral diseases. Standardized descriptions are necessary to zero-in on the cause of tissue loss. Was the damage really caused by a parrotfish or was it a bacterial infection? Ultimately the CDHC created a flow chart of symptoms for researchers to use when investigating coral diseases in the field.  It suggests things like fire worms, snail predation and various “band” diseases. The chart is a starting point for in-depth laboratory analysis. Unfortunately, this tool is of little help to an aquarist with a tank suffering from coral tissue loss. For many reef hobbyists the only symptom observed is tissue loss with no obvious cause. The coral may also display a brown jelly-like mass on the decaying tissue (Brown Jelly Syndrome). If the disease progresses slowly, we call it STN. If quickly, we name it RTN.

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Brown jelly surrounds a surviving polyp that suffered from tissue die off.
Single surviving scroll coral polyp after an RTN episode surrounded by brown jelly.

Diagnosing STN and RTN?

It’s important to remember that not all tissue necrosis is related to the “mystery disease” of STN and RTN. Coral tissue can be damaged from poor handling, stinging from a nearby coral and other known causes like parasitic copepods and flatworms. For this discussion let’s assume your water quality is perfect. All your corals have been thriving for a long time. One day you notice some odd-looking tissue on a coral. You observe progressive tissue loss with a clear distinction between the healthy tissue and the white skeleton. For no apparent reason the tissue continues to decay, often at a very fast pace. The disease may kill a section or the entire colony. Search tissue necrosis online and you’ll read the horror stories and view the images – welcome to STN – RTN.

What causes STN and RTN?

There are many hobbyist theories about what causes this disease. You’ll also find research papers about “coral disease” on the reefs. It is easy to get lost in all the information and make generalizations that have little application for home reef aquarists. Here’s a summary of the latest information:

  • No one knows exactly what the root cause is for what reef aquarists describe as RTN and STN.
  • Researchers have found that when ocean temperatures rise above normal, bacterial strains of Vibrio invade coral tissue and cause damage. However, this does not explain the appearance of tissue necrosis in healthy reef tanks with stable water temperatures.
  • There are hobbyist and research reports suggesting the ciliates Helicostoma and Philaster are responsible for tissue necrosis and Brown Jelly Disease in aquariums. Researchers compared captive diseased corals with healthy specimens from the same tanks. They observed Philaster ciliates on corals suffering from BJD and RTN. The ciliates were seen eating tissue and ingesting symbiotic algae. But they could prove the ciliates were the cause of the disease. It is unknown if the ciliates play a direct role in the infection or simply like to gather and consume damaged coral tissue.
  • The same aquarium study took a look at the makeup of bacteria growing on the sick corals. Normally one pathogenic bacteria type would be found dominating the diseased tissue and point to the main pathogen. While diseased corals had a higher population of bacteria, no one type was found in the wounded tissue.
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While there is a link between tissue necrosis and the appearance of certain microbes at the wound site, it is unknown which organism is responsible for the initial tissue damage. There is also the chance that all of these microbes simply take advantage of a weak coral and consume tissue and symbiotic algae. More research is necessary to answer these questions.

Can it be prevented or cured?

The obvious answer is to do all you can to maintain water quality and keep the aquatic environment stable. We like definitive answers to problems but unfortunately with STN and RTN there are no set of actions that guarantee your corals will never suffer from tissue necrosis.

Hobbyists and coral farms have tried many things to cure infected corals and stop the spread in the aquarium. You’ll find many suggestions online. Here are two that appear to consistently provide a chance of recovery.

Fragging and dipping seem to be the most successful way to treat tissue necrosis in coral
Fragging and dipping seem to be the most successful way to treat tissue necrosis in coral
  • Attempt to save the coral by fragging. Cut the coral at least ½ inch ahead of the creeping necrosis or jelly. Discard the bleached section. If you can, siphon off dead tissue before disturbing the coral.
  • There are several dips and treatments you can try which are widely used in the industry to treat corals and help prevent disease problems. Check out how to dip, here.

Final thoughts

The study of coral diseases in nature and aquariums is taken seriously by researchers throughout the world. Hopefully in time we’ll find out what causes coral tissue loss and how to prevent it. Until then, keep working to provide your corals with the best possible conditions. Do all you can to eliminate stressors like high water temperature and poor water quality. If there are any ground-breaking developments, we’ll be sure to post an update.

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