Recently as more and more aquarists are growing stony corals, particularly the small polyped stonies (SPS) there have been reports of rapidly progressing diseases that cause the coral tissue to disintegrate. Some hobbyists observing this for the first time confuse the condition with "bleaching," which is just a loss of pigment or photosynthetic zooxanthellae, a less serious ailment. The rapid loss of tissue leaving behind white skeleton is not bleaching, it is death of the coral due to a pathogen. It is contagious and can sweep through and kill the corals in an aquarium in hours or days. Hobbyists discussing this condition in forums via the Internet have been calling it rapid tissue necrosis or RTN.
The clear brown gelatinous blobs on this Acropora are destroyed coral tissue teaming with protozoans that eat zooxanthellae. The principal pathogen in this case has been identified tentatively as Heli-costoma nonatum (see volume one page 328). J. Sprung
In nature a condition of this sort called "white band disease" has been known for many years. More recently as scientists have become more aware of coral diseases, the white band condition has been subdivided into several distinct conditions. Although it is not yet proven, it seems apparent that the diseases affecting captive corals are the same as those in the wild.
After the "brown jelly" is siphoned off, the exposed bare skeleton is flushed with a strong jet of water or brushed with a toothbrush, and then underwater epoxy can be applied as a bandage of sorts. The epoxy should be applied partly onto the living tissue to be sure to cover the exposed area. J. Sprung
One type of tissue necrosis that has been common in reef aquaria for years is caused by a protozoan, Helicostoma. It causes "brown jelly" infections in which the tissue is destroyed along a rapidly progressing front of gelatinous mass teeming with the protozoans, which consume zooxanthellae. It can affect small polyp stony corals, other stony corals, soft corals, zoanthids and corallimor-pharia. It is a common cause of rapid death in Euphyllia species.
Other forms of tissue necrosis have more mysterious causes. It is a general opinion (ours included) that most are caused by bacterial infections, but little is known about this. Dr. Craig Bingman (pers. comm.) recently halted the progress of a particularly virulent tissue necrosis event in one of his aquariums by administering a dose of Chloramphenicol to the water. This antibiotic, while difficult to obtain because of its recognized hazards to human health, is quite effective in the treatment of pathogenic bacteria that live in saltwater.
One type of RTN essentially is identical to the white band disease in nature. White skeleton is exposed along a front of rapidly disappearing coral tissue. In front of the skeleton the tissue appears healthy, behind it the tissue is gone. Another type affects whole branches at once, and causes the tissue to drip off the skeleton. Often the tissue appears perforated before it falls off. This latter condition usually spreads very rapidly, and may wipe out all Acropora in the aquarium in a short period.
In these closeup photos one can see that the loose tissue becomes quite like a network strung over the bare skeleton. J. Sprung
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