RSS Survival of the Fittest of Aquarium Corals

MASA Admin

8 May 2007
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To paraphrase what Charles Darwin wrote approximately 150 years ago, survival of the fittest is the process of natural selection that allows living things to have the greatest likelihood for producing successful copies of itself in the future.  So I’m guessing you are wondering right now, what this has to do with successful reefkeeping.

One of the benefits of doing this for a while is that over time you get to sit down with a lot of different individuals, share adult beverages and talk about just everything hobby related and otherwise. I have been fortunate in this regard in that I have had many such discussions. One of which included Richard Ross, Sanjay Joshi, Joe Yaiullo and Mitch Carl. The discussion focused on are we, for lack of a better term, “selectively breeding” the best corals for our tanks. That is, is the act of keeping a reef tank successfully artificially selecting for the corals most likely to do well in that tank?

A tank “selectively” grown by placing lots of frags in the tank and allowing it to grow with some obviously thriving and some failing

Needless to say this was a lively discussion, and I have been thinking about it for quite some time ever since. We have known for as long as we have been keeping corals that there are some corals that simply do not do well in some tanks and some that fail to thrive, while others thrive spectacularly. But up until this discussion and several later discussions with others I had never really looked at it in the context of selection. 

I am not talking about rare corals or corals that were known to be difficult to keep, but actually corals that did well in my friend’s tanks, but in my tanks nothing.  Also with how much experimenting I do in some of my tanks as well as the opportunities that I have had to see other’s tanks why some corals thrive has always been on my mind. I have begun to try to look at this concept and understand it better with the hope that it would make me a better hobbyist.

The coral that got me to think of this were the common stony corals of the genus Hydnophora. While there are several species of this coral in the genus, in my case it doesn’t really matter as they all are skeletons soon after I have placed them in any of my sps dominated tanks. I have tried wild colonies, aquacultured colonies and frags from friend’s tanks where their frags had grown into dinner plate sized colonies. It did not matter and the process was the same for each. 

Hydnophora ‘Horn Coral’ is a hardy and very easy to keep SPS coral

Within a week of placing them in one of my tanks, and this occurred in many different tanks over a decade, none of them has ever survived for more than a week. And I should note, that these are tanks where just about every other type of sps and lps corals have grown and thrived. I have done all the testing, placed them in different areas under different lights and with different flow and it has not mattered. So for whatever reason the conditions in my tanks select out Hydnophora.

These failures and the above noted discussion caused me to think and ask questions of my fellow hobbyists. This has resulted in my asking a question that we generally do not ask each other: what has died off, bleached or failed to thrive in your tank? We love to deal with our many successes, but rarely do we ever talk about what has failed or what we killed off and even less about what hasn’t really lived up to expectations and with that why this has occurred.  I would love to be able to write that I have never killed a coral or lost a fish, but that would be lying. More importantly I have come to realize that we do not only learn from our successes, we learn from our failures as well, and possibly more so. 

As a result of these questions about failures to my fellow hobbyists I have been able to learn a few things that I think may be helpful to all of us. I know there will be some folks out there that will disagree with me and what may lead to problems, and I appreciate your input, but these results were what we found. So I am only describing my findings if I had four or more people I respect agree with what I found.

After salinity and temperature, testing alkalinity or carbonate hardness is one of the single most important water parameters for a healthy reef aquarium,

As we all know there is debate about everything in this hobby and how to do things and the reality is there is no perfect way. If there was we would all be doing things exactly that one same way. Alkalinity is one such aspect that still produces a lot of debate. The debate typically centers around which is better: high alkalinity (above 10dkh) or low alkalinity (between 7-10dkh) [ed. note: natural seawater has an alkalinity of 7dKH so 7-10dKH is normal to above natural values].

A recent paper by Hylkema, Wijgerde and Osinga showed that both high and low alkalinity values can be right or wrong and are more dependent on the nutrient levels than we realized. That is, when nutrient levels were low, the coral they studied did fine at a lower alkalinity level, however when nutrient levels were increased calcification was diminished unless the bicarbonate concentration was also increased. So in order to maximize calcification the nutrient and bicarbonate levels need to be balanced with each other. 

For most of the corals we keep, maintaining this balance works fine, however after much conversation we concluded that some corals do better under high alkalinity and others at a lower alkalinity even when the nutrient levels are in balance. We found that Hawkins Echinata [actually an Acropora turaki], the Red Dragon and other Dragon Acros as well as A. turaki and the A. echinatas all tend to bleach and fail to thrive if we let our alkalinity levels get above 13dkh and they crashed and burned at 15dkh.

The Hawkins Echinata in the center did not tolerate high alkalinity and died back to nubs from a full colony when exposed to high alkalinity

Yes I know these are not common alkalinity levels, but accidents in the form of high alkalinity do occur.  Needless to say, none of us tried to get our alkalinity levels that high, but due to various equipment malfunctions we all had experienced these levels at one point or another. We also found at these levels chalice corals began to melt away from their skeletons and that this continued even once the alkalinity levels were lowered. 

Chalices like this one from World Wide Corals will seemingly melt when exposed to high alkalinity for longer than a week

So we found that at these levels we artificially selected for the corals that could handle high alkalinity. The corals that seemed to do best at a higher alkalinity included the bulkier stony corals in our tanks like A humilis, A. monticulosaA. globiceps and A. gemmifera, as well as Porites and Pocilloporas.

When these corals were exposed to lower alkalinity in our tanks for a week or more, under 9, they started bleaching at their bases and would bleach out completely if the alkalinity was not increased and took months to recover once the alkalinity levels were increased if they recovered at all.  So as a result of this we now know that we can select out which corals will thrive or not based on our alkalinity levels.

Acropora globiceps is a sturdy acro that can handle higher alkalinity levels

Similarly, our discussions also led us to understand that differences in light and even the mode of lighting can determine which corals thrive and which fail. In the early days of keeping sps corals the theory was to blast everything with as much light as possible and it would thrive. Now that we are keeping a wider array of sps corals from multiple depths and locations this is no longer the case. 

With the advent of high powered LEDs, and their more focused nature, proper acclimation to the light also now can significantly impact which corals survive, which just hang on, and which totally fail. One of the surprising things that we have found is that Acropora in the millepora group tend to fare less well under LEDs than they did under intense halides.

In all of our tanks they survived, but unlike in the past under halides, they do not show the explosive growth like they had. In all of our tanks they grew at about 10% of what their old growth rates were. Several frags that were frags a year ago still for the most don’t look much different. In the past in our tanks one year old Acro milleporas or prostrates would be overwhelming their neighbors with their considerable growth.

A colony of A millipora grown at Penn State from a frag in short time under metal halides, this coral does not show the same rapid growth under LEDs

 Montiporas also can be affected adversely when not acclimated properly when exposed to the bright lights of LEDs. The mantra with these corals has typically been moderate light, moderate current and they will do great. With LEDs’ light seemingly penetrating our tanks better than occurred with halides it is now necessary to get them accustomed to this light over months in order to diminish the likelihood of bleaching.

In my own experience before I realized what was occurring I lost a couple of colonies soon after placing them in the tank even though other Montiporas were thriving right next to them. I didn’t realize that the bright light of the LEDs would select out corals that did not acclimate to it quickly.

One other aspect of “ artificially selecting” out our corals that we often overlook is the inhabitants we choose to house with them. What I am talking about here are the fish and other invertebrates that we place with our corals and how they interact. While I love the look of a well-stocked tank full of fish and vibrant corals I have also come to the realization that fish are a pain and given the opportunity if I don’t feed enough or feed the right food any one of them can turn in an instant and start eating the corals in the tank.

As a result of understanding how artificial selection works I now focus on what fish and inverts I keep with my corals and what can happen in the worst case scenario. Needless to say I myself now employ more artificial selection when picking a fish to put with my corals. I will admit that I have made one huge mistake in adding a Red Sea Regal angel to my sps tank 14 years ago. 

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