[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.