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The Elusive Blue Tipped Acropora Eric Borneman Aquarium.Net May 1997

Eric Borneman discuss coloration in acropora sp., Aquarium Net has numerous articles written by the leading authors for the advanced aquarist

The Elusive Blue Tipped Acropora

And the Last (but not final) Word on Coloration

By Eric Borneman

(Staghorn, Branch, etc.)

Subclass: Zooantharia Order: Scleractinia Family: Acroporidae Genus: Acropora species: over 350 named species


Ah! Finally! Acropora ! The real reef keeper's coral, right? Not necessarily. Not in my humble opinion. Well, sort of.

If you pay too much attention to the "experts" in the hobby, most new or even experienced reef keeper's would get the impression that you are not a "real" reef keeper until you have a tank full of Acropora. Not only that, but you must be able to maintain the coloration of the branches through expensive, obscure, untested and overwhelming methods. Well, to those readers that are not doing so, I say you are no more and no less a "real" reef keeper than anyone else. In fact, you are probably a lot smarter and have significantly more coin in your pocket as a result. Tanks without Acropora are every bit as authentic and exciting as those SPS dominated tanks that the "reef studs" keep. There! I said it. With that out of the way, I would like to lightly brush on the care of Acropora , and provide a little background on them. I will not be able to cover the species in depth, since I would like to concentrate on coloration. However, rest assured that Acropora will be covered in great depth in the near future. I hope this article will be even more enlightening.

Acropora : A Background

In the Atlantic and Caribbean basins, two corals bear the name Acropora. Common staghorn, or Acropora cervicornis , and Elkhorn, or Acropora palmata , are the sole members of a one thriving community that has been reduced by over 80% in the past two decades. When I first started diving, the first fields of coral from shore were massive gardens of staghorn and elkhorn, and they continued to thrive all over the reef. Today, because of widespread bleaching, white band disease, poor boating practices, collection, sedimentation, and a host of man made disturbances, Acropora on this side of the World is a pretty rare sight. It is quite sad and disturbing.

In the Pacific, the outlook is somewhat brighter, at least in terms of Acropora , where 368 species or more exist throughout the Indo-Pacific. In this vast region, the number of reef building corals is truly staggering. This is not to imply that the Pacific reefs are not subject to the same duress, but the sheer number and size of the reef systems somewhat dilutes the man made atrocity for the present. Staggering in number, shape, color and size, these highly successful corals are the fastest growing of all genera, and are well adapted to their environment.

Types, shapes, and facts.

All Acropora are fast growing small polyped scleractinia. They are unique in that a specialized axial corallite is responsible for most of the growth and differentiation of the colonies into their many forms. Acroporids can be found in table-like formations, branch-like colonies, bushy formations, finger-like projections, etc. Similarly, they grow among each other in every shade of the rainbow, always overtopping, outgrowing, overshading and digesting their way to the finish line in the "race for space" on the coral reef. Because of their incredible growth rates (up to 6 inches or more per year), Acropora is a highly studied coral in research. Similarly, it has many unique characteristics that make it such a fascinating subject. There is so much to say about these fragile beautiful corals, but I must not digress at this time.

Aquarium care.

As with knowledge of Acropora in the wild, knowledge of Acropora in the aquarium is one of the most passionate subjects in the hobby. I could quite literally go on for a hundred pages or more communicating all the thoughts, theories and passions of hobbyists behind the growth and propagation and care of Acroporids, but please be patient for future articles on the subject.

Suffice for now to know that, of the corals that can be successfully kept in aquariums (excluding the corals I consider to be general no-no's like Goniopora, Stylaster, Dendronephthya, etc.), Acropora species are among the most difficult to manage. They require mature tanks and an steady experienced hand. They require excellent water quality, strong lighting, turbulent water flow. They do not tolerate rapid changes in water chemistry, temperature, and lighting. These corals have a very thin tissue layer that they seem all too willing to expunge at the drop of a hat.

Bleach? a heartbeat. And there is the dreaded contagious tissue peel known as RTN, or rapid tissue necrosis, which is the bain of many Acropora keepers. Shut down syndrome, progressive bleaching, white band disease, Vibrio infections and more all are common occurrences with Acropora . So why the fascination? Is it a challenge? Yes. Is it for their beauty? Yes. Is it for the mystique? Sure. Does it make sense? Of course not, but they sure do look good for some unexplainable reason.

I usually do not recommend keeping Acropora for at least a year after setting up a reef tank so that the system can properly mature. Too many people who have excellent systems, lighting, and crystal pure water find that they cannot keep Acropora for the first year or so. Yet, many people with older tanks throw these animals into comparatively less reef-worthy conditions only to have them thrive. I beleieve that there are too many changes happening in new systems to allow for widespread success with these corals. I am very sorry that I can not go into more depth on captive care and identification, but it will have to wait for another article. Other general do's and dont's?

Do provide a lot of light, preferably metal halide, and from 5- 10 watts of light per gallon or more. Do slowly acclimate these corals to lighting and water conditons. Do provide a lot of water movement around the colonies. Do make room for growth. Do provide lots of bioavailable calcium for them to grow. Do not expose them to rapid temperature changes, rapid water parameter changes in pH, etc. Do not let specimens showing any tissue peel into the tank. Do not change their position frequently in the tank. In fact, do not do anything but try to provide stable excellent water conditions. Do try to pick smaller specimens or fragments with no signs of bleaching, thicker branches, extended polyps, for the best chance of success. Also, brown specimens are easier to keep than the brightly colored ones.

But, no one wants the brown ones. So what's in a color?

The Miracle of Color, pt. 3

It's funny how many people tell me they have a blue tipped Acropora in their tank as a serious attempt to identify it. Using color to identify a species of Acropora is entirely useless, as there may be fifty different species of blue tipped Acropora within the same reef. What is important, though, is the overwhelming desire of most hobbyists to maintain the coral's brilliant colors in their aquarium. Unfortunately, most people have found that the corals turn brown in captivity...or at least become less brilliant in color.

After countless hundreds of hours hashing these points out in countless emails and message boards, it has become the mass opinion that higher temperature metal halide bulbs, like the 10000K and 20000K bulbs seem to maintain coloration of blue and purple tipped Acropora, while 6500K bulbs seem to maintain yellow and green Acropora better. Others have suggested that it is because of the higher incidental UV radiation produced by these bulbs. Still others say that use of small amounts of blacklight (maximizing UV wavelengths) has resulted in maintenance of coloration. However, just as many people have watched corals fade to brown under identical light regimes.

I personally have seen better color retention using 10000K bulbs in SPS corals than I did with 6500K bulbs. However, through much reading and theorizing, I have reached a conclusion that lighting is not directly attributable to much of the color of corals, and may be indirectly contributing at best.

To explore this further, it is necessary to recap what was mentioned in the previous articles in the series, and also to understand the role of light, once again, in the biophysiology of corals. In the first article, I explained how within the marine dinoflagellates and including the zooxanthellic species, there were literally dozens of pigments used in the photosynthetic machinery of the algae. In fact, there are enough pigments known to exist that could be responsible for almost all hues seen in the reef. When coupled with non algal photosynthetic pigments, the array becomes truly amazing. These pigments all have specific roles in the energy transfer of light into photosynthetic processes. Some pigments changed color as they are oxidized in the photosynthetic process. Others become predominant, especially certain carotenoids and PCP compounds (see article 2 in the series) under certain light conditions where they are found in greater amounts to allow for the more complete utilization of certain wavelengths of light that the actual photosynthetic pigments, the chlorophylls, cannot use on their own. Thus, coloration to some degree may be seen as coupled to the overall coloration of the dinoflagellates.

A problem, however, arises in that irrespective of light regimes, the dinoflagellates are shades of brown. If there are a lot of zooxanthellae, the coral becomes "browner" If you vary the light conditions to the zooxanthellae, they may shift slightly to one shade of brown or another, but the overall effect is still brown. The non-photosynthetic pigments, GFP, PFP, and Pocilloporin, among others, may predominate in effect over the brown coloration of the zooxanthellae and impart either fluorescent or non fluorescent shades of pinks and blues and greens, but they do not account, in any combination, for what is being seen and measured on the reef and in the aquarium. And, because light effectively only changes the distribution and amounts of either a) the zooxanthellae or b)the clear UV protecting substances of MAA's, Palythine, S-320, and others, which do not have any perceivable effect on the coloration of the corals except perhaps to allow certain ratios of the brown zooxanthellae pigments to be maintained, light seems to be somewhat least theoretically. Lighting also seems to be irrelevant in that almost all the brightly colored Acropora species on the reef are being collected from the same type areas where both red, yellow, green, purple, blue and brown specimens of many different species are found in close proximity to each other and exposed to practically the same conditions. So, while various light spectral qualties from varying depths, amount of suspended matter in the water, latitude, time of day, etc., may play an important role in inducing change in zooxanthellae composition, pigment ratios, and amounts, it does not seem to be directly correlated to the bright colored pigmentation that is the desire of the reef aquarium keeper.

So what's the deal? Where do we go from here? Well, perhaps thinking about possible roles of coloration and their adaptive significance may be in order. Corals are animals, not plants. They are special animals, granted, but are still animals; so perhaps thinking in terms of coloration in animals rather than plants may be an avenue of importance. In nature, coloration serves one of three roles. It can increase conspicuousness, decrease conspicuousness, or be unrelated to appearance. Generally, bright coloration in animals that serves to increase conspicuousness can be related to reproductive success. An example is mating, where brightly colored animals are perceived as better mates than less brightly colored individuals. Bright colors often serve as warning to potential predators. They can also be utilized in mimicry. Mimicry is very important in nature, as animals may try to attain similar recognition to other species to decrease the possibility of predation and/or increase the chances of successful reproduction. Coloration, either by increasing of decreasing conspicuousness either increases or surpresses contrast in their environment so that they are more or less visible to potential predators. Optical ambiguity is seen everywhere in nature, and a prime example is the eyespots on the tails of certain butterflyfish who have these false eyes to confound would be attackers.

Trumpetfish change color and orient themselves vertically among gorgonians to avoid being noticed. In fact, cryptic coloration and patterning is more a rule than an exception.

Indeed, because of the always conservative role of nature, color * must * be based on either the vision of others * or * be present so that the color has an actual functional role to the animal. Otherwise, coloration would be an energy expenditure that could not be justified in either physiological or evolutionary terms. No matter how much we may like to think that the reef is brightly colored so we can take pictures and explain how pretty it all is, this is simply not the case. There is always an underlying reason for coloration, and light simply bringing out bright colors in corals is not a valid would not happen unless there was an adaptive significance.

What could the adaptive significance of coloration in a reef habitat consist of? To be sure, the reef is a myriad of brightly colored fishes, corals sponges and other life forms. Perhaps all the lifeforms are in a constant mimicry of each other, knowing the high degrees of specialization are what allows for such incredible biodiversity in these areas. For example, a species of angelfish eats corals and a type of red sponge, but not a blue sponge, so certain corals take on a blue coloration in mimicry of the blue sponge to avoid being eaten. This is a drastically simplified and highly unlikely example, but it illustrates the point. Perhaps more likely is that certain predatory nudibranchs eat red morphs of species "A" of Acropora , but not the blue morphs of the same species, so species "B" of Acropora that grows nearby adopts a blue coloration to mimic the uneaten variety. While this is more likely, it is still questionable and unproven for corals.

What is proven is that animals can see in many colors, and that coloration is important in attracting and repelling certain species from others. A classic example is the ability of many pollinating and predatory insects to see in the UV regions of the light spectrum. To insects, certain flowers, appearing simply yellow to us, have the equivalent UV of a neon sign that says "land here and take my pollen!" Indeed, there has been little research conducted to demonstrate that this highly likely scenario exists underwater. Yet one study showed that fish have extremely acute vision in the UV spectrum that was presently unknown. Mantis shrimp have astoundingly complex eyes able to see in both UV and IR wavelengths. It is thus likely that the more "insect-like" organisms, including zooplankton, crustaceans, and others share this trait perhaps even more so than fish. It even makes sense that reef animals would favor having vision in this spectrum as color vision consisting of reds, oranges, and yellows would be quickly filtered by the water itself, while green, blue, violet, and ultraviolet would penetrate to the limits and beyond the depths at which stony corals predominate. While there is no "pollination" that need happen with corals, this possibility has great significance in terms of potential predation, commensal animals and attraction of food animals. I will return to this topic shortly.

Coloration in animals is much different from that of plants in that animals cannot synthesize pigments. Coloration is either due to reflecting pigments called biochromes or microchromes (schemochromes). These pigments, and in fact all pigments in animals, must be obtained from diet. Pigments responsible for typical bright coloration are found in the chromatophores of animal cells. Generally, red, orange and yellow are genuine pigments while green, blue and violet are formed by other factors and combinations. Contrary to plants, pigments in animals have no importance in UV radiation protection, light induced reactions, etc., but may play a role in thermal and hydroregulation in terms of the wavelengths that are reflected and/or refracted back to create the appearance of color. This type of regulation has been most studied in amphibians and reptiles who change color to control their body temperature and with concomitant water conservation, but the possible application to corals and other sessile invertebrates should not be overlooked. Indeed, such an effect seems highly plausible and likely. Most coloration in animals is due to diet obtained carotenoids and carotenoproteins, although quinones, xanthins, pterins, hemocyaneins and many of the other pigments that are synthesized and found in plant cells may be assimilated into animal tissues through ingestion. Thus, all (or most) of the pigments available to plants may be found in animals as well, though not in a directly functional role to metabolical needs.

Tubipora and certain Alcyonia (soft corals) are red or yellow from unclassified pigments that contain iron. The blue skeletal color of the non scleractinian coral Heliopora also results from an iron containing pigment. Certain anemones contain a blue pigment that is an oxygenated acidogenic carotenoid known as astaxanthin. Cyanoproteins have been isolated and found to be part of the formative color in Distichopora , Allopora and Stylaster corals. And most notable is the fact that although unclassified or unnamed, most corals and invertebrates seem to have predominant coloration due to the presence of one or more pigments called astaxanthins. And, they all obtain these pigment produced colors from their diet. As an example, certain nudibranchs get their color from the type of sponges they eat. Raised without the sponge, the nudibranchs lose their color. This type of experiment has been repeated in many cases, the notable exception seeming to be the lack of trials with corals.

General Animal Pigment Qualities of Coelenterata






astaxanthin and esters


varying degrees

few, esters of astaxanthin and/or metridioxanthin, minor carotenes


little to none

considerable, metrodioxanthin,esters, xanthophyll esters, carotenes, free xanthophylls

red with brown

varying degrees

many, esters of metridioxanthin and astaxanthin



many, metridioxanthin esters, free or esterifiedastaxanthin, free metridioxanthin, carotenes, xanthophylls

Thus, I raise a semi-supportable hypothesis that in order to keep our Acropora with the coloration they maintain in the wild, diet and not lighting, may be the answer we are all looking for. What is the proper diet?

I don't know, but before concluding with that topic, I would like to point out some evidence that I feel supports this theory. Although some Acropora and other SPS colonies have truly uniform coloration, the majority of specimens seem to have colored tips. The general growth pattern of branched fast growing corals, especially Acropora with its specialized axial corallite, involves pale colored tips which are the areas of fastest growth and highest calcification. Although this is a paradox, since these areas are sparsely populated (if not devoid) of symbiotic zooxanthellae which enhance calcification, there are many papers which cover the possible explanation of the dichotomy. Among these are the most plausible theories that intercolonial transport of nutrition supplies the rapidly growing tips with the energy needed for calcification. If this were the case, then not only would heterotrophically met nutrition be a means of transporting pigment to this area, but the relatively lack of brown zooxanthellae would not be present to obscure diet obtained coloration present in the more or less white tips.

Furthermore, this would seem justified by conditions in the aquarium. It has been shown that many species of corals, especially Acropora , Stylophora , etc., may meet up to 150% of their energy requirements by their zooxanthellae alone. Yet, it has also been found that Acropora are vigorous consumers of zooplankton, nanoplankton, bacteria and other organisms which meet up to 70% of their daily energy needs. Although this finding may be surprising in light of their ability to grow with light alone, these "seemingly" innocuous nonaggressive small polyped corals are in fact quite deadly, heterotrophically speaking. With that in mind, let us suppose that a brightly colored Acropora is relocated to a captive and brightly lit aquarium which will not likely (especially in heavily skimmed aquariums) provide significant conditions for heterotrophy, and even less likely provide the theoretical dietary component responsible for the animal pigment that provides it color. The coral would then turn toward either a new source of heterotrophic nutrition which may cause a change in available pigment input and resultant color, or, more likely, become more dependent on its overly capable zooxanthellae to meet its energy needs...and thus begin to turn BROWN. Perhaps the zooxanthellae mask the underlying color, perhaps the animal pigment is used up without heterotrophic replenishment, or perhaps a combination of both may effect the change.

There remains a problem in this hypothesis, however. Why, in some cases, has a change in lighting intensity and spectral quality been successful in maintaining coloration of some corals? It has been shown in other studies that lighting absorption spectrums for pigments P-560 and P-590 did account for coloration in certain species of Acropora , and thus light obviously plays an important role in coloration. But, as is usually the case in biology, there is never a simple answer to a question, and the answer lies most often in synergistic webs of biomechanics and biochemistry. I contend that all of the following occur in corals:

1 . Sunlight allows for the proliferation and possibly selective diversification of plant pigments in the zooxanthellae. The fact that most brightly colored corals are found in areas of extremely intense light suggests that most spectral qualities are present and thus can provide for highly adaptive pigmentation characteristics found in these corals. The presence of full spectrum and highly intense lighting in the aquarium can provide similar means for corals to adapt to light and produce pigmentation of the zooxanthellae proper for the present conditions. The use of higher intesity and more selective temperature characteristics most closely approximates the conditions found in the wild for those corals which maintain coloration using those specific bulbs, but not for corals which lose coloration with a given bulb type.

2 . Diet provides the majority of coloration of corals through heterotrophic pigment procurement, and the coloration * may * be used to attract certain food sources directly, or indirectly through lack of predation of corals or coral food sources, that will help maintain the adaptive coloration. In the aquarium, these food sources are likely unavailable and the pigmentation is lost or changed to meet whatever heterotrophic food stuffs exist, if any significant amounts at all, in the captive conditions.

3 . Due to the nature of translocated products between symbiotic algae and coral animal, the manufacture or translocation of pigment may exist. The true nature of all translocated products has yet to be ascertained, although glycine, amino acids and carbohydrates seem to be common.

I think that further suggestive evidence is found in those corals which maintain solid coloration within the skeleton itself. In some corals like a hot pink Montipora digitata in my own aquarium, in Tubipora , and in Heliopora , it has been shown that the pigments responsible for skeletal coloration are those acquired by dietary uptake. Thus, the coral animal must assimilate these pigments in the calcification process which is closely tied to the translocation of the photosynthetic products of the zooxanthellae.

Furthermore, in solid colored Acropora , the tissue itself maintains the pigment, and it is not only found in the tips but the entire branch. This would also tend to suggest that such consistent coloration could not be met if the zooxanthellae (and thus light) were solely responsible for coloration, and massive ingestion of animal product containing the responsible pigments must almost certainly be occurring to a degree such that the brown coloration of a highly light dependent coral is totally masked or almost not present.

This implies that heterotrophy in such corals is likely to be a major constituent of energy needs. Finally, bleaching further supports the theory of heterotrophic coloration. The zooxanthellae are found within vacuoles in the gastroderm of the coral animal. When corals bleach, they expel the zooxanthellae and become white. However, in corals which bleach, there is no known cellular mechanism which can selectively expel some intracellular contents of that size and not others. Thus, it would have to figure that the cell would expel the cytochromes as well. If this were not the case, and the zooxanthellae alone were expelled, then the cytochromes would remain and the coral would remain colored, if not a different color entirely, and not be white at all. Of course, if the zooxanthellae were the sole intracellular constituent responsible for coloration, then their expulsion would also result in a white coral. But isolated zooxanthellae are always brown and not red, yellow, green, blue and purple. Furthermore, the existence of animal pigment in corals and the existence of cytochromes would have to not exist at all, and this is simply not the case.

In conclusion, I am suggesting that diet may be as important, if not more important than lighting in maintaining coral coloration in the aquarium. As to what the actual constituents of proper diet are is entirely beyond me. I have seen relatively little research in this area, outside analysis of gut contents of Montastrea annularis , Acropora cerivicornis ( a brown species), and various other unrelevant studies of anthozoans and cnidarians. It would be interesting to gain feedback from hobbyists who are presently feeding their corals, or those who may begin to do so after reading this article.

This way, perhaps some understanding of these complex animals can be fed to the scientific community without the destructive methods usually employed in coral studies. I would also be very interested to know from those who have spent time among the shallow water Acroporids of the Pacific, if anyone has noticed or can relate any discernible patterns in coloration on the reef itself that may elucidate a specific food source employed by corals of a similar color. While I do not think that, given the incredible flux of nutrients through a shallow reef environment, that attraction of pelagic zooplankters and the like could be significantly altered, perhaps certain corals are more selective and finicky than we imagine.

OK. That's it for this month, and I hope this was as exciting and enlightening for you as it was for me. I have some coral foods to prepare, so.........just be careful not to go overboard and fed the tank so much that you reduce water quality and jeapordize the inhabitants!!!

Eric Borneman Eric

References : (lots)

Burtt, Edward H. Jr. The Behavioral Significance of Color. Garland Press:New York, 1979.

Cott, Hugh B. Adaptive Coloration in Animals. Methuen & Co. Ltd.: London, 1940

Davies, P. Spencer. The Role of Zooxanthelaae in the Nutritional Energy Requirements of Pocillopora eydouxi. Coral Reefs, 2: 181-6, 1984.

Falkowski, Paul, Light and the Bioenergetics of a Symbiotic Coral, December, 1984, pp. 705-10.

Faust, Maria A. Response of Prorocentrum Mariae-Lebouriae (Dinophycae) to Light of Different Spectral Qualities and Irradiances: Growth and Pigmentation. J. Phycol, 18: pp. 349-56, 1982.

Fox, Denis L. Biochromy. University of California Press: Berkeley, 1979.

Fox, H. Munro and Vevers, Gwynne. The Nature of Animal Colors. Sidewick/Jackson Ltd.: London, 1960

Gil-Turnes, Sophia, Studies of Phtoosynthetic Pigments of Zooxanthellae in Caribbean Hermatypic Corals. Proceedings of the Fourth ICRS. 1981, pp. 52-3.

Hinde, rosalind. Symbiotic Nutrition and Nutrition Limitation. Proceedings of the Sixth ICRS, 1988, pp. 199-204.

Jeffrey, S.W. Chloroplast Pigment Patterns in Dinoflagellates. Journal Phycology, 11: 374-84, 1975.

Leletkin, V.A., et. al. Photosynthesis of Coral Zooxanthellae from Different Depths. Proceedings of the Fourth ICRS, 1981, pp. 33-7.

Lipkin, Richard. Sight in the Sea: Exploring Light and Color in Coral Reef Ecosystems. Science News, 148: pp. 184-6, September 16, 1995

Muscatine, L. et. al. Reef Corals: Mutualistic Symbioses Adapted to Nutrient Poor Environments. BioScience, 27: July, 1977, pp. 454- 60.

Portman, A. Animal Camouflage. University of Michigan Press: Ann Arbor, 1959.

Shibata, Kazuo. Pigments and a UV-Absorbing Substance in Corals and a Blue Green Alga Living in the Great Barrier Reef. Plant & Cell Physiology, 10: pp. 325-35. 1969.

Sorokin, Yu. Aspects of the Biomass, Feeding and Metabolism of Common Corals of the Great Barrier Reef, Australia. Proceedings of the Fourth ICRS, 1981.

Taylor, Denis L. Intra-colonial Transport of Organic Compunds and Calcium in Some Atlantic Reef Corals. Proceedings of the Third ICRS, 1977. pp. 431-6.

.....and many various cumulative personal communications, intangible sources, etc .

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Last modified 2006-11-18 20:01