"In the end, we will conserve only what we love, we will love only what we understand, and we will understand only what we have been taught." Baba Dioum, In Nature


The wise prelude above offers, I think, elegant words for a simple philosophy that most of us practice. Some of us love what is popular, and conformists are taught that popular is what we want. If you ask a general question on most 'Net message boards about what to keep in a new aquarium, or if you browse the online and printed picture galleries in our hobby's literature, you will see the same reef species and the same aquascape styles over and over again. Traditional choices are so pervasive, yet beautiful, that most of us accept these familiar organisms and designs without exploring beyond them for newer or more creative options. Mind you… this is not a matter of right versus wrong or good versus bad display choices. But simply stated, I am sure many folks would readily try alternate specimens, designs and techniques if only they were encouraged to do so. There is far more to the aquarium hobby than keeping the traditional staples.

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The topic of this article, as you can see, is fluorescence. I dare say it's an issue about which most aquarists feel they understand what "little" there is to know. Some corals fluoresce and some don't, right? Certain light bulbs produce more dramatic colors and "effects" of fluorescence than others, right? Yet the truth of the matter is that what we have been conditioned to expect from fluorescent organisms under popular hobby lamps (blue-weighted) is merely the tiniest fraction of what is actually going on with fluorescence! Whether you are a casual aquarium hobbyist, a scientist or simply an intelligent and curious reef aquarist, the world of fluorescence is amazing and remarkable to explore with applications to satisfy aesthetic as well as academic interests.

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Sarcophyton: a comparison - with and without daylight.

I think it is safe to say that, to many people, reefkeeping is truly a marriage of scientific and avocational interests. The complexity of relationships between the organisms we choose and other life forms, their environment and the hardware that supports them, necessarily drives us to want to learn and observe more and more all the time. To the point, we (reef aquarists) have both the potential and the drive to investigate some amazing dynamics of aquatic science through our hobby. With proper instruction we can do it with an elegant weave of chemistry, physics and biology applied in our husbandry. In many ways some of us are practicing amateur scientists. And we have tremendous resources, namely livestock and disposable income, that are a unique treasure for work in science at large to utilize.

Quite a few professional scientists know this to be true; we count many among our hobby's friends who eagerly work, or otherwise interact, with the private aquarium community. Some hobbyists had the pleasure to see Dr. Charles Mazel, for example, present the topic of fluorescence at the 2004 IMAC conference. We were truly inspired to hear what Dr. Mazel had to say, and I think he was impressed in kind by the knowledge and potential of reef hobbyists that he met. The equipment I use to study and photograph fluorescence on land and under the sea is from his company www.nightsea.com. Dr. Mazel has spent a lifetime on his work with coral fluorescence. His research is fascinating, his passion for the sea apparent, and his products are offered from a love of studying the same organisms that reef hobbyists admire. Please be sure to explore the resources and references of Mazel, et al that I have cited below. They contain a lot of fascinating reading.

In this article I intend to detail some of the benefits to aquarists of observing the fluorescence of marine organisms. My hope is to inspire folks to explore even just a tiny bit more of the potential for discovery, wonder and enjoyment in the learning centers that we call reef aquariums.

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With tentacles extended or not, this Sarcophyton makes quite an impression by night.


So what exactly is fluorescence? For starters, not all things that luminesce in our aquaria are fluorescing. Some organisms bioluminesce, such as the small worms or Ophiuroid starfish that we see suddenly beaming brightly at night when stimulated. Bioluminescence is a product of a chemical mix that produces light. One form of energy is converted into another. We see this in some bacteria and among various macroscopic life forms. Some fishes and squids house such bacteria in symbiosis (e.g., "Flashlight" fishes). The process of bioluminescence can be intracellular or extracellular, but its chemical origins distinguish it from fluorescence.

Another form of light emission that we see, in minerals, for example, is phosphorescence. Phosphorescence is the glowing emission of light from absorbed radiation (a source of excitation). Unlike fluorescence, though, the glowing emission (phosphorescence) continues for a period of time after the source of light energy has ceased.

There are other "shiny" issues, too, that we must contend with. Some of the very colorful reef creatures have no remarkable distinction under a light that excites fluorescence. This is often because of still different artifacts in how we perceive color and light… namely reflective or iridescent qualities of various structures (spicules, e.g.), pigments and scales.

But fluorescence, in layman's terms, is the absorption of light at one wavelength and its re-emission at another (without heat). Note: the difference in wavelengths (re-emission) is an important distinction here from mere reflection. Fluorescence changes the wavelength (color); reflection does not. Yet not all fluorescence is apparent to the naked eye; hence the excitement you hear from folks fascinated with the tools used to help see and record fluorescence in reef organisms.

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This Tongan Acanthastrea is so unremarkable by day that you'd easily swim by its discreet brown visage. But at night with fluorescence... it comes alive on the reef!

Seeing Fluorescence:

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The fluorescing visible in this nudibranch is two-fold: the slug's own green pigments, and the red color of the algae in its 'gut' (tassels of cerata).

Indeed, many corals have no hint at all of (visible) fluorescence by daylight. But seeing the "big picture" with fluorescence is not about daylight, or even reef corals solely. Sponges, algae, worms… even some fishes and many other organisms will fluoresce with the right source of light and filters to see or record it. This is one of the first things to really astound aquarists when they begin exploring with fluorescence tools. Everything that has been so familiar to us - diving reefs by daylight (or night with a flashlight) and viewing our aquariums under traditional hobby lamps - is cast in a dramatically different light, quite literally, in the dark with fluorescence. You feel like you are exploring an unseen new world because, in a very real way, you are.

While it's true that green is the most common fluorescence color, fluorescence explorations reveal a bounty of other magnificent colors in red, orange, yellow and cool spectrum colors alike. One of the very first things to impress aquarists is… red. Red, red… crimson red, everywhere! The reef is covered and coated in a most beautiful red color from the fluorescence of chlorophyll (the primary light-absorbing pigment in plants and algae). Interestingly, a number of gorgeous Xeniids such as the blue, green or silvery pulse corals also reveal a magnificent crimson color (photo below); it's quite a stark contrast to what we are familiar with seeing in such soft corals. Some feather dusters that do not appear to have any remarkable color by day also come alive in fluorescence. These are but a few examples.

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To better view fluorescence in aquaria, we need a proper source of light (a specific excitation spectra) to stimulate the fluorescent emissions. Special yellow glasses will filter the extraneous light for blue light fluorescence. You'll also want to turn off aquarium lamps and indirect room lights. This is even more important for your camera, which does "see" some reflected light that we cannot. For imaging, a barrier filter is likely available for your camera if your lens is threaded. Otherwise, get creative on a do-it-yourself project to capture the weakly fluorescing emissions. Thus, with glasses for viewing aquaria, or a lens to fit over a diver's mask, and a filter for the camera and flash… all eyes can now see the bigger fluorescence picture!

All we need now is a source of excitation (other than Rod Stewart classics and too much wine). For this, our traditional actinic lamps and blue spectrally weighted bulbs are useful, but specialized excitation filters for camera flashes and intense LED flashlights can provide a much more focused source of excitation. Photo tip: you may need to leave a tiny bit of lingering daylight (a weak desk lamp, for example) or red light on in the room for your camera's lens to focus on objects; the requirements vary by camera or lens. Best fluorescence viewing, however, is done through the yellow glasses/lenses in darkness with a focused source of (blue light) excitation such as a flashlight and/or filtered camera flash. Once the glasses, lenses/filters and lights are all in place, the fun begins! Be prepared though… fluorescent colors are quite vivid; it's an aspect of their very narrow wavelength (a rather pure spectral product).

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The tools used to assist the human eye and camera lens to see and record fluorescence are
rather affordable, with supplies ranging from roughly $100-300.

Why Fluoresce?

Coloration in reef organisms at large is quite a complex phenomenon. Much attention has been paid to such issues by aquarium hobbyists and researchers alike. The matter is complicated further by the very limitations of how different people interpret the same colors. That, however, is a matter of psychology, to some extent.

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This gorgeous corallimorpharian could not be much less attractive by daylight! Very pleasant discoveries in a whole new realm appear when using fluorescence imaging tools.

We know that corals specifically produce proteins that fluoresce while hosting symbionts known as zooxanthellae. The fluorescent proteins (FP) may be utilized in part to reflect or re-emit light as needed. Whether you know it or not, most aquarists are quite familiar with this process. But first, let's examine the purpose of FPs.

Numerous organisms that might not otherwise be discovered in the recesses of the reef, such as this tiny feather worm, are apparent with fluorescence tools.

Some pigments that make corals appear colorful to our naked eye may be used to reflect excess/"unwanted" light away from shallow-water adapted corals. Or, if the proteins are fluorescing they are probably not "reflecting," but rather the pigments absorb (and then fluoresce) to remove the excess light. In lower light, such absorption and re-emmision expands the spectral range of useful light (range of PAR) for the hosted endosymbiotic algae.

Notice the worms fluorescing just in front of the Acanthastrea polyp.

In either capacity, the presence of FPs produces the re-emission of sharp spectra of light that we can see as fluorescing. So, when truly shallow-collected corals are no longer kept under adequate light (less intense/less UV… whatever) they may "lose color." What we are then seeing, at least in part, is the reduction of FP density in the absence of peak light: a significant biological savings. Similarly, some deep(er) water corals - such as blue or green Plerogyra corals or bright green Nemanzophyllia Fox corals - may lose color when placed under brighter lights. They too are reducing their density of FPs and may appear browner colored from the now more apparent and "unmasked" populations of (brown) zooxanthellae. These are not the only possible reasons for various corals losing color, but they are influences that often contribute.

Whichever, if any, of the above-listed roles FPs play - reflector or light-harvester - the application of specific qualities of light stimulates their production and presence. Much to the chagrin of reef gardeners, the exact light required for producing the beautiful range of colors seen on a reef varies for different corals with different FP. This is but one of the many challenges of maintaining a garden reef aquarium under standardized parameters of light, water flow, feeding, etc. It is also one of my favorite rants to fellow aquarists in encouraging more folks to get away from impractical, if not impossible, garden reef aquaria with very unnatural mixes of specimens from different oceans or parts of the reef. I'm encouraging friends to focus on more natural and compatible groupings, if not specific biotopic displays. Well… it's either that, or I'd at least like to hear less folks complaining that all their corals are not all thriving with all of the original colors they were purchased with… in their homogenized garden reef display. Sheesh! It's an unrealistic expectation.

Frequent readers of my writings might recall the popular example that I like to use to illustrate this dilemma: Acroporids. The family Acroporidae is… er, huge. And an aquarium dedicated to keeping so-called SPS (small polyped stony) corals even so "specifically" as Acroporids is really not so specific at all. Diving the reefs reveals that the two genera from this family that are principally used in the hobby, Acropora and Montipora, largely contain species that have such widely differing requirements that it is no wonder that members of both groups in the same aquarium usually do not fare equally well in color or growth. The optimal water flow and lighting requirements are really quite different among members of Acroporidae and many commonly kept (together) corals in general.

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Reef aquarists often realize with chagrin that the lighting requirements of even related family members like Acropora and Montipora are so very different as to stymie the optimal keeping of both groups in the same aquarium.
Photos courtesy of Robie Sayan (ROBZ).

Uses for Fluorescence Imaging Tools:

One of the many interesting uses for fluorescence technology is for studying the early life history of settled cnidarians (Mazel, C. H., M. P. Strand, M. P. Lesser, M. P. Crosby, B. Coles, and A. J. Nevis, 2003). This one has applications for coral farmers and reef scientists alike. With specialized underwater optics and imaging equipment, we can observe and record specific fluorescence and reflectance patterns of rather minute or diminished emissions. That is to say that by noting unique occurrences of distinct emissions of light, we can spot the very tiniest and earliest stages of cnidarian (and other fluorescing organisms') settlement. I must admit, this was one of the first surprises that I had when exploring my tank with NightSea goggles and a flashlight. Not only could I spy the almost microscopic settlement of hundreds of planulated Pocilloporid larvae (albeit not an uncommon occurrence in aquaria) sooner than usual, but I also noticed tiny buds of several other corals in the display from which I had never harvested polyps! It made me wonder if other coral polyps or planulae have been produced regularly in the past, but were so tiny and unseen that they were encroached upon and killed by other organisms before they could grow large enough to be observed and collected. Arghhhh! Those blasted so-called "reef-safe" shrimps and crabs!

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Nearly microscopic stages of settled larvae can be observed with proper fluorescence tools long before they can be spotted with the unassisted eye. Pocillopora damicornis, pictured here, planulates regularly in captivity.

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Some cnidarians which are less colorful than mud by daylight, such as these brown zoanthids, are remarkably fluorescent!

You can imagine just the same that researchers can scan tracts of wild reef and interpret the data with software to get a far more accurate assessment of species representation than divers who tediously (and at great expense) "eyeball" the reef to do surveys. It's a fluorescent fingerprint, so to speak, used for mapping and assessing the world's reefs! However, it is true that many fluorescing cnidarians share the same or similar fluorescing characteristics. But many fluorescent emissions are still unique and useful to observe.

Another fascinating use for fluorescence technology is for studying the causes of coral "bleaching." There seems to be a correlation between corals with higher FP densities and lower incidences of photoinhibition (Hardy, J. T., F. E. Hoge, J. K. Yungel and R. E. Dodge, 1992). Thus, FP densities may be a reliable indicator in the identification of key morphs of the same species that endure light stress better than others. As we know, other environmental factors such as temperature increases are also stressful influences on bleaching events. Identifying the mechanisms that subsequently force the expulsion of FP can be key in understanding some bleaching events. The employment of fluorescent imaging technologies is a nondestructive means of measuring such events and the variously stressed photochemical efficiency of some zooxanthellate organisms.

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The crystal clear tentacles of this nocturnal Dendrophylliid cup coral are nearly impossible to spot at night by (white) flash light, yet stand out strikingly with fluorescence in the recesses for the curious explorers.


So what does it all mean for the reef hobby? Good fodder to ponder, yes. It's intriguing science for the more curiously minded aquarists. Beyond the novelty and aesthetic of using such tools, we can discover dimensions of life forms, new and unseen, in our aquariums that we did not even realize existed. For even more beautiful images of fluorescence, see this month's ReefSlides here.

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Corallimorphs that are beautiful by day can be equally attractive when fluorescing.

As for the controversial issues of keeping or cultivating select colors in corals… it really is quite a conundrum. The practical application of light over typical reef aquaria alone is nearly impossible to standardize. With regard for unstable water clarity, inconsistent delivery of light (clean or dirty paths between emission of light and the photosynthetic creatures… namely dust, dirt and debris on lamps, lenses and aquarium covers for starters), lamps aging and spectral shifts, plus more than a few other influences, we may never be able to thoroughly predict artificial reef lighting to finesse coral coloration optimally.

Coral coloration (within reason) is not even a very accurate if at all meaningful indicator of health. In fact, there may be no function to some coral coloration at all! Some theories indicate that it may all have more to do with the "properties of the pigments, with the color being an adaptively neutral by-product." (Mazel, C. H., and E. Fuchs, 2003) If so, it puts an interesting spin on the evolution of higher order species and their coloration. It reminds me, in fact, of a fascinating theory that Eric Borneman was relating to me some years ago that, to summarize, "what if" colorful reef fishes and other motile creatures evolved their gaudy colors and patterns as more of a means of camouflage (!) against the already gaudy and colorful invertebrates and lower order organisms? Perhaps adding credence to this notion is the lack of any strong evidence that many, if any, reef fishes can see fluorescent emissions preferentially. We must not forget, too, the sometimes-great difference between human-perceived color and what many sighted reef creatures are, or are believed to be, seeing.

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A gorgeous free-living Mussid polyp in my nano-reef.

In hindsight, though, I frankly don't care if my shrimp, fishes or crabs agree with me on the creatures I keep and which they think are pretty. I am attracted to this hobby in large part for the aesthetic beauty of the creatures we study. Fluorescence is but one beautiful dimension of that study. Enjoy, my friends.

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For taking pictures of fluorescence in aquaria, always use a tripod and somewhat longer shutter speeds. Experiment with consideration of the subject's movement, and turn off the pump's water flow temporarily while photographing. Also, use the strongest flash you can find with an exciter filter to stimulate fluorescence. Use two flashes if you can… the more excitation light available, the better!

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A photo tip for capturing fluorescent images: use a somewhat faster film speed so that you can increase
the depth of field in such low light environments (darkness is best for viewing fluorescence); perhaps as fast as
400ISO to start with for beginners.

All photos copyright Anthony Calfo, except where otherwise noted.

* Note: if you really have some time to invest or kill… do a general www.Google.com search for "coral fluorescence" and note how many hits are returned! Then, shift and refine your search with an http://scholar.Google.com search and see how many even still are reported: an amazing list of content pages.

If you have any questions about this article, please visit my author forum on Reef Central.

Bibliography and Recommended Reading:










Delvoye, L., 1995. The histological basis of tissue fluorescence in the hermatypic coral Agaricia agaricites (Linnaeus, 1758), in Proc. Sixth Intl. Conf. on Coelenterate Biology, pp. 143-150.

Doubilet, David, 1997. A new light in the sea. National Geographic, August, 192: 32-43.

Drafahl, Jack, and Sue Drafahl, 1999. Photographing fluorescent corals. Skin Diver, April, 24:25.

Fuchs, E., and C. H. Mazel, 1998. An experimental method to separate the fluorescence and reflectance components of the spectral signatures of corals. Proc. Ocean Optics XIV Conference, Hawaii.

Fuchs, E., and C. H. Mazel, 1999. Unmixing coral fluorescence emission spectra and predicting new spectra under different excitation conditions. Applied Optics, 38:486-494.

Gilmore, A. M., A. W. D. Larkum, A. Salih, S. Itoh, Y. Shibata, C. Bena, H. Yamasaki, M. Papina, and R.Van Woesik, 2003. Simultaneous time resolution of the emission spectra of fluorescent proteins and zooxanthellar chlorophyll in reef-building corals. Photochem. Photobiol., 77:515-523.

Gross, L. A., G. S. Baird, R. C. Hoffman, K. K. Baldridge, and R. Y. Tsien, 2000. The structure of the chromophore within DsRed, a red fluorescent protein from coral. Proc. Natl. Acad. Sci. USA, 97:11990-11995.

Hardy, J. T., F. E. Hoge, J. K. Yungel and R. E. Dodge, 1992. Remote detection of coral 'bleaching' using pulsed-laser fluorescence spectroscopy. Mar. Ecol. Prog. Ser., 88:247-255.

Heikal, A. A., S. T. Hess, G. S. Baird, R. Y. Tsien, and W. W. Webb, 2000. Molecular spectroscopy and dynamics of intrinsically fluorescent proteins: Coral red (dsRed) and yellow (Citrine). Proc. Natl. Acad. Sci. USA, 97:11996-12001.

Logan, A., K. Halcrow and T. Tomascik, 1990. UV excitation-fluorescence in polyp tissue of certain scleractinian corals from Barbados and Bermuda. Bull. Mar. Sci., 46:807-813.

Lukyanov, K.A., A. F. Fradkov, N. G. Gurskaya, M. V. Matz, Y. A. Labas, A. P. Savitsky, M. L. Markelov, A. G. Zaraisky, X. Zhao, Y. Fang, W. Tan, and S. A. Lukyanov, 2000. Natural animal coloration can be determined by a nonfluorescent green fluorescent protein. J. Biol. Chem, 275:25879-25882.

Manica, A., and R. W. Carter, 2000. Morphological and fluorescence analysis of the Montastraea annularis species complex in Florida. Mar. Biol., 137:899-906.

Matz, M. V., A. F. Fradkov, Y. A. Labas, A. P. Savitsky, A. G. Zaraisky, M. L. Markelov and S. A. Lukyanov, 1999. Fluorescent proteins from nonbioluminescent Anthozoa species. Nature Biotechnology, 17:969-973.

Mazel, Charles, 1988. Underwater fluorescence. Sea Frontiers, 34: 274-279.

Mazel, Charles, 1991. Black night, black light: underwater fluorescence. Ocean Realm, summer: 63-68.

Mazel, C. H., 1995. Spectral measurements of fluorescence emission in Caribbean cnidarians. Mar. Ecol. Prog. Ser., 120:185-191.

Mazel, C. H., 1997. Coral fluorescence characteristics: excitation - emission spectra, fluorescence efficiencies, and contribution to apparent reflectance. Ocean Optics XIII, SPIE Vol. 2963:240-245.

Mazel, C. H., 1997. Diver-operated instrument for in situ measurement of spectral fluorescence and reflectance of benthic marine organisms and substrates. Optical Engineering, 36:2612-2617.

Mazel, C. H., 2001. Guide to Underwater Fluorescence Photography. www.Nightsea.com

Mazel, Charles, 2002. Neon photography: Shedding new light on the reef. Advanced Diver Magazine, issue 6. [Reprint available on-line.]

Mazel, C. H., and E. Fuchs, 2003. Contribution of fluorescence to the spectral signature and perceived color of corals. Limnol. Oceanogr. 48:390-401.

Mazel, C. H., M. P. Lesser, M. Y. Gorbunov, T. M. Barry, J. H. Farrell, K. D. Wyman, and P. G. Falkowski, 2003. Green-fluorescent proteins in Caribbean corals. Limnol. Oceanogr. 48:402-411.

Mazel, C. H., M. P. Strand, M. P. Lesser, M. P. Crosby, B. Coles, and A. J. Nevis, 2003. High resolution determination of coral reef bottom cover from multispectral fluorescence laser line scan imagery. Limnol. Oceanogr. 48:522-534.

Mazel, C. H., T. W. Cronin, R. L. Caldwell, and N. J. Marshall, 2004. Fluorescent enhancement of signaling in a mantis shrimp, Science, 303:51.

Salih, A., A. Larkum, G. Cox, M. Kuhl and O. Hoegh-Guldberg, 2000. Fluorescent pigments in corals are photoprotective. Nature, 408:850-853.

Schlichter, D., H. W. Fricke and W. Weber, 1988. Evidence for PAR- enhancement by reflection, scattering and fluorescence in the symbiotic deep water coral Leptoseris fragilis. Endocyt. C. Res., 5:83-94.

Vermeij, M. J. A., L. Delvoye, G. Nieuwland and R. P. M. Bak, 2002. Patterns in fluorescence over a Caribbean reef slope: the coral genus Madracis. Photosynthetica, 40:423-429.

For fluorescence tools, images/info and information:

Dr. Charles Mazel
20 New England Business Center
Andover, MA 01810 USA
877 436-9262 (toll free)
978 685-6410
Fax 978 689-3232

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Magnificent Fluorescence! Aquaristic Perspectives by Anthony Calfo - Reefkeeping.com