Coral tissue necrosis Stn/Rtn
By Justin Hester
Have you ever had a coral that lost its tissue and bleached right before your eyes? Have you ever noticed a coral losing tissue at its core and watch this spread out to its branches over a few days or weeks? What does this mean? What causes it to happen? Can it be prevented or even stopped?
Trial, observation, and I’m sure many errors have given me a glimpse into perhaps what is happening with some of these events in our reef tanks over the years. I will try to share some of my thoughts and observations with the following stipulation. I am not a scientist who conducts research in a controlled lab environment. I have done to my best ability to control the reef environments I have created and nurtured for 15 years. When testing a variable, I do my best to keep all other things the same.
What is Stn Rtn?
Ok, what the heck can cause our corals to smoke down to their white skeletons almost right before our eyes? It turns out that while many things can cause coral bleaching, it looks like a good chunk of bleaching that occurs in captive coral reef tanks is caused by bacteria. Specifically, as I looked into it, a specific strain of the bacteria in the vibrio family. Now ironically, this turns out to be the most prevalent bacteria family in the oceans. So if that is true, what makes them suddenly able to affect corals in this way?
Why does it randomly effect some corals and not others?
So, here comes the part where I have to rely on what I have read and what I have seen in my own tanks over the years. 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 our corals.
My speculation is that either their numbers suddenly explode in our reef tanks or that some other factor weakens certain corals to a tipping point where we see either Stn or Rtn occur. This could be an alkalinity spike, temperature or salinity swing, or probably anything that shocks our corals from their normal flat line stasis of water parameters. I also believe that this weakening is invisible for the most part to most reef aquarists. After all, the corals can’t shout and tell us what is wrong or how hey are feeling! We have to pay attention to the most subtle of clues like polyp extension, growth rates and coloration changes which almost all happen over a longer period than Stn/Rtn usually occurs in.
How does Stn/Rtn occur?
Typically Stn/Rtn will reveal itself not through a visual change in the corals health, but rather in the sudden tissue loss on a certain location of the coral. I have seen this start with the tips, but more often, I have seen it begin in the core of a coral. These bleached sport 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. This appears to be where the mass of the bacteria are living and consuming something as food from the coral tissue as they spread. This typically but not always will progress throughout the entire coral and can also spread from one coral to another if their tips are Touching each other. This has been observed in well over half the species of corals we’ve kept (154 species) at one time or another.
How do we stop or more importantly prevent Stn/Rtn?
The million dollar question, and unfortunately, there are a bunch of methods here I and some can be used in combination with others. If you see stn/Rtn occurring on one of your corals, here is what I would do.
-First confirm it’s Stn/Rtn and not just one coral stinging another
-Second, try as best you can to frag that coral at least .5 ” or more AHEAD of the spreading brown jelly mass of bacteria
-Next carefully remove the bleached part with the bacteria jelly on it. You don’t want those to drip onto another coral and start all over. Sometimes siphoning it out before removal is a good measure.
-Check all your water parameters and take notes of what you find, and what you may have had stress the corals in the previous few days/weeks
We have tried many dips to try and halt these outbreaks over the years with varying degrees of success. From Coral Rx, to Bayer, to blasting the jelly off outside the tank with a turkey baster. So far, this results have been spotty at best. The one product that has seemed to work is dipping the corals for 5-10 minutes in Melafix. We learned of this trick from a LFS that had tried it and seen success. Just make sure you wear gloves and eye protection for any dips as these products can ALL product an allergic reaction or worse in humans.
Temperature & salinity
In doing some internet reading on the life of vibrio and the studies that have been done on it, I found some interesting information about where its comfort zone lies. Apparently, it I isn’t very fond of temperatures lower than about 78 and doesn’t like salinity higher than 36. While a salinity above 36 is also not too favorable for our corals, a temperature below 78 is not a problem for most corals. In fact, most can go down to 70-72 and only show signs of slower growth rates.
So we decided to try this during a Stn/Rtn flare up we had a few months back. Ironically, all the water parameters we can test for are super flat lined since our calcium reactor etc are all on controllers ( more about this in a bit). We did keep our temperature up around 80 though so our coral growth was as fast as possible. So we lowered this over a week down to about 76 and performed a dip to kill the majority of the bacteria on the corals and give them a chance to fight back. Knock on wood, 4 months later, the 4 species of corals that we’re exhibiting Stn have all been healing and growing over their old area of tissue loss. Also interesting was that in all cases, the STN occurred on our mother colonies, but not once on the smaller frags that were sitting right next to and even touching these colonies! This was the part that really has me scratching my head.
The species effected for those that are curious, we’re:
-Some, but not all of our rainbow acropora colonies, not small frags
-Acropora Gemifera colony, but not
-Mother colony but not frags of our fruity pebbles acro
-Hollywood Stunner Echinopora Colony, but no smaller pieces
-Acropora Granulosa mother colony, but not frags
Many of the methods I have mentioned above have been introduced to me through you all in the reef community at large. If you choose to try any of the things I outlined, please well verse yourself in some good ole fashioned Googling before you try anything. Having multiple perspectives on treatments can be priceless in preventing overdoses and unintended results. Research allows for you to refine and formulate how these methods can most successfully be implemented in your particular reef tank.
With over 2000 gallons in our 2 facilities and 3000 frags, we have seen some interesting things over the years as our coral tanks mature and our corals fight with their neighbors to compete for light. STN/RTN has definitely been one of the most challenging things about this hobby since signs of its forthcoming are so subtle and elusive. We will continue to keep our temps down a little and nurture our corals for all of you out there looking to stock your tanks with some sustainably grown rarities. We will also continue to test other variables associated with our tank lighting to see if they have any effect on coral overall health.