Coral coloration FAQ

[EricHugo]

Most aquarists are very concerned about maintaining or keeping colorful corals in their tanks. The following is a very brief summary of the nature of coral coloration.

Corals are a symbiosis between animal polyps and one or several types of many species and strains of single celled protists (dinoflagellates) collectively called zooxanthellae.

Zooxanthellae can vary in color from golden to dark brown. They multiply in response to a non-polyp abundance of nutrients, specifically nitrogen which is limited by their host. They can also photoacclimate by producing pigments that make them darker or lighter in response to lower or higher light levels, respectively. In general, low light and/or high nutrients will result in a coral becoming darker brown colored as the zooxanthellae multiply and/or photoacclimate.

Coral polyps (sans zooxanthellae) have their own set of animal pigments, too. There are non-light related pigments, such as those that form in the skeleton of Tubipora musica, Heliopora coerulea, and Pocilloporids. Also, there are the vividly bright colors of the skeletal sclerites in soft corals like Dendronephthya spp. and Scleronepthya spp. Colored skeletal elements are also formed in gorgonians, often in their axial rods.

Also, there are colored tissue pigments, such as those found in the tissues of many azooxanthellate corals, many which are not found on coral reefs. Examples are Tubastraea spp., Diodogorgia nodulifera, Swiftia, most Pacific sea fans, etc.

Finally, and most likely to be of interest to aquarists, are the light-related fluorescing proteins. Some corals produce these in response to high light and they act as photoprotective shields, such as some pink morphs of Pocillopora damicornis and some blue morphs of Acropora spp. Other corals produce them in response to living where the light is very low where the proteins act as photoreflectors, such as in Trachyphyllia geoffroyi, Cycloseris spp., and many faviids.
It has recently been found that some species seem to produce them for no currently known reason at all.

The fact that not every colony of a species responds the same in terms of fluorescing protein production under the same light conditions also suggests that there are specific genetic components invovled.

The overall coloration of corals is therefore a complex function of genetics, light-related fluorescing proteins, dietary derived or metabolically produced tissue and skeletal pigments, and their combination with the various hues of brown that mix into the palette in all corals with zooxanthellae (a function of both light and nutrients, primarily).

Most reef corals are shades of brown, and green fluroescing protein is the most common, probably followed by orange, then reds, blues and yellows. There are no rules for an aquarist to follow to maintain the color of corals. The reasons they were one color in one environment (tank or reef) may not be present where they are now, and without the history of those conditions, may be very difficult to reproduce.

In general, low nutrients and high light will likely allow for the production of bright colored high-light fluorescing proteins int hose corals that have the genetic machinery to produce them. Which ones are expressed (blue, green, orange, etc. is probably not possible at this point to predict). Low nutrients and low light will allow for the expression of low-light fluorescing proteins in those corals with the genetic machinery to produce them.

High nutrients will probably result in a masking effect by the zooxanthellae becoming darker brown in most cases.

There is, to my knowledge, no correlation between polyp size and coloration whatsoever. In other words, light related fluorescing proteins are not specific to "SPS" corals or "LPS" corals, and there are almost limitless examples of high and low light fluorescensce in both. However, it seems tobe a general truth that of the corals available in the trade, many seem to produce high-light fluorescing proteins under sufficient light, and it takes little effort to maintain those colors in corals that produce the low-light fluorescing proteins.

[coralite]

Originally posted by EricHugo


Corals are a symbiosis between animal polyps and one or several types of many species and strains of single celled protists (dinoflagellates) collectively called zooxanthellae.

[/B]

Oculina, Tubastrea, Astrangia. These have no Symbionts, are they not corals?

Zooxanthellae can vary in color from golden to dark brown. They multiply in response to a non-polyp abundance of nutrients, specifically nitrogen which is limited by their host. They can also photoacclimate by producing pigments that make them darker or lighter in response to lower or higher light levels, respectively. In general, low light and/or high nutrients will result in a coral becoming darker brown colored as the zooxanthellae multiply and/or photoacclimate.

Coral polyps (sans zooxanthellae) have their own set of animal pigments, too. There are non-light related pigments, such as those that form in the skeleton of Tubipora musica, Heliopora coerulea, and Pocilloporids. Also, there are the vividly bright colors of the skeletal sclerites in soft corals like Dendronephthya spp. and Scleronepthya spp. Colored skeletal elements are also formed in gorgonians, often in their axial rods.


Are you considering pocilloporin a non light related pigment or are you referring to the rusty brown color of eydouxi and other skeletons? Is there a name for this pigment?

Also, there are colored tissue pigments, such as those found in the tissues of many azooxanthellate corals, many which are not found on coral reefs. Examples are Tubastraea spp., Diodogorgia nodulifera, Swiftia, most Pacific sea fans, etc.

.

[EricHugo]

>>Oculina, Tubastrea, Astrangia. These have no Symbionts, are they not corals? <<

Actually, Oculina and Astrangia are facultatively zooxanthellate so long as we are splitting hairs.

>>Are you considering pocilloporin a non light related pigment or are you referring to the rusty brown color of eydouxi and other skeletons? Is there a name for this pigment? <<

Pocilloporin is now known to be one of the fluorescing proteins..is the same thing. The name pocilloporin was used prio to the more recent and complete characterizations.

The skeletal pigments are different - I'm not sure that one in some Pocilloporids has been studied...its really interesting though, because it is variable - can be almost bright brick red.

[BIGBOB]

Why is it that when corals bleach they release all of their colour, if only a portion of the colour is from zooxanthallae?

[EricHugo]

The don't. All their color is lost only during prolonged or sever bleaching events. Many bleaching events result in changes so minor its not noticeable to the human eye. Often, when corals "lighten up" or "color up" in aquariums, its a result of partial bleaching. Also, if you look at my article on bleaching at advanced aquarist, you will see a photo of a Euphyllia that is very bleached but still has its fluorescent proteins. Bleaching is a continuum.

Also, the method of bleaching can involve variously "strenuous" methods. The zoox can be released a few at a time into the coelenteron, they can be purged in pellets, or in streams. During very acute events, most of the cell contents can be purged, including the fluorescent proteins, and sometimes the entire endodermal cell is released.

Hope that explains it a bit.

[dwestfall]

I have several acropora that have completely bleached and are totally white, but the polyps are extended and seem to be doing fine. There is even a little pink colour to the corals themselves. I'm assuming that most if not all of the zooxanthellae are gone. Is there anyway to provide new zooxanthellae for reincorporation into the corals?

[photobarry]

It's very likely that there are still low concentrations of zooxanthellae in your coral (they can't get rid of them completely). If you can feed the coral that may help it recover more quickly.

[dwestfall]

Thanks Barry,

The tank is getting Combisan and a live nanocloropsis culture. They've been this way for several months now with no improvement. We are adjusting some of the other additions (Sr mainly), but I only expect this to affect the growth rate of the stonies.

[photobarry]

Let's start a new thread on this in this forum so we don't take this one off track. (For food I would suggest trying golden pearls soaked in Selcon.)

[Dazz]

By Nutrients do you mean more along the lines of Nitrogen or actuall food like phytoplankton?

[EricHugo]

Nitrogen

[Dazz]

thx

[crlkeep]

Not to keep the topic off track, but do SPS corals actively feed on particulate foods? Specifically Acropora sp. corals.

Most of what I have read so far leans towards dissolved nutrient uptake. I appologize for not recalling the source, but I also remember reading an article that claimed no evidence of feeding was or has yet been found in the guts of these corals.

The reason I ask is that after months of feeding Phytoplankton, I saw few if any results. After only a few short weeks of reduced skimming, which allowed my Nitrate level to elevate slightly, I noticed a dramatic shift in coloration and growth in all of my (Acropora sp.) and other SPS corals. My LPS where happy as well, but I anticipated that.

[EricHugo]

Not to say your sources of information are wrong, but....they are wrong.

Acropora are voracious feeders and ingest more per mass than large polyped corals, in general, and among those studied. But, they feed on animal material (zooplankton), particulates, bacteria, and, of course, dissolved nutrients. They do not, insofar as has been found, feed appreciably if at all, on phytoplantkon. In fact, insofar as is known, very few corals feed on phytoplankton at all.

[Rebels23]

crlkeep - What type of color shift did you see in your corals with reduced skimming? I am thinking about upgrading my skimmer, but am afraid of over skimming.

Eric - Is it safe to assume that over skimming might have an effect on the coloration of my corals? Brighter, darker?

Thx

Rebels23

[EricHugo]

Likely darker, at least initially as nitrogen levels increase temporarily. Ultimately, I think, for example you'll see more contrast in dark and bright colored areas....and they look healthier.

[Rebels23]

Thanks Eric. That is strange, I thought over skimming would decrease nitrogen levels as you remove more organics from over skimming? So less nitrogen = less nutrients = brighter corals?

[crlkeep]

Abosolutely darker. Browns are deeper, and the fluorescent green hues at night are rich and amazing.

As Eric mentioned, the colors contrast each other quite nicely.

Eric, I was referring to Phytoplankton uptake by Acropora's. Sorry if I only half explained myself earlier, but thank you for the answer that I was looking for. The bacteria and other trapped particulates, do they attach or grow on the slime coating of these corals?

Likely darker, at least initially as nitrogen levels increase temporarily.

Are you referring to reduced skimming, or "over skimming".
These are the reactions that I have noted due to reducing my level of skimming.

[EricHugo]

I'm sorry - wasn't very clear was it? Underskimming or not skimming will likely result in an at least temporary deepening of color. Not sure how to quantify overskimming but anything that reduces dissolved nitrogenwill likely result in the same or reduced zoox levels and cause the coral to become paler. Whether or not colors become brighter won;t have much to do with aything but the masking effects of the brown zoox, because the colored pigments in zooxanthellate corals are fluorescin pigments and seem to be primarily a factor of light irradiance.

[crlkeep]

Interesting...

I run 2 Berlin Classic skimmers rated at 250 gallons (tank capacity) each, on my 125 gallon reef. After narrowing down the possibilites for the "paling and Recoloring of some of my acropora sp.[/i] corals, I was left with either my method of filtration, or some dreaded unknown.

In order to keep the aeration effect provided by having 2 skimmers, I decided to reduce the air-bleed on the second skimmer. It still collects a thick film of gunk on the Collection chamber walls, but very little makes it to the cup.

Within a week, without any other changes being made, a couple corals started to "glow" much more vividly under actinic lighting. Now, these green hues can be seen during normal afternoon lighting levels. The shade areas near the base are very dark and rich in color.

In the future, I may readjust the skimmer back to its old settings, to see if this was a fluke or something to look at a bit deeper.

Thank you for your insight Eric. As always it was a pleasure.

Charles Poole

[Rebels23]

Ok thanks Eric! Sorry for being a knuckle head on this. I think I wasn't clear before.

When I say 'darker' I guess what I was thinking of was what I refer to as the coral "browning out". Brighter or Deeper colors I guess the way you are describing, I would have no problem with.

i.e. My green monti Cap is now a deeper green or a brighter green, but not brown.

I am afraid if I under skim, my corals will brown out because of excess nutrients. I am not sure what the effects of over skimming would be, brighter, deeper, brown colors??

Ok, I relaized I just sounded like a babbling idiot. lol

Rebels23

[crlkeep]

What I gather from the results I have seen in my tank is that if you "over skim", you may actually remove not only vital Nitrogen compounds, but micro organisms and algaes that may also serve as food. This effectively could result in starving the animal.

If you "under skim", and your system is not capable of processing all that your skimmer would normally be removing, then you may cause harm to tank with an excess of these nutrients.

It makes good sense to me, but without more testing on the levels of nutrients absorbed or otherwise attained by these animals, it is hard to say what is best overall. Since most of our corals come from many different reefs, they likely have very different needs.

[joereefer]

i bought an open brain coral last week from my lfs,i noticed that it appears to be bleaching. its under a pc 2- 96 watt bulbs. what do you think is going on?

[Rovert]

Eric, thanks for the great read! I'm often asked by friends what makes corals so colorful, and have to stammer through the "pigments to protect the photosynthetic organism" speech. Now, I can just hand them a copy.

Now, here's my question.

A while back, there was a hypothesis going around that suggested a correlation between coral coloration and alkalinity levels. What's your take on that?

[EricHugo]

well, a hypothesis is a tentative explanation for an observation, phenomenon, or scientific problem that can be tested by further investigation - as far as I know, this hypothesis has never been tested, and remains a weak anecdotal observation...

my feeling is that higher alkalinity spurs faster growth and hence longer growth tips that are low in zoox but high in pigment, lending the appearance of being more colorful. Personally, I don't think there is any correlation between the two.