Reef Food

This month's article varies from the subject of corals, strictly, and is more concerned with coral reefs, in general. I have fielded many questions in "The Coral Forum" about feeding corals, and have provided talks to several groups on coral nutrition. I plan to discuss this in more depth in upcoming articles, but felt an overview of nutrition to a coral reef community and coral reef aquarium would be a beneficial prologue to develop a more complete view of the subject.

A coral reef supports a tremendous variety of life, all of which are dependent on energy sources for their survival, growth and reproduction. There are two basic types of organisms in terms of their method of gaining energy: heterotrophs and autotrophs. Autotrophs are the primary producers; they use sunlight, converting its energy through photosynthesis into energy rich products (reduced forms of carbon, usually in the form of simple sugars) that are used by the organism. In this way, they form the beginning of the food chain, as they are the original or primary source of dietary energy for all other organisms. Photosynthetic bacteria or cyanobacteria, may also be considered to be primary producers, and their biomass on and near coral reefs, including in the water column, is enormous. Heterotrophs are those organisms that must attain at least some nutrition from feeding or absorption to acquire a reduced source of carbon. Even primary producers need more than sunlight to survive, and this is part of a great misconception; that being, that autotrophs can "do it all." Consider the houseplant that dies without nutrients from soil or fertilizer; it obviously needs additional nutrients besides light and water. Consider, as well, that fertilizers and soils are commonly described by their nitrogen and phosphorus content; these are also among the most important nutrients required by heterotrophic organisms. The main difference between autotrophs and heterotrophs is not that one "eats" and the other "just needs sun," but that one can provide various amounts of required carbon by using light energy.

The word "nutrient" is often misunderstood. The terms "high nutrient" and "low nutrient" can be taken in many contexts. In general, nutrients are those organic and inorganic compounds necessary to sustain life. While this comprises a very large group of potential compounds, nutrients are often simplified in terms of those elements that are major "building blocks" for fats, amino acids, and carbohydrates. Furthermore, they are frequently those elements which tend to limit further growth by their availability and ability to be procured. In general, carbon, nitrogen and phosphorus are often used to describe the "nutrient" condition of reef organisms (and others, as well). Plants and animals with photosynthetic symbionts tend to be nitrogen and/or phosphorous limited under normal conditions, since photosynthesis usually provides non-limiting carbon source material. Coral reef waters are typically "nutrient poor" as they contain very low levels of nitrogen and phosphorus (they are both precious commodities and any excess is usually taken up quickly). In nearshore areas where there is significant organic loading from land runoff, waters tend to be rather nutrient rich. Both types of environments sustain their own flora and fauna with varying amounts of habitat overlap in terms of the organisms that can exploit the continuum of nutrient conditions. The nutrients available in water to coral reefs can be dissolved in the water, in the form of particulate material, or as living biomass.

The coral reef is a place of both high primary productivity and consumption of nutrients, with a great deal of nutrients being recycling within the community. For many years, coral reefs were thought to be "nutrient poor deserts." In fact, this is not the case. It would be a very poor assumption to imagine that any species-rich community was not highly dependent on nutrients. While measurement of the water column shows it to be relatively devoid of organic and inorganic dissolved nitrogen, carbon and phosphorous and, therefore, "nutrient poor," it is largely because of the efficiency of the reef community that such water conditions are attained. Waters around coral reefs are rich in nutrients in the form of various types of microplankton; these are largely removed by coral reef organisms. It should be noted that most of the plankton on coral reefs is produced by and lives within the reef or nearby communities, and is not borne into it in great quantities by the open ocean.

An adaptation that has allowed for such diversity, to a large degree, is the symbiosis of animal and plants (algae and cyanobacteria) to make efficient use of each other's limitations. Such symbioses occur commonly in sponges, corals, nudibranchs, anemones, clams, hydroids, foraminiferans, and many other invertebrates that make up a large portion of the total reef community. These organisms are not autotrophs, no matter how efficient and substantive the contribution of their symbionts, and they must be fed. So, what do they eat?

Reef Food

Coral reef inhabitants have widely varied diets, and most aquarists are familiar with the often highly specific dietary needs of some of these animals. Motile invertebrates may be predatory, like fish. Others are scavengers of decomposing material, or they can be "filter-feeders" by any number of mechanisms. Some employ numerous methods of nutrient uptake. Both "filter-feeding" by passive means and active prey capture are used by many of the sessile invertebrates commonly maintained in aquaria. At various early stages of their life, the diet of reef organisms may require planktonic organisms, and they, themselves, may be planktonic at some part of their life. Some of these animals (and all algae) are also capable of acquiring nutrition through the absorption (or direct uptake) of dissolved organic and inorganic nutrients. Normally, the levels of these substances on a coral reef are very low, and such nutrients are often a limiting aspect of the growth of any one life form. Because of the number of species present on a coral reef, most any food source is often a source of fierce competition, even if not directly. Often, simple competition for space is enough to limit nutrient availability.

Nutrients enter a coral reef from a variety of sources. They can arrive from freshwater or terrestrial sources; rivers and rain can both wash land based nutrients out to sea. Cooler water from deep in the ocean moves upward, bringing nutrient rich water upwards to the reef. This water is nutrient-rich because of the "downfall" of organic material into ocean depths and a comparative lack of planktivory in the deep ocean compared to that which exists in the upper photic zones. Currents, tides, storms, and waves bring plankton and nutrients from various distances to wash back and forth over the reef. The production and waste material of the reef organisms also provide important nutrition to other animals on the reef, and they are part of what is know as the detrital food chain. Detritus, marine snow, particulate organic material, and suspended particulate matter are all names for the bits of "dirt" that flow around the reef; material that is composed of fecal material, borings, algae, plant material, mucus, associated bacteria, cyanobacteria and other particles. Decomposers (mainly bacteria and associated flora and fauna) break down waste material in the water, on the reef, and, primarily, in the soft sediments. The result of their presence and action is not only a food source in and of itself, but provides raw material for channeling back into the food chain, largely through the benthic algae and phytoplankton.

Phytoplankton are small unicellular algae, or protists, that drift in the water column. They may be very abundant in and around coral reefs, and they are capable of absorbing large amounts of organic and inorganic nutrients. When conditions are proper, they can reproduce very quickly, and areas of high nutrients will often have a greenish, reddish, or brownish, cast and lower water clarity, mostly resulting from high phytoplankton populations. Some of the reef animals can feed directly on phytoplankton; many soft corals, some sponges, almost all clams, feather-duster worms, and other filter feeders utilize phytoplankton directly as a food source. Small animals in the water column, termed zooplankton, also utilize phytoplankton as a food source. For the smaller zooplankton, phytoplankton and bacteria are the primary food source.

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Both of the photos above are from reefs on the Great Barrier Reef, Australia. The left photo shows the clear "nutrient poor" (oligotrophic) waters of the outer reefs. The right photo is of an inshore "nutrient rich" lagoon reef off Townsville. Notice how coral coverage in both systems is high, and even though the green phytoplankton-filled lagoonal reef is nutrient rich, it supports a high density of Acropora. Photos by Eric Borneman.

Zooplankton are of various sizes. Truly pelagic zooplankton constitute the vast majority of zooplankton in the ocean, but not on the reef, and are composed largely of the calanoid copepods and the larvae of many marine organisms. Zooplankton are larger than phytoplankton, and compose the primary diet of many marine and reef organisms, from fish to corals. Stony corals, for example, rely heavily on the capture of zooplankton to meet their energy needs. Zooplankton can be grouped into various categories, depending on size, location, behavior, and other characteristics. Larger pelagic marine organisms (such as fish, jellyfish and others), or those that are not associated with the water column (benthic animals such as echinoderms, crustaceans, mollusks, and others, can also be prey, or food, to various organisms. Contrary to what is commonly believed, there are many small benthic crustaceans, like some amphipods, that are not considered zooplankton as they do not migrate into the water column. However, demersal zooplankton, or those with vertical migration from the reef benthos into the water column (generally at night), primarily copepods and mysids, comprises the majority of zooplankton available to coral reefs.

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Prey capture of polychaetes by Diploastrea sp. at night at Tomia Island, Indonesia. Photo by Eric Borneman.

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These coral polyps are feeding on small crustaceans, probably mysids, which were swarming in the water column. Several captured crustaceans are visible. Photo taken in Cozumel. Copyright 1981, Ronald L. Shimek

Coral reef food sources, then, are largely produced by the ocean. Bacteria, detritus, phytoplankton, zooplankton, small benthic fauna, mucus, and dissolved organic and inorganic material of various types and sizes are what comprise the majority of food on a coral reef.

Are We, As Aquarists, Providing It?

In a word, No.

What we provide to, and what is provided by, our aquariums are extremely limited in both quality and quantity. Yet, many of us are troubled by high nitrate and phosphate readings. As a result, many aquarists resort to minimal feedings, in an attempt to keep water quality manageable. In terms of aquaria, which are closed systems, we do not have the luxury of billions upon countless billions of gallons of water to dilute and wash away high nutrient loads, nor do we have the bountiful biodiversity (for the most part) that maintains the "nutrient poor" water quality of a coral reef. In return, when our water tests "high" for nutrients, we are often plagued by those aesthetically undesirable organisms that are most adept at utilizing such resources as dissolved organic and inorganic material; the algae and cyanobacteria.

Filamentous, slime, smear, and macroalgae are highly efficient at absorbing such material, and they grow rapidly. In most circumstances, the microalgae and macroalgae, while very useful as part of turf scrubbers or small algal communities within a reef, often become problematic as they overtake the more aesthetically and, in some ways, functionally desirable crustose red algae (coralline), corals, and other sessile invertebrates. It should be noted, though, that these organisms might also be capable of significant nutrient uptake. Bacteria and phytoplankton are also extremely proficient at removing this material. All these organisms are quite valuable to our captive reef communities. They not only "purify" water by the utilization of nutrients, but also are all part of a beneficial food web, both in coral reefs and in aquariums.

Many elements are already drastically under-represented in aquaria, not only because of the limited size and productivity of the average reef aquaria, but because of the incredibly high bioload relative to the water mass present in even the most barren tank. I have used an analogy of how a reasonable facsimile of true natural bioload could be thought of as a one inch coral fragment in an Olympic-sized swimming pool of seawater. Even that example, relative to the oceanic volume, is probably "overstocked."

As a result of the often unnaturally elevated nutrient levels in aquaria, we employ a number of nutrient export devices, such as filters, ozonizers, and protein skimmers (foam fractionators). We also tend to add these devices to avoid or limit another common nutrient export mechanism, the water change. Unfortunately, it is a serious and probably deleterious compromise in many ways. Such devices actively strip the water column of the very bacteria, detritus, mucus, and plankton that exist, limiting the effectiveness of our captive community to deal with the nutrients and, in return, providing food sources within the food web. When the water column is "stripped" of its productive elements, the populations of filter feeding and predatory sessile invertebrates are compromised, as is the productivity of the substrate communities - including the live rock and live sand with their associated microbial, floral, and faunal components. However, if we do not "purify" the water, we may encounter nutrient problems and react with limited feeding schedules. It is quite literally a Catch-22.

In aquaria, we are faced with several realities. Our phytoplankton and zooplankton populations are generally negligible to non-existent in comparison with coral reef communities. Those which do exist are either rapidly consumed without having a chance to reproduce, or they are rapidly removed or killed by pumps and filtering devices or suspension-feeders. Coral mucus, bacteria, detritus, larval benthos and other "psuedo-plankton" might be present in a reasonable amount if the water column were not stripped. On the other hand, dissolved organic and inorganic material levels are frequently much higher than they are in the ocean. For an excellent, detailed analysis of sampled aquarium water refer to It's (In) the Water and It Is Still in the Water by Ron Shimek, Ph.D. Even very well maintained aquaria are generally found with much higher levels of nitrogen and phosphorous than wild communities. Even though many desirable organisms are able to utilize these nutrients, levels in most aquaria are very unnatural, and coral reefs under such conditions often wane or die - a process known as eutrophication.

It is the lack of water column-based food that results in limited success with the maintenance of some desirable animals, such as crinoids, flame scallops, clams, certain corals, sponges, bryozoans, and many other invertebrates. Even the symbiotic (zooxanthellate) corals suffer, despite many obvious long-term successes with these animals. However, sexual reproduction in corals is not common. Some of this may have to do with the lack of proper spawning cues (moonlight, temperature, etc.), and some may have to do with the small sizes of corals not being of sufficient area, age, or polyp density to be reproductively viable. However, heterotrophic nutrition, especially in the acquisition of nitrogen, is very important in gonad development, whereas the nutrition provided by the symbiotic algae (zooxanthellae) is largely used in their metabolic needs and growth through the production of large amounts of carbon. If we fed our corals more often, and with proper food sources, without the stress of being in a high nutrient environment, would we see more spawning events? Quite possibly.

Can We Provide Enough Food In An Aquarium?

Yes and No. To a degree, some of the limitations of a closed system are insurmountable. In a wonderful analogy using some feeding rate data of reef communities from scientific literature, Dr. Ron Shimek (Shimek, WMC 1998) noted how it would take 250-350 ml of wet food per 100 gallon of water per day to approximate food availability on a coral reef. Using a similar analogy, based on nutrient and water dwell times, I would add that the coral reef gets a 100% water change 2-3 times per day! This degree of nutrient availability and water exchange is coupled with the fact that we have relatively little data on the exact feeding requirements of various animals. However, we do know some specifics, and many generalities. For many filter feeders, it is not even so much the constituency of the food, but a requirement based largely on size. In other words, many filter-feeding and prey capturing animals will capture whatever particle size is manageable by the mechanics of water flow and capture mechanism only. Other animals are far more specific, and may depend on complex chemorecognition. Simple observation of the life in our tank gives us some clue that we are not providing the right stuff, or enough of it, and/or too much of the wrong stuff. Furthermore, we simply do not have access to many of the species that exist on coral reef. Yet, we do have access to many (often beautiful) species that, perhaps, we shouldn't, as they are still too difficult to maintain.

What can we do to make our situation better? There are many solutions. One way is to purchase a plankton net, and perform plankton drags in the ocean. However, this is not an option for those without easy access to the sea - and it is not very convenient, either. Still, I have found occasion to grab a net full of plankton on trips to the beach, and the animals one finds are simply fascinating. Another way to provide food sources is to culture plankton. It is certainly possible to begin producing batch cultures of plankton and/or plankton substitutes. Culture materials are generally simple, and various algae, rotifers, Artemia nauplii, ciliates, mysids, Gammarus, etc. are readily available and easy to grow. These food sources are not only nutritious inputs for reef aquaria, but may be enriched with vitamins, minerals, trace elements, medications, antioxidants, etc., and used as biocarriers of such substances. Cultured food sources, I feel, are far more valuable in both time and expense than many of the other products and devices we operate and use.

Our use of "live sand" has provided another important contribution to food sources. These areas are breeding grounds for many of the worms, crustaceans, microbes, and algae that later directly feed grazers and predators, or add food to the water column with their larvae and gametes. Furthermore, the action of the sand and live rock communities as decomposers and consumers of organic and inorganic material is invaluable. Live rock is also an important source of detritus and other reef food. We have also begun to make use of refugia, small areas or separate tanks separated from, but connected to, the main tank. Refugia provide areas where continual cultures of small flora and fauna can be produced without the intrusion of predators. I find refugia to be both fascinating sub-communities and very important for the main community.

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Notice the high level of particulate material in the water column; a perfect food source for this Sarcophyton sp. soft coral. Photo by Eric Borneman.

Finally, we can feed the tank more often with conventional foods. This is the area where the most care must be taken. One of the biggest problems with early aquariums was overfeeding, as there were not significant or sophisticated means of nutrient export, uptake, or recycling. Today, with reef systems and natural fish systems, there is significant decomposition occurring in the tank, without solely aerobic breakdown. Furthermore, nutrient export mechanisms, like protein skimmers, along with the "many mouths to feed" (in terms of the abundance of life forms other than fish), make overfeeding a less troublesome occurrence in today's aquaria. It is not, however, absent; overfeeding and poor nutrient management is still an area that could stand improvement. Often, conventional food is too large to be utilized by most reef organisms, except larger predators (brittle stars, fish, anemones, etc.) and large-polyped corals. Since the food is not alive, it starts to decompose immediately after being added to the aquarium, and will eventually be reduced into its constituent organic and inorganic components - substances of which we already have enough. Some of this material does, in fairness, contribute to larger populations of beneficial microbes and deposit feeders. But, it is a food source that is not self- limiting, and it is less desirable than live food cultures.

In terms of previously mentioned export mechanisms, it really does little good to be cultivating or adding more food material in the water column if it is all being rapidly removed by filtration devices. Live rock and sand provides abundant filtration, and some of the articles in past issues describing the set-up and use of unskimmed tanks are, in my experience, something that should be seriously considered. Algae Turf Scrubbers are also viable systems that provide low ambient water nutrient levels while maintaining higher amounts of food and particulate matter in the water. I also feel that if protein skimmers are used, they should probably be used in an intermittent fashion. I realize this is contrary to the advice that many others may offer, and it may sound like a reversal of thought and progression over the past year's trends towards increasingly efficient protein skimmers. However, I feel today's powerful skimmers are certainly able to provide adequate nutrient removal to maintain aquariums with very low nutrient levels without running "around the clock." However, I also think that as our understanding of the biology of captive systems and natural communities has increased, and our experiences have accumulated, that some important contributions may no longer be quite as important as they once were.

In fact, I feel the most beneficial nutrient export mechanism is the "old-fashioned" water change. Not only does this simple procedure remove excess nutrients and toxins, but also provides a more balanced replacement of water constituents to a baseline level. Yet, most of the "food items," such as plankton and particulate matter, are conserved in the remaining water, continuing to exist as both immediate food and as reproducing plants and animals. Filtering devices are not so gentle, as they process all of the tank's water over and over again. Most aquarists dread water changes, but they are simple, effective, and inexpensive. After having come full circle, I have found small water changes to be less work than what is involved with performing the additions, purchases, and maintenance of so many products and equipment available in the market. Furthermore, I have found mature, well managed, diverse reef communities to be fairly self-sufficient.

A Word of Advice and Experience

It has been my experience that the following pattern emerges among aquarists that begin "upping the volume" of food to their aquarium: Increased addition of prepared foods begins, followed with a concomitant and fairly rapid increase in measurable nutrient levels in the tank water. Soon thereafter, the aquarium begins to experience blooms and growth of cyanobacteria and filamentous algae. At this point, the aquarist typically ceases feeding at the increased rate, worried that the nutrient level will remain elevated and cause the demise of the health of the tank inhabitants at the expense of the algae. I stress that this is in all likelihood not the case. When first setting up an aquarium, levels of uptake and decomposition are low. As live rock "cycles," and dead plants and animals decompose, a nutrient spike is seen in all cases. Following this, various algal successions occur, usually in the order of diatoms, cyanobacteria, filamentous algae, and finally crustose coralline algae. Nutrient levels drop over time and the reef becomes a stable low nutrient place. The same process is occurring with increasing food sources to an aquarium. The nutrient levels spike, and various algal successions occur, until a new steady state is reached with a larger number and diversity of life than at the previous level. This process can take time, and food can be slowly increased over longer periods of time, allowing for such development to occur and bring measurable nutrient levels down to previous water column levels. It is my experience that perfectly "obscene" levels of food can be added to well stocked and diverse reef aquariums over time without high nutrient levels in the water column. To be sure, algae growth will also increase even over the long term with the added nutrient inputs, even though measurable levels are low. This is easily countered with the addition of more herbivores. Grazing has been shown to be the primary means of both filamentous and fleshy algae control on reefs. Ambient nutrient levels are far less important in algal-dominated reefs than the lack of herbivory. Even if a reef aquarium is highly mismanaged and has aberrantly high nutrient levels that result in prolific and undesirable algae growth, it can be controlled with additional grazing. However, I stress that such conditions may also act to the detriment of other organisms and is not encouraged. I make the point simply to illustrate the importance of adequate grazing.

Do We Need to Provide All This Food?

I think we do. There are many ways to do be a successful reefkeeper. I think such a diversity of thought and method should be encouraged. I also think the understanding and provision of proper food sources is an important and relatively recent school of thought in keeping aquaria; one that is just beginning to be realized by many. It is a key aspect of natural communities, and it has provided me with visible and tangible evidence of its importance in aquariums. I have crystal clear water and no problem algae with healthy fish and thriving corals. "So what," the reader may say, "Certainly the same can be said for those keeping stony coral galleries with powerful foam fractionators." Yes, it could. Indeed, I was once one of those people and I considered myself to have a very successful aquarium. But now, I have "reef snow" in my tanks, I have copious natural sponge growth, and I have communities of animals that never existed (or did not thrive) in the absence of these food sources. I also feel it is important to utilize food sources that provide maximal nutrition with minimal volume or unused components. In other words, high protein sources (e.g. "Golden Pearls") live or cultured live sources (e.g. Artemia, Mysis, rotifers), unicellular algal cultures (or live phytoplankton products such as DT's phytoplankton), and fresh whole food products (e.g. blenderized seafoods and algae), along with the intentional growth of a biodiverse community acting together as predators, prey, producers, and decomposers, is vital to success in keep coral reef communities in aquariums.

It is my personal belief that reef aquaria should be a thriving community of biodiversity, representative of their wild counterparts, and not merely a collection of pretty specimens growing on tidy clean rock shelves covered in purple coralline algae. By intentionally depriving many of these animals of natural food sources, I think we become lax in our responsibility, even if we did not spend money to acquire them. Dinnertime is a happy time for all, and nutrition is a universal requirement for survival. We may never be able to duplicate the coral reef, but we can get closer and closer as we learn more about closed systems and the natural communities.

Links to Part 2, Part 3, Part 4, Part 5, Part 6, Part 7

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

Some Further Reading

Anthony, K.R.N. 1999. Coral suspension feeding on fine particulate matter. J. Exp Mar Biol Ecol 232: 85-106

Bak, R.P.M. et al. 1998. Bacterial suspension feeding by coral reef benthic organisms. Mar Ecol Prog Ser 175: 285-288.

Bythell, J.C. 1990. Nutrient uptake in the reef-building coral Acropora palmata at natural environmental concentrations. Mar Ecol Prog Ser 68: 65-69.

Hamner, W.M., et al. 1988. Zooplankton, planktivorous fish and water currents on a windward reef face: Great Barrier Reef, Australia. Bull Mar Sci 42(3): 459-479

Hamner, W.M. and Carleton, J.H. 1979. Copepod swarms: attributes and role in coral reef ecosystems. Limnol Oceanogr 24(1): 1-14.

Hatcher, Bruce Gordon. 1988. Coral reef primary productivity: a beggar's
banquet. TREE 3(5): 106-111

Hatcher, B. G. 1997. Organic Production and Decomposition. In: Life and Death of Coral Reefs (Birkeland, C., ed.) Chapman and Hall, New York. 140-174.

Porter, James W. 1976. Autotrophy, heterotrophy and resource portioning in Caribbean reef-building corals. Amer Nat 110 (975): 731-742.

Sebens, Kenneth P. 1997. Zooplankton capture by reef corals: corals are not plants! Reef Encounter 21: 10-15

Sorokin, Y.I. 1995. Reef Environments. In: Coral Reef Ecology: Ecological Studies Vol. 102 (Heldmaier, G. et al., eds.). Springer Verlag, Berlin: 34-72.

Sorokin, Y.I. 1995. Plankton in Coral Reef Waters. In: Coral Reef Ecology: Ecological Studies Vol. 102 (Heldmaier, G. et al., eds.). Springer Verlag, Berlin: 73-126.

Sorokin, Y.I. 1973. Trophical role of bacteria in the ecosystem of the coral reef. Nature 242: 415-417.

Wilkinson, Clive R. 1986. The nutritional specturm of coral reef benthos; or sponging off one another for dinner. Oceanus 29 (2): 68-75.

Wilkinson, Clive R. et al. 1988. Nutritional specturm of animals with photosynthetic symbionts - corals and sponges. Proc 6th Int Coral Reef Symp 3: 27-30.

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