RTN and STN possible causes and Treatments

[coralfreakcr]

Everybody in these hobby had suffered of RTN (rapid tissue necrosis) or STN (slow tissue necrosis) in their priceless Acroporas or Montiporas corals but its hard to find information about how to deal with this problem, all the information its always spread in the web and in short articles, so I take some time to write this article about the topic with possible causes and treatments. I hope be a help full tool when you deal with RTN and STN.

 

What is RTN and STN?

 

RTN: Rapid tissue necrosis is a rapid degradation of the coral tissue on a certain location of the coral in just a matter of hours or days, also known as Shut Down Reaction. The symptoms generally involve sloughing off of large chunks of the coral tissue while the coral still shows healthy polyp extension in other areas of the colony. This disease could start in the tips, but more often it begins in the core or base of a coral. These bleached spot will also usually have a brown or black jelly like ring around the outside edge of the bleached area that expands as the bacteria wins the battle over your weakened coral and spreads across the tissue. RTN can reduce a beautiful, small-polyped stony coral (SPS) dominated reef tank to a glass box full of bleached, tissue-denuded coral skeletons.

 

STN: Slow Tissue necrosis is sudden tissue loss on a certain location of the coral in a matter of weeks or months. Its begin in the core or in the base of a coral.

 

What causes this coral disease?

 

It is thought to be caused by different strains of Vibrio sp bacteria. Coral pathogens are often known to be found in healthy tissue and only become pathogenic when environmental conditions and host pathology changes. Stressors such as increased temperature either modify the structure of the coral microbial symbiotic community or trigger the production of virulence factors. Temperature stress can increase chances of coral disease development in several ways by creating stress in the coral holobiont (zooxanthellas) and decreasing its resistance to infection, increasing growth and virulence of opportunistic coral pathogens, and decreasing the production of antimicrobials by symbiotic bacteria in the coral mucus, thereby facilitating the growth of opportunistic and potentially pathogenic bacteria. It has been shown that corals harbor species-specific bacterial communities, and the composition of the microbial communities might also play a role in susceptibility to disease. Given these vibrio and other bacteria are always present in the oceans and in our aquariums, there must be some sort of trigger or triggers that enables them to exploit in our corals. The possible triggers are:

- Alkalinity spike

- Temperature spike

- Salinity spike

- Low dissolved oxygen

- Poor water quality related with phosphate levels up to 5 ppm

- Change in water flow

- Additions of sand

- Changes in brand of salt

- Bad test kits giving faulty results

- Levels of minor elements such as Iodine, Potassium, Strontium

- Light intensity

- Metals in contact with water such as copper wires from pump windings, decaying magnets

- Changes in water flow

- Addition of new corals

- Stress caused by heavy fragging

- Pesticides

- Airborne Contaminants

 

But while the causes will continue to be debated, certain procedures can minimize the probability of this occurrence, and can also slow or stop its progression.

 

Procedures to minimize the probability of this occur:

 

When introducing a new colony to a tank, it is best to pre-dip the coral for 10-15 minutes in a mixture of 5-10 drops of Lugol's iodine solution per liter of tank water, or to simply follow the directions on commercially available iodine-based coral dips. The dip should be performed in an isolated bath with an airstone to provide some water motion. The coral can then be given a quick rinse with tank water to minimize introduction of the dip chemicals into the tank. Ideally, newly acquired specimens should also be quarantined in a separate and isolated system until they are well adapted to tank conditions. Ideally corals should be left in quarantine for 3 months. During the quarantine time you should view the coral on a daily basis and watch for any anomalies.

 

Procedures to slow or stop its progression:

 

There is no possible way to implement all the varied advice that worked for someone. Taking multiple actions just confounds the problem and will never expose the impact of any single action or get us close to figuring out what might actually work. In this situation it is best to be open to ALL advice, but BE SKEPTICAL of all advice. Here is a short list of some of the advice that people recommend:

 

1. Check all your water parameters at least twice and take notes of what you find, and what you may have had stress the corals in the previous few days/weeks (Double-check any unusual results from a test kit. Some values that I've seen quoted by hobbyists are, quite frankly, chemically impossible in a reef tank. Look at the critters in the tank, and if they look okay, chances are that nothing is very far off. Check expiry dates on your test kit's reagents and make sure they are all fresh; often, odd readings can be traced to old reagents. Perform any test at least three times and average the results, this will reduce the impact of imprecise technique.)

2. Dip corals (In Revive, Iodine, Coral RX, etc)

3. Frag the corals at least 5 inches of it

4. Remove the bleached part with the bacteria jelly on it. You dont want those to drip onto another coral and start all over. Sometimes siphoning it out before removal is a good measure.

5. Water Change: Perform a 20% water change every day

6. Drop the temperature from 75 to 70F

7. Increase flow

8. Lower light intensity

9. Treat entire tank with antibiotic (Doxycycline or Chloramphenicol)

 

My recommendation is to take all the advice, but evaluate it with respect to its potential to impact the desired result. In addition, evaluate it with respect to feasibility, rationale, risk, applicability to your situation, and cost. The corals we grow are capable of nearly infinite vegetative propagation. Take a fragment of EVERY "irreplaceable" coral and give it to an experienced reefkeeping friend. He'll likely share some of his fragments, adding to each other's coral diversity in case something goes wrong. The same goes for those who have multiple tanks. If space is available, put at least one tiny fragment of every "irreplaceable" coral into a completely isolated system, preferably on a different electrical circuit. When the next wave of nasty, chemical-resistant, Acropora eating "monsters from the deep" sweep through the hobby, this isolated tank will contain the new seeds to re-populate the system.