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Colouring / Browning of Corals

By Eric Borneman, Ron Shimek and Dallas Warren. Posted to the ReefKeepers emailing list, Thursday 20th and Friday 21th May 1999.


Well if the coral's natural coloration is white or yellow, why would it turn orange/tan in my tank? None of my other corals have changed color since I got them.

What do you call a coral's "natural colour"? The colour it was in the store? Colour you see it at a particular location on a reef? What about that specimen that is at 10 metres higher depth? What it is in your friend's tank?

The fact is that the colour of corals is highly variable. It relies on a large number of environmental factors, such as lighting, spectrum, nutrition, and alkalinity. If you change any one of those variables then the colour of the coral will change.

The colour of a photosynthetic coral is dominated by the amount of zooxanthellae in the tissue. These are typically brown/gold in colour. The higher the number of zooxanthellae in the tissue, the more brown the coral is. The extreme of this is when a coral bleaches, it goes white as all of the zooxanthellae has been expelled from the tissue. As they slowly repopulate the tissue then the brown colouring returns.

I had heard that turning brown was a symptom of insufficient light, but in PGC it says cup corals only need medium light, and mine is right under a 175w halide and two NO actinics so it seems strange he doesn't have enough light. 10 watts/gallon in my tank. He is about four inches under the water surface. If anything he should have too much light. I hear a lot of people with 400's say brown corals color up under more light so it seems odd that he would turn brown if I gave him a lot of light.

There are two things that are interacting when you talk about the number or density of zooxanthellae present in the coral tissue. First off you have the amount of light available, the more light you have, the more algae cells that can be present to use that light. Second you have the coral which controls the population of zooxanthellae in its tissue so that it gets enough nutrition from them, less nutrients being produced by the algae the more it allows to live in the tissue.

The over-riding influence is that of the coral, if the zooxanthellae are not producing enough nutrients for it, then it will allow the population to increase. If they are producing too much then the coral will expel some from its tissue. The extreme of this is of course bleaching. You suddenly put a coral under much more intense light than it is use too and the algae produce too much oxygen. To avoid poisoning the coral then expels it.

So, how does this translate? Well, the more light you give a coral then the lighter in colour it should be.

But you say, why do the corals go browner up in my tank? It is under a huge amount of light. Well, what are you measuring it against? The colour it came to you from the store? After being shipped half way around the world, placed under huge amounts of stress, then under typically substandard light? If so, then you have to expect the coral to colour up after that, there is no way it will have its full allotment of zooxanthellae after that.

Well, that is how I see and understand it at the moment .... will be interesting to hear other peoples thoughts on this.


Well I am certainly no expert but when I researched cup corals I found references to green ones, yellow ones, and even (here on this list recently) white ones, but never orange-tan ones.

I wish I had kept up with this thread, because it seems as though you have already received plenty of commentary. Just a few thoughts:

Coral coloration is too complex to be understood by single variables like light. It depends on light, zooxanthellae strains, nutrients, dietary inputs, etc The change in coloration is perfectly normal. Furthermore, you really have no idea what light intensities the coral was exposed to, nor could you ever expect match its water quality or food inputs. All these things would serve to change its color. Finally, even if the coral was exposed to high light (doubtful), a 175 watt metal halide is not very much light by comparison, unless the coral were out of the water and a few inches away from the bulb. ;-) So, what you are seeing, I suspect, is an increase in zooxanthellae number or pigment content due to higher ambient water nutrient levels, probably nitrogen. But thats a guess based on likely scenarios in closed systems.

Once again, a brief dissertation of photoacclimation and photoadaptation.

Depending on the depth range of a coral, the depth at which it was taken, the amount of shadowing, clarity of the water, blah blah blah, it was exposed to and regularly received a given amount of lighting every day, varying with clouds, growth of other nearby life, etc.

It was also exposed to a certain amount of food, and lived in water with a given amount of nutrient. I say probably N, because recently P has not been shown to have much of an effect on zooxanthellae densities. It does affect calcification, but we aren't going there now.

In our tanks, there are a few givens. One, that the corals are not feeding on the same amount or type of food as they do in the wild, and both pigments and precursors for animal pigment biosynthesis are often provided for (if not almost entirely) by diet. For example, look at asymbiotic sponges, Dendronephthya, and the azooxanthellate tips of Acropora spp. OK, got that part?

Two, that the light intensity and quality, while theoretically possible to match if we knew the readings where the animal was taken, will be quanitatively different, no matter how hard we try. On a positive note, we may actually be capable of providing a more optimal light than the coral was receiving - for example, a coral which settled under a big -ol tabletop Acropora and it was not exactly in prime real estate, scleractinia-wise. For many, especially those tolerant of various lighting, and who are less likely to have light induced changes - say, Euphyllia and Plerogyra, the effect of different light quality seems to have less effect, and one would assume that these guys are predominantly colored by animal pigment (see above). Perhaps I shouldn't have used Plerogyra as an example, because it actually photoacclimates with its bubble-vesicles, but you all get the point. For others - like some of these nutty brilliant tight-space branched reef crest Acropora, we can pretty much forget about providing the conditions they are exposed to.

Third, we have N content as ammonia, nitrite, nitrate. In the ocean, most N is ammonia but in our tanks, it's nitrate (corals don't use nitrite, and it barely exists in the ocean or our tanks under normal circumstances). Now, even in tanks with untestable nitrate, the nitrate level is usually orders of magnitude higher than the reef. A level of .1micromolar NO3 (about .1ppm) is usually given as the level where coral reefs turn into algal reefs. My tanks are significantly higher than NSW NO3 levels, although they are still darn low.

Now then, corals want to grow and so do their captive algae. They hang out because they help each other and are more likely to survive in each other's company, but given the chance, most corals would sooner be without these symbionts, and the algae would definitely rather be alone if conditions were optimal. Nonetheless, they both try to keep a short leash on each other, and keep the other working hard. So, if a coral finds itself in a less than optimal light regime, it wants more energy for growth, so it releases some more of its waste to the algae to get them to grow - either in number or size or pigment content. As mentioned, zooxanthellae are brownish to golden brownish, not pretty colors. If they get to grow in one of these parameters, they get browner. If the coral is getting boatloads of light, so much that the algae are producing too much oxygen, the coral ditches some of them. Low level bleaching is not visually evident as a bleached coral, but some of the brown tinted algae stop masking other coral pigments - like fluorescing proteins, pocilloporin, and intracellular animal biochromes. The coral, with reduced zooxanthellae, looks "prettier" even though it is just lighter in the algae department - could be optimal, or could be deleterious - hard for us to tell without speaking coral-ese or doing lots of tests to measure productivity/accretion, etc. (anyone wanna donate their coral to a lab?).

There's another way the algae get to thrive, too, besides reduced light. Stop relying on those stingy ass corals to give them their daily dose of nitrogen....if the water has higher nitrogen, they can just absorb it as it crosses the outer tissue membrane and enters the coral cells where the zooxanthellae dwell in vacuoles - they can suck it up, and start growing, reproducing, or changing their pigments....mostly growing. Then the algae stop giving up their photosynthesis goodies to the coral cause they need to feed the increased numbers of their own kind. Mr. Coral gets a little hacked off - his growth rate is slowing, he's not getting his dinner, and something has to be done...he bleaches or expels ssome of them. If conditions continue, the coral is weakened, and some tissue loss may start, and the next thing you know, all hells' broken loose and the macroalgae are buying property in the neighborhood, property values are falling, the water is getting green, light isn't penetrating, so the zooxanthellae get to reproduce more, corals are getting sick, aand the 'hood is falling aprt in eutrophy. Anarchy!

Anyway,if your corals are turning brown, they are just doing their thing to maximize the conditions they are in - and they are good at it, too. So long as your tank isn't covered in problem algae, and things aren't growing (or, horrors, actually receding) then don't worry about it - most reefs are predominated by brown corals anyway. If you are set on forcing corals to look pretty, slam them with light, but you may be harming some of them, given other factors. Also, if your corals bleach all the way, some also go through some significant internal changes that result in them purging other things, too - like their biochromes, too. One more note on brown corals - some like to eat excess zooxanthellae, so keep that in mind, too. Last possibility - various zooxanthellae strains are present - with all the implications (see my Aquarium Net article on this). So, Donna, your brown corals in high light are growing well and I say leave 'em be. As I recall, you have had some amount of biomass from the flatworms returned to the tank - this may be part of it...could also just be your relative levels of things. And don't starve the tank to make them pretty - our corals are already pretty hungry for some prey material, and without that, they won't reproduce. So, as I have said before, we should probably stop dinkering with our tanks to get our favorite corals to look pretty and start thinking about the whole community, the overall health, and let them do what they do so we can strat hearing more about spawn collecting than trading fragments!!

If 0.1 ppm is where coral reefs turn to algal reefs, why aren't our tanks (with 50-100 times the nitrogen content) doing the same thing? In my tank, for example, the tanks eat the macro, but don't touch the micro algae. There are a few snails, but not enough to do the job.... it's seemed to me that the small crustaceans on/in the rock are what's doing most of the job, but I could be wrong. Anyway, how are we able to keep our tanks relatively algae-free in such relatively high nitrogen levels?

Well, that is a very good question. A couple years back, Marlin Atkinson pondered out loud the same question. He couldn't really understand it either. Want my theory? OK, you get it anyway!

I think that, through trial and error and years of blundering about with some successes here and there, we have gotten pretty good at forcing success through a number of ways - skimming, water changes, the use of buffers and CaOH2, lighting that doesn't overheat the water and limitation of wavelengths that stimulate algae. If we go back not too long ago to wet/dry's and 5500K bulbs, we pretty much do have cases where corals were growing (but brown), and there was a consistent battle with nuisance algae. I also think that our limitation of natural recruitment through inputs has a lot to do with it....both sides of the knife, here. In one sense, we don't have the pluses associated with constant renewal, but nor do we have the minuses. Third, we choose organisms that do not promote natural reef destruction - the parrotfish, the boring sponges, the corallivorus snails, etc Whenever there is injury or insult in the wild, the algae move in, but are limited under oligotrophy by biodiversity and that there is life whenever there is death, death whereever there is life. We limit both the renewal and the destruction. Fourth, the high biomass/water volume (i.e. the stocking density) probably extends to non-selected life, too. Micrograzing, in the absence of overachieving macroalgae pruned by macrograzers, is the main limit to algal overgrowth. All the amphipods, etc. are doing their thing at densities which are perhaps smaller in diversity but probably greater per mass/volume than the wild. Maybe Ron would confirm this from his investigations? I know that worm densities and bacterial densities can be equal to or greater than the wild. Perhaps also why long term tanks seem to have much less trouble with problem algae than newer tanks? Nice pseudo-ecological balance of established predator/prey (if one considers algae prey in this case)? Also, most reefs don't have one herbivorous snail per gallon of water. Thankfully, too, I might add! Although such a population of escargot might be a possible solution to feeding the masses ;-)

So, that's my theory. :::tossing it out there::::


In most cases in our systems, I doubt the worm densities reach natural densities in the rocks - studies by Kohn and Lloyd about 30 years ago showed that normal reef rock normally has between 40,000 and 60,000 worms per square meter of surface area (for rock about 10-15 cm thick). In fact the presence of these worm borrows sans the worms is one of the reasons for the porosity of "live" (really mostly dead) rock. I think the next big breakthrough in management will come when we can get actually live rock in from distributors.

I think the bug densities can probaby reach and exceed normal densities, and I think that sand infauna (worms, bugs, et al.) can also reach and exceed normal densities - Although I must state that I think it is a rare reef tank where this occurs. Most folks simply don't work the system to promote diversity or abundance (as this requires the presence of various (horrors!!!) algae).

In natural systems these animals are distributed in patches of varying abundance. Most of our tanks are on the same size scale as some of the patches - and can maintain good populations indefinitely. If such populations occur, I agree that these systems have enough herbivory to prevent algal take over. Also, we have have to have some method of mass export of nutrient out our systems - algal removal, skimmate removal, it really doesn't matter. If the nutrient is efficiently exported, then the algae can be kept under control.

In many of the natural reefs where algal take over has occurred, the nutrient level remains high, and is not effectively reduced by export (consider that a bay with sewage effluent entering one side and water leaving the other can be considered as a steady state system retaining high nutrients, even though a lot is exported). These examples are examples on the large scale of the failure of nutrient export pathways.

Created by liquid
Last modified 2006-11-24 18:40