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Rapid Tissue Necrosis

By Various Authors. Posted to Reefkeepers emailing list, Friday the 24th to Tuesday the 26th of March 2000.

Gary Robinson

Well my purple tip Acropora colony basically dissolved in a little over 24 hours. I fragged a few branches off - the ones I put in the soft coral tank are still alive, the ones I put elsewhere in the hard coral tank are dead.

The RTN does not seem to be spreading to any other coral, at least not yet.

Interestingly, I lost a frag colony of this same coral about six weeks ago. I went to Pensacola for the day to go coral shopping and when i returned I saw this small colony was bone white. I had assumed it got stung by something or eaten by a hungry crab but now that I saw the mother colony dissolve I am wondering if RTN took out the daughter colony too, six weeks before. Seems odd that the same coral would get the same disease six weeks apart and none of the other corals are affected.

Has RTN ever been linked to stinging? My maze brain has had some hellaciously long sweepers lately and I am wondering if maybe he landed a punch on the Acropora and triggered the RTN that way.

Mike Kirda

Well my purple tip Acropora colony basically dissolved in a little over 24 hours. I fragged a few branches off - the ones I put in the soft coral tank are still alive, the ones I put elsewhere in the hard coral tank are dead.

Not that unusual. Did you do any sort of water change, run carbon or a Poly-filter in the meantime in the hard coral tank?

I am wondering if RTN took out the daughter colony too, six weeks before. Seems odd that the same coral would get the same disease six weeks apart and none of the other corals are affected.

Not really... Frags will be stressed as they have to shunt their energy into growth/repair. They will be prone to stress...

Has RTN ever been linked to stinging? My maze brain has had some hellaciously long sweepers lately and I am wondering if maybe he landed a punch on the Acropora and triggered the RTN that way.

RTN has been described in the wild as 'Shut-down reaction' by Antonius as early as the late-70's. So far, I have not seen a paper that describes this as a proper 'disease', per se. (i.e. fulfills Koch's postulates...) I have seen recently a reference to a recent paper that identifies a compound that binds the tissue to the calcium carbonate matrix- it makes me wonder if, in stressed corals, this compound does not break down and 'cause' what we call RTN to occur. Certainly, it is a stressor, and it is my belief that it is related to the secondary metabolites that are released as chemical warfare occurs in the tank. The only way to really reduce these levels are water changes, carbon usage and Poly-filters (and the like). I assume that protein skimmers are also marginally effective, however, on the compounds that are not removed by skimmers, it would seem that they would just accumulate... I wonder if ozone usage would not also help, but this is mere speculation- there are many, many more reasons to argue against its use... As far as a maze brain being the cause, I somehow don't think so. Typically the sweepers are much more concentrated in their sting and the area of damage is obvious. If the coral was already stressed, it could have been the final piece of straw though.

James Wiseman

Isn't it interesting that "RTN" took down just that _genotype_ of coral, but none of the others? Are any of your other corals affected?

I've seen this in my tanks too. I will have one coral, with multiple "daughter" clones in the tank, and when one goes, all the rest will go to, while none, or maybe one or two of the other corals are affected.

Wonder why...

Kevin McDonald

Ditto James. I've lost a few corals under these same kinds of circumstances. I had a Colt coral that was just stunning, grew like a weed. Something happened to the mother colony, started turning to mush, and all the props I had in the tank (about 4 at the time), we're gone in a week as well. I even removed some of them to my Sump.

Very strange indeed. All my other softies were just ducky.

Gary Robinson

None other affected as far as I can tell. There are several green Acropora colonies and fuzzy brown Acropora colonies which so far have been unaffected, as well as Porites , Seriatopora , Pocillopora ...

Anyone every try to "recolonize" the dead skeletons with new frags? How long after the wipeout must you wait before it is safe to "land" there and start a new colony?

Steve Wolfe

To me, that would indicate something in the water column - toxin, pathogen, etc.. I'm sorry to hear of your loss, I know (from experience) that it's not a very fun experience.

In this case, I think the clue is that the frags left in that tank died, while those in another tank survived. Unless the sweepers would have been able to reach the frags, then I would guess that isn't the culprit.

Gary Robinson

Not that unusual. Did you do any sort of water change, run carbon or a Poly-filter in the meantime in the hard coral tank?

I ran carbon because it was easy but I am really skeptical about the idea of a water change stopping RTN. I have never heard of anyone using a water change to stop it Not that I am an expert on RTN....

Not really... Frags will be stressed as they have to shunt their energy into growth/repair. They will be prone to stress...

This frag colony was several months old and had grown quite a bit.

secondary metabolites that are released as chemical warfare occurs in the tank. The only way to really reduce these levels are water changes, carbon usage and Poly-filters (and the like).

But in that case wouldn't it affect the whole tank? It sounds like you are saying it is not caused by bacterial infection, but by chemical warfare. In that case why have people reported getting it after introducing a new wild coral to their tank? Or stopped it by dipping the coral in antibiotics?

damage is obvious If the coral was already stressed, it could have been the final piece of straw though.

But does stress go hand in hand with growth? I would think a stressed Acropora would not be growing as well as this one was.

Steve Wolfe

Well, by putting the frags in the soft-coral tank, you basically did a 100% water change, and didn't that stop it?

Dipping a coral in antibiotics isn't guaranteed to eradicate all pathogens, just like taking a single pill of penicillin isn't going to cure you of every disease that you might have. In either case (biological or chemical), it's all about susceptibility - hence the remark about how the unknown cause affected only one *genotype* of coral.

Gary Robinson

The situation is a little misleading. I took the biggest and best frags and put them in the soft coral tank in the middle of a piece of slab rock where they are easy to see. I scattered some tiny ones in the hard coral system, trying to avoid getting them close to other SPSs. The tiny ones are now missing or bone white. It may or may not have anything to do with what tank water they are in. The hard coral tank has a lot of LPS in it with sweepers, as well as shrimps and crabs which could have knocked the frags around and into cracks, so the demise of the tiny fraglets may have nothing to do with the water in the hard coral tank.

Mike Kirda

I ran carbon because it was easy but I am really skeptical about the idea of a water change stopping RTN I have never heard of anyone using a water change to stop it. Not that I am an expert on RTN....

'RTN' is, I believe, not a bacterial infection, but something else. Eric Borneman put forth the idea that it was more an allergic reaction. No matter what the exact mechanism involved, I do believe it is due to some stressor in the tank. In this case, I believe that it is chemical in nature, most likely due to secondary metabolites.

If you have high ammonia in the tank, what would you do? Water change, right? Why is this so different from secondary metabolites? You want to lower the concentration of any compound that is elevated above NSW concentrations, you should replace it with new salt water.

I was in the middle of replying to James' earlier post when my computer decided to freeze and I lost the post. I will try to recreate it here...

Given that the stressor is in the water column and that you use an aquarium, you can be reasonably sure that the concentration of the compound is essentially equal in all parts of the aquarium. Therefore the stressor will act equally upon all corals in the system. I postulate that different coral species have different tolerance levels to secondary metabolites, making Acropora species, for example, more susceptible to them than Montipora species. Given that RTN reports are for Acroporid corals mostly, I think that this is a reasonable assumption...

Next, take a given Acropora species. Frag it, leaving the mother colony and daughter colonies in the same tank. Because of increased tissue mass, the mother colony will have more energy stores available in case of stress than the frag would. Bathe both of them in water that contains a stressor, the frag would succumb to the cumulative stress before the mother colony would. Reason fragging the coral would force it to shunt part of its metabolism into repair/growth, rather then energy storage. Also, as chemical warfare starts, the coral will slowly ramp up it's own defenses and offence- meaning it will also start producing secondary metabolites. In a coral already short on energy reserves, having to heal and work hard on producing chemicals, the coral will be stressed. This is why I believe that the fragment checked out before the mother colony. You also can keep in mind here that the frag/mother colony are genetically identical.

But in that case wouldn't it affect the whole tank? It sounds like you are saying it is not caused by bacterial infection, but by chemical warfare In that case why have people reported getting it after introducing a new wild coral to their tank? Or stopped it by dipping the coral in antibiotics?

Bacteria are endemic to the marine environment. Vibrio sp. have been implicated in only a very few cases. Recent reviews of coral disease literature have shown that Vibrio is usually not the causative factor, but merely an opportunistic feeder, in most coral 'diseases'.

Now, if RTN was caused completely by bacteria, then application of an strong antibiotic should eliminate it, right? Why then does it not work as often as it works? Success rates of dipping corals into an antibiotic like CAM or Vancomycin are reported to be quite low, even though these antibiotics are quite capable of killing most, if not all, Vibrio sp. you are likely to encounter.

I am going to use another argument here- we'll see how people react to it... You have fish in a tank and all are ok with each other. You add a new fish and suddenly aggression begins. In a similar fashion, corals seem to be able to sense what other corals are around them. And if something new is added, their 'aggression' will take the form of secondary metabolite production. If the levels are at a stressed, but sub-lethal level, the addition of another species of coral *could* tip the scales to increase the production to lethal levels. We know that macro algae have the ability to kill corals, and we know that some corals have the ability to kill other corals merely by the secretion of chemicals, so I don't think that this is such a stretch.

But does stress go hand in hand with growth? I would think a stressed Acropora would not be growing as well as this one was.

Growth would be stressful only if the energy reserves were decreasing. Typically, growth is used as a fairly good proxy indicator of health.

In your case, if you had known that the fragment had died of RTN, but there were still no apparent signs on the mother colony, I would have recommended that you perform a series of water changes and start using carbon aggressively. This is probably the best method of reducing secondary metabolite levels, and would lower the stressor levels. I have 30 gallons of salt water mixed up at all times, just for such emergencies, plus carbon and a Polyfilter or two. Luckily, I have not yet needed it. And nowhere do I have any antibiotics.

Gary Robinson

Now, if RTN was caused completely by bacteria, then application of an strong antibiotic should eliminate it, right? Why then does it not work as often as it works?

Well that assumes it does not in fact often work. I thought Craig Bingman had shown that it did work, but that the right antibiotic was hazardous and hard for people like me to get a hold of.

corals are around them. And if something new is added, their 'aggression' will take the form of secondary metabolite production. If the levels are at a stressed, but sub-lethal level, the addition of another species of coral *could* tip the scales to increase the production to lethal levels.

It is an interesting concept. As a counterpoint I encountered some "lore" in #reefs recently to the effect that SPS corals shipped from a certain dealer should be dipped in strong iodine concentrations before adding them to your tank to reduce the risk of introducing RTN into the tank. Someone who normally did that, failed to do it on one occasion and sure enough got RTN almost immediately. Coincidence? Quite possibly, but I thought it was interesting that groups of people out there are doing this to reduce the incidence of RTN, apparently with some degree of success.

Growth would be stressful only if the energy reserves were decreasing. Typically, growth is used as a fairly good proxy indicator of health.

Yes, my point was this coral was growing fine till the day it died, which indicates it was not stressed prior to the day of death.

In your case, if you had known that the fragment had died of RTN, but there were still no apparent signs on the mother colony, I would have recommended that you perform a series of water changes and start using carbon aggressively.

Well, it's funny you say that, because shortly after the daughter colony died, I started the anti-flatworm campaign and did the Marin Oomed dosing which was followed by a 60% water change, running "flow through" carbon, followed by a series of daily water changes and more carbon. Maybe this saved the mother colony for awhile and I didn't even realize it, but if it did, it wasn't for long.

I will note that prior to February of this year I ran carbon 24/7. Recently I have only been running it about once every two weeks or so. The daughter colony died before I stopped the constant carbon running. The mother colony died after I switched to the occasional carbon regime.

Has anyone ever showed a link between RTN and carbon/lack of carbon or water changes/lack of water changes?

Has anyone ever been able to induce RTN in the lab?

Craig Bingman

'RTN' is, I believe, not a bacterial infection, but something else. Eric Borneman put forth the idea that it was more an allergic reaction.

Eric has a largely unsubstantiated hypothesis about what people call RTN. In a nutshell, the idea is that the coral autolyzes (degrades itself) in response to some stress.

To say that it has been shown that there is no bacterial involvement in this is completely incorrect. An argument as strong or stronger could be made for a bacterial origin of this problem, that may also have some autolytic component. The two hypotheses are *not* mutually incompatible, at least in their more general form.

My personal involvement in this ended when I became aware that people had secured grant money to study the problem. I felt there was no way that I could be competitive in that area if people had salaries and some money to swing at it. There was another component as well. I'm not qualified to ID bacteria. Bacterial identification once was a matter of running a number of biochemical and histological tests. Recently it has become clear that lateral exchange of genetic information across what were formerly called species of bacteria is rampant. Fundamentally, I don't know what the species concept means when applied to bacteria anymore, and I'm not alone in that confusion. The more authoritative bacterial IDs are now made on the basis of PCR'ing up marker genes (usually ribosomal RNA genes) and identifying the bacteria based on that. I'm not set up to do that, and my former collaborator wasn't set up to do it either, so this was another reason for my losing interest in the project. Bacterial taxonomy is in rapid flux, and it is a full-time job to keep up with it. I have another full-time job.

A number of the arguments against bacterial involvement don't hold water very well. Eric tried culturing up bacteria from a number of sick corals. He didn't wind up isolating the same bacteria from all the colonies. So what? A visitor from another planet would wind up mighty disappointed if they were trying to understand all of human disease by attempting to culture a single causative organism from all sick humans.

A number of other arguments that have been made against bacterial involvement are equally sloppy. Eric and his coworker found that not all diseased colonies responded to chloramphenicol treatment. Some did, but not all of them. That argument was used against the bacterial involvement hypothesis. Actually, that is exactly the pattern one would expect if there was bacterial involvement. It will work ins some cases, and not in others. They suggested that CMP was decreasing host protein synthesis, and that was reducing the severity of autolysis. If this were the case, it would be expected that either CMP would work on a given coral species (certainly on all clones of a coral) or not.

I could comment further on this, but really, there isn't much point.

Mike Kirda

To say that it has been shown that there is no bacterial involvement in this is completely incorrect. An argument as strong or stronger could be made for a bacterial origin of this problem, that may also have some autolytic component. The two hypotheses are *not* mutually incompatible, at least in their more general form.

A recent review of coral 'diseases' authored by Laurie L. Richardson (TREE vol.13,no.11 pg.438-443) lists 13 separate 'diseases' documented at least partly in the scientific literature. Of these, only four have a known agent, one a fungii, the other three bacterial or cyanobacterial.

The one 'disease' that most resembles 'RTN' is 'Shut-down reaction', and "no microbiological studies were conducted." So, we are kind of at a loss for what the cause is.

I could comment further on this, but really, there isn't much point.

Not sure why you feel this way. You have my attention. I would ask you then how to explain why clones (frags) so infected with bacteria would then not die when moved to another separate tank. This is the part where the entirely bacterial argument breaks down for me. If the cause was entirely bacterial AND the frag was infected, it would not matter if it was moved to a new tank... Or am I missing something?

Craig Bingman

Not sure why you feel this way. You have my attention.

Yes, well, I get the feeling oftentimes that "the hobby" makes up its mind about a given topic, and once that has happened, there is tremendous inertia... which could be overcome I suppose, if I were willing to invest a lot of effort in it. Most of the time I'm not, though.

I would ask you then how to explain why clones (frags) so infected with bacteria would then not die when moved to another separate tank.

I'm not sure what you mean by "so infected." I think that there is a latent hypothesis here that hasn't been addressed the spatial distribution of a putative pathogen in an infected coral. It might be all over the colony, or it might be mainly spatially restricted. If I have an infection in my finger, then the infection starts out in my finger, and if you whacked it off, the infection would be done. Of course I'm a lot different than a coral colony, so if I have an infection in my right hand, you can't just whack off fingers on my left hand and start new little clones of Craig. ;-) Which is probably a very good thing, by the way, but I digress. ;-)

If the infection goes long enough, it can become systemic, spread by my circulatory system to my entire body. At that point, you can't amputate anything and save me.

This is the part where the entirely bacterial argument breaks down for me. If the cause was entirely bacterial AND the frag was infected, it would not matter if it was moved to a new tank... Or am I missing something?

So, another 'sharpening' statement has been made here, and that is *entirely* bacterial. I don't think I've ever said it was entirely bacterial. There are compounding factors. Stress plays a role in how susceptible any organism is to diseases.

As far as fulfilling Koch's postulates goes, that can be trickier than people might imagine as well. There are some bacteria that are so virulent that a person will almost always get sick if they take in a few of the bacteria by a normal route, like ingestion. There are other bacteria where you really have to ingest large numbers of them to blow past the host immune system in a large number of cases. There are some bacteria that won't make me sick no matter how many of them I eat, but might make me sick as hell if I inhale just a few of them, or if a few of them are injected subcutaneously. So it can be quite complicated to set up a proper experiment, and one might have to play with various ways of delivering the bacteria to the corals to make them sick. This is why I often don't know what to make of null results. Perhaps one would need to abrade the corals. Perhaps one needs to stress them otherwise. It gets even more complicated if these bacteria are frequently present on asymptomatic corals. Then if you abrade or stress a control group, corals will wind up getting sick and you might come to the conclusion that it is the stress itself that is causing the problem, and that there is no bacterial involvement. That is exactly the case for some bacteria that live on human skin, by the way. Most of the time the host coexists with them with no problem.

Mike Kirda

So, another 'sharpening' statement has been made here, and that is *entirely* bacterial. I don't think I've ever said it was entirely bacterial. There are compounding factors. Stress plays a role in how susceptible any organism is to diseases.

Sorry, when I wrote the above, it was late and I was really tired. Not meant to 'sharpen' or even be 'pointy' LOL. Reading it now, I see it is in dire need of clarification...

RTN, as I have seen it, consists of a rapidly moving band that seems to start from the base or center of a colony and moves outward. Rapidly. Like an inch or more in an hour. Just in front of the band, the coral seems to be blissfully unaware of what is in store, polyps extended fully, etc Behind it, white skeleton, formerly covered in tissue. At the band, the tissue seems to just detach and dissolve.

Now, the only disease that comes to mind in human terms is the so-called 'flesh-eating' disease, which is bacterial in origin. Nasty stuff...

Given that bacteria are present in the water column and on the coral's surface, it seems a bit strange that RTN always seems to start from the bottom/center and move outwards to the branch tips. The likely place of infection should be at the tips, which are easily abraded. So why the base or center as the point of infection? (I ask this mainly as a rhetorical question...)

Possible explanations for RTN would be that it is

  1. Entirely bacterial in origin
  2. a stress-induced reaction
  3. some combination of the two and
  4. something else not yet discovered.

I am not entirely familiar with the protocols that Eric used, but will assume that they are standard and that he would use a standard petri dish with an appropriate culture medium, that he would sample a coral exhibiting RTN, place it in the media, then wait to see what grew. I have no access to his data, but he has characterized it pretty simply- they found no bacteria that was consistent from one RTN sample to another. The ones found were consistent with ones you would find if you took a scraping of a normal, healthy coral. I don't believe that this is a mischaracterization- if there is any doubt at all, I will e-mail him and ask him to clarify.

This methodology is crude and inconclusive. If there was an anaerobic bacteria responsible, the culture medium might have been inappropriate. It is a good first study, but I would not want to bet my academic career on it- there is a lot more that needs to be done before they can entirely rule out a bacterial cause.

The other argument used was that antibiotics were not always effective. Well, let's see... I would not want to assume that the antibiotics were administered in exactly the same way each time. Also, I can not rule out that the water used was not different at different times. (If an RTN coral was moved to some water that had a different source than where it came from, this also changes another variable... However, it could account for why antibiotics worked sometimes but not other times...)

So it can be quite complicated to set up a proper experiment, and one might have to play with various ways of delivering the bacteria to the corals to make them sick. This is why I often don't know what to make of null results.

Join the club...

Getting back to my original point, I am curious what you believe is the difference, then, in the two tanks, one of which the frags survive in, the other of which they succumb in. Corals do have a circulation system, albeit not nearly as complex as ours. If we frag ahead of the RTN band, I do not believe that the hypothesized bacteria would be inside the tissue of the frag. I also find it difficult to believe that two tanks in the same house would have entirely different populations of bacteria in them- I think it safe to assume that the hypothesized bacteria would be in both of them. (Again, asked as a somewhat rhetorical question...) Why, then, would frags survive in the second tank, but not in the first?

Have to say, Craig, that this is an interesting exchange for me. My mind is far from being made up, even though I lean toward the stress-induced reaction hypothesis as it just makes more 'sense' to me with my current level of understanding coral biology.

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