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Aquarium.Net March 97

Eric Borneman writes on Poccilipora, Aquarium Net has numerous articles written by the leading authors for the advanced aquarist

Pocillopora- The Cauliflower Coral

and the beginnings of coloration pt. 1

By Eric Borneman

(Bird's Nest, Branch, Brush, Cluster, Finger, Lace)

Subclass: Zooantharia Order: Scleractinia Family: Pocilloporidae Genus: Pocillopora species: damicornis, eydouxi, meandrina, verrucosa, woodjonesi, danae

I have wanted to cover some of the more renowned SPS (small polyped scleractinia) corals for quite some time. I have hesitated to do so, since there seems to be such a focus on these corals in the hobby. Everyone wants to keep them. I am always disheartened at this, since not only can they be difficult to keep, but the keeping of SPS corals seems to be a notch on the belt of reef keepers...and to not be successfully keeping them seems to imply a overall lack of respect and adequacy in the hobby. This is entirely unjust, as many corals are subjectively more beautiful and every bit as natural an addition to an aquarium as SPS corals. In fact, most are far more colorful, and they maintain their colors under less than perfect water conditions. However, it is the coloration of these corals which prompts me to begin this series with Pocillopora and an overview of pigmentation.

Location and Background

Pocillopora is found as one of the most widely distributed corals in the Pacific. It is found from the Eastern Pacific, throughout Oceania, Australia, Japan, the Indian Ocean, and the Red Sea. It has been estimated that Pocillopora is the largest contributor to reef structure in the world simply by virtue of its incredible range, limited number of species, and abundance.

Morphologically, Pocillopora is distinct among most small polyped corals. Their polyps are quite large and fuzzy, giving it a very distinctive cauliflower-like look with all polyps extended from its usually branching (arborescent) skeleton. Many species of Pocillopora have raised growths that somewhat resemble Acropora's corallites, but they are rounded and more "warty." These are called verrucae, and are a distinguishing characteristic of Pocciloporids. The tissue of Pocillopora at the base of the corallite often has distinctly darkened "eyes" that look like little rings from which the polyps emerge. Although some have noted that polyps extend mainly at night, I have found my specimens of P. damicornis to be extended mainly during the day. Distinctions among the common species are as follows: P. damicornis has its verrucae as part of the branches. P. verrucosa is branched with separate verrucosa but a regular branching pattern while the branches of P. meandrina adopt a twisting fused pattern that resemble in shape a more sinuous flattened Acropora palifera (Cat's Paw). P. eydouxi is distinct in having separate verrucosa and flattened individual branches that would almost look like Acropora palmata (Elkhorn) if the colony were not so rounded and "cauliflower-like." One thing most notable about Pocillopora is its capacity for morphologic change. Perhaps more than any coral, variations in latitude, light, and water movement vary its structure in both color and skeletal growth patterns. It is highly variable, though the obvious skeletal cues allow for its identification.

Pocillopora damicornis has a very engaging history in the companionship of humans. It has, for years, been the most studied coral in terms of coral research. It is also the first coral, as far as I know, to have reproduced in captivity. In aquariums, sexual reproductions of Pocillopora are becoming more common, but Pocillopora also reproduces asexually in a more common way called polyp bail-out. Sometimes sporadically, and often in response to stress, Pocillopora is noted along with a handful of other corals, to simply release it's polyps from the skeleton. The polyps then settle, begin calcifying a new skeleton, and perhaps many new colonies can thus be born. Fragmentation, as in many branching SPS corals, is a common method of reproducing colonies. Because of its relative hardiness in the reef aquarium, Pocillopora is now regularly offered for sale as captive bred specimens from several locations. My highly successful colony of P. damicornis has a light fluorescent halo at the tips of the polyps and was purchased from Scientific Corals in Atlanta. I have found that, in general, captive bred colonies are much hardier than wild harvested specimens.

In the aquarium, Pocillopora can adapt quite well to water movement and light conditions, provided water quality is excellent. There are some reports of those keeping Pocillopora under normal output fluorescent lights, but I do not think this is viable for the long term survivability of most species. In fact, almost all Pocilloporids grow the quicker and healthier when exposed to very strong but random water currents and high intensity lighting. It seems as though in this case, at least, water movement may actually be more important than lighting for the success of Pocillopora in captivity. In terms of aggression, these corals are fairly "middle of the road." Their sting is not great compared to many corals, but I have noticed some very "cute" sweeper tentacles where a lower branch of my colony has come very near a branch of Montipora digitata. The sweepers are elegantly thin and transparent, and about two or three times longer than a normal polyp tentacle. Pocillopora is more aggressive in terms of its ability to grow rapidly and crowd out and/or overshadow nearby species.

Some quick trivia about Pocillopora before moving on to more in depth discussions: There are several species of common communal crustaceans (hey! an alliteration! I love it when a writer's device like that just happens! Maybe I should try an onomatopoeia next?) that may be discovered within the branches of wild Pocillopora. Pistol shrimp (popping, snapping shrimp) are common inhabitants of the branches, using their claw-induced sonic shock waves to capture small prey while maintaining safe harbor in the branches. Other small communal shrimp may also be present. There are two types of crabs also worth mentioning. One type is presumably symbiotic. It maintains refuge within the branches, may eat parasites, bacteria and slime from the corals, or leftovers that escape the polyps. In turn, it chases away potential predators of the coral. The other type of crab is a mimic. It also inhabits the branches, but actually forages off the living tissue of the coral. It is up to the aquarium keeper to determine whether or not a found specimen is beneficial or not...but, be warned! I have found the incredible inter-branch dexterity of these crabs to be the equivalent of a sugar glider in the branches of trees. They are nearly impossible to get out without breaking branches of the coral. Perhaps close inspection prior to purchase would be warranted here!

Now, on to the topic du jour...or would that be topic du mois?

Pocillopora, while not a garishly colored coral, can be quite striking in its common pink variety. As I mentioned, Pocillopora is a commonly researched coral, so it should not be surprising that one of the first non-photosynthetic pigments to be isolated from a coral came from this genus. The pigment is called pocilloporin. Interestingly enough, pocilloporin is not only found in Pocillopora, but many corals. It is responsible for pink coloration and is often seen in Stylophora pistillata and Seriatopora hystrix, among others. This pigment is not strongly fluorescent, and so differs from the FPP (fluorescing pink pigment) that shows up strikingly under actinic lighting in other corals like Actinodiscus sp. (Mushrooms, Corallimorphs) and Trachyphyllia geoffroyi (Open Brain).

What is the function of pocilloporin then? Well, this is a good question. In fact, the entire topic of coloration in corals is a good question. I believe that the reef hobby is doing a tremendous job in elucidating some of the most cryptic patterns of coloration seen in underwater fauna. That aside, I now besmirch this tantalizing question until future articles in the series. Instead, I will concentrate on some basic information involving pigments in photosynthesis and coloration. Because of the uncharted territory involved in such discussion, I would like to offer the following: I have read a great deal of the research being done in this area, and I do not want to miss any points I may have overlooked involving current knowledge. If anyone has information that I have missed, or experiences in this area, I would really like to hear from you. I would also like to state that, despite my own small experimentation in this area, I am going to pose some questions and theories that will hopefully spur some feedback from my readers.

Most corals are photosynthetic and much of their primary coloration is a result of the pigments used by all photosynthetic cells. Some pigments are used directly in the photosynthetic process to capture light energy from the sun and transform it into organic compounds in the light reactions of photosynthesis. Other pigments are used indirectly as accessory pigments. Some have oxidative abilities and are used in the electron transport chain that is part of both the light and dark reactions of photosynthesis. Some even have duplicate functions adding other levels of function to the photosynthetic cell. Marine algae contain a preponderance of all pigments known to exist. Unarmored marine dinoflagellates (several species) are the marine algal symbionts known as zooxanthellae that take up residence within the gastric mesentery of the coral polyp. These algal symbionts have been covered in much depth by many authors and researchers.

Chlorophyll-a is a pigment largely responsible for green coloration. It is found in almost plant cells, including the zooxanthellae. It's primary absorption peaks are at 430 nm and 670 nm. corresponding to the blue and red areas of the spectrum. The fact that it absorbs these colors, means it reflects the colors it does not absorb, hence the perceived color of green. The pigment itself is not green, only its appearance. This is an important lesson in perception. Under different types of light, the perceived color of an object may be remarkably different than the wavelengths it absorbs and reflects. Color is a complex phenomena that not only involves actual light absorption, but also our sense and perception of the color. The fact that absorption is different from reflection, and that the many pigments present in various quantities can absorb several wavelengths further complicates matters. Finally, pigment absorption spectrums are commonly carried out after elucidation and dissolution in a solvent. The use of solvents, such as alcohol, may skew the actual absorption peaks present in the cell.

Carotenoids are accessory pigments that are responsible for predominantly yellow and orange coloration. Their color is usually masked by the presence of chlorophyll. They are present in all photosynthetic cells. If chlorophyll disintegrates for any number of reasons, these pigments may be oxidized to a red state. Most commonly seen in trees turning color in the fall, this oxidation has other implications in the marine environment to be covered in future articles. Carotenoids are composed of carotenes and carotenols (xanthophylls). Carotenes have secondary functions as antioxidants. Xanthophylls consist of oxygenated carotenes such as neoxanthin, violaxanthin and lutein, all which have provide characteristic coloration through absorption, either functionally or incidentally. Fucoxanthin is a yellow pigment with primary absorption around 530 nm.

Phicobilins are pigments first discovered in red marine algae. Primary types include the phycoerthrins, red pigments that improve and widen the absorption of chlorophyll in the middle of the spectrum. Blue pigments, called phycocyanins absorb at about 630 nm and narrow the absorption gap in chlorophyll-a in its respective spectral area.

Flavins and flavenoids are yellow pigments that function as redox intermediates in respiration. They also contribute to color. Black and brown coloration is provided from the oxidation of the flavenoids and related phenolic compounds.

The anthocyanins are pigments that are the most important in providing coloration to flowers. Although I have not found sources indicating their presence in the zooxanthellae, the significance of these pigments will present themselves in a possible explanation of the adaptive mechanisms of colored corals. They provide many of the red, blue and violet colors of flowers by their mixing effects.

Other accessory pigments found in photosynthetic cells are peonidin, delphidina, cyanidin, flavanol, aurones, chalkones, and pelargonidin. They all contribute to the photosynthetic process and to coloration in distinct ways.

Quinones consist of over 200 pigments associated with colors ranging from pale yellow to black. From the obviously enormous palette of pigments found just in the photosynthetic cells, it is apparent that coloration is an incredibly complex phenomena, and that not only are there functional aspects to these pigments to the photosynthetic reactions, but there are also adaptive functions to the organism as a whole. There are probably scores of unknown functions, as well.

The following is a table of pigments found to be contained within all the zooxanthellae, comprising several species of dinoflagellates:

Color Name Absorption
peak
(nm)
orange beta carotene 447, 449
blue-green chlorophyll a 662, 429
orange-yellow unknown  
yellow diadinoxanthin 476, 446
brick-red peridinin (64%) 472
light green chlorophyll c 627, 578
pink unknown 464
yellow dinoxanthin 470, 440
brick-red neo-peridinin  

and within all photosynthetic dinoflagellates: pyrrhoxanthin astaxanthin peridininol diatoxanthin pyrrhoxanthinol P-457 ( a unique hexoside) fucoxanthin venificum aureolum gyroxanthin

There are also combined units of carotenoid-chlorophyll-protein complexes (PCP complex) consisting mainly of peridinin, chlorophyll a, and one of 12 to 20 proteins all forming unique complexes that can alter the ultimate coloration of the living coral through the zooxanthellae. The sheer number of substances found solely in the dinoflagellates is daunting enough. But, there is more to the story.

Also present are many pigments found in corals that are non photosynthetic in nature. Pocilloporin is a pink pigment, absorption of 560 nm, that is found in many SPS corals whose function remains unclear, but seems to serve as an adaptive mechanism that ensures the superiority of pocilloporin containing colonies over non-pocilloporin containing colonies, albeit for presently unknown reasons.

MAA's are clear substances that function as UV protectants for some highly photoadapted species with an absorption peak of 320 nm. It may be through their effect on altering the UV spectrum that certain pigments can predominate in function, causing a change in color. GFP, or Green Fluorescing Protein, is found in many corals and may be linked to photoadaptation to UV wavelengths. It also has the ability to alter the light reaching the chromophore and affects colors throughout almost the entirety of the visible spectrum. There are also PFP's, or Pink Fluorescing Proteins, that predominantly cause a pink color to coral's that is an entirely different pink pigment from pocilloporin and other pink accessory photosynthetic pigments. It appears that these fluorescing proteins are indirectly involved in photosynthetic pathways, therefore they may also be a type of accessory pigment.

Finally, there are other pigments, such as the purple pigment found within the tissue of certain species of gorgonians (used as a dye by many indigenous peoples) along with many other substances in specific species, or in general, whose function is much less clear. This is not to mention a potential myriad of pigments of unknown in structure and function, and perhaps as of yet completely unidentified. Some examples are the pigments responsible for the color of non-photosynthesizing corals such as Tubastrea and Dendronephthya.

Early research showed the presence of non photosynthetic pigments in SPS corals with still unidentified names corresponding to their absorption peaks, named P-440, P-480, P-500, P-560, and P-590. P-480 was found to be the predominant color of Pocillopora.

With such an array of agents responsible in countless interactions and complex amounts, it is little wonder that we marvel at the splendid wash of colors found within the coral community and, indeed, in our own reef aquariums. It should also be obvious that an understanding of the processes and requirements of these colorants whose existence depends on the presence of light, may "illuminate" some questions as to the nature of maintaining colors in our tanks...and why it may or may not be possible. At the very least, corals seem to have the proverbial "bases covered" when it comes to maximizing light absorption. And then some.

Next month, I will describe another species in depth, and it's relevance to the ongoing exploration of the mysteries of coral coloration.

Eric Borneman

PCF Coral or EricHugo@aol.com

References:

Bingman, Craig. Green Fluorescent Protein. Aquarium Frontiers. Summer, 1995, pp. 6-9.

Fossa, Sven A., and Nilsen, Alf Jacob. The Modern Coral Reef Aquarium. Birgit Scmettkamp Verlag, Germany, 1996.

Foyer, Christine. Photosynthesis. Wiley and Sons, New York, 1993.

Goodwin, T.W. Chemistry and Biochemistry of Plant Pigments. Academic Press, New York, 1965.

Gregory, RPF. Biochemistry of Photosynthesis. 2nd edition, Wiley and Sons, New York, 1977.

Jeffrey, S.W. et. al. Chloroplast Pigment Patterns in Dinoflagellates. Journal of Phycology. 1975: Vol. 11, pp. 374-84.

Johansen, J.E., Svec, W.A., et.al. Carotenoids of the Dinophyceae. Phytochemistry. 1974: Vol. 13, pp. 2261-71

Nielsen, E. Steeman. Marine Photosynthesis with Special Emphasis on Ecological Aspects. Elselvier Scientific Publishing, Amsterdam, 1975.

Rabinowitch, E. and Govindjee. Photosynthesis. Wiley and Sons, New York, 1969.

Shibata, Kazuo. Pigments and a UV-absorbing Substance in Corals and a Blue-green Alga Living in the Great barrier Reef. Plant and Cell Physiology. 1969: Vol. 10, pp. 325-35.

Spector, David L. Dinoflagellates. Academic Press, New York, 1984.

Takabayashi, Misaki, et.al. Isolation and Partial Characterization of the Pink and Blue Pigments of Pocilloporid and Acroporid Corals. Biologic Bulletin. 1995: Vol. 189, pp. 288-97.

Veron, J.E.N. Corals of Australia and the Indo-Pacific. University of Hawaii Press, Honolulu, 1986.

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