Reef Food

[jhnrb]

THIS SERIES OF ARTICLES IS NOT COMPLETE UNTIL THIS NOTICE IS REMOVED. THANKYOU FOR YOUR PATIENTS.


Reef Food PART-1

BY: ERIC BORNEMAN

This 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.

(CONT)

 

[jhnrb]

Reef Food Part-1 (cont)

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.

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.

(CONT)

 

[jhnrb]

Reef Food Part-1 (cont)

-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.

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.

(CONT)

 

[jhnrb]

Reef Food Part-1 (cont)

-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.

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.

END PART-1

cont. to part-2
POSTED jhnrb

 

[jhnrb]

Reef Food Part-2

From the Food of Reefs to the Food of Corals

by: Eric Borneman

-Food sources for corals.
Coral feeding is part of a well-orchestrated “three-part harmony,†because corals are supremely adapted to utilizing all manner of the available food sources on coral reefs. The three parts to this story, or harmony, are light, prey capture, and direct absorption. This month, I will cover only the first part, the nutritive aspects of light.

-The Energy of Light
Before becoming concerned about a repetition of a bevy of other articles on the subject by many authors, this will not be a discussion of aspects of lighting, qualities of light, suggestions for lighting, or anything of that nature. Perhaps such subjects are interesting; they certainly have been well discussed, and presumably because of the vital importance of light to many corals. Rather, it is my intention here to have the readers understand exactly why lighting is an important subject in reef aquaria.

There are two basic types of organisms: autotrophs (mostly photosynthetic organisms) and heterotrophs. Corals are heterotrophs, with a big caveat. Most reef building corals, or hermatypes, and many non-reef building corals, or ahermatypes, maintain symbioses with various dinoflagellate algae called zooxanthellae. While the coral polyp itself is not autotrophic, its nearly obligate association with these dinoflagellates provides polyps with a built-in autotroph that it can, to some degree, control. Therefore, reef corals with polyps maintaining symbionts have characteristics of both autotrophs and heterotrophs. Lighting provides the energy for zooxanthellae to photosynthesize. It may or may not come as some surprise that light, to corals, is simply food.

Waters around coral reefs usually have extremely low levels of various nutrient sources, largely because of fierce competition for those same nutrients amongst the vast numbers of species found there. A common and successful strategy to allow successful competition for habitat space in such an environment is to utilize an energy resource that is not generally limited in tropical waters... sunlight. Corals are not the only organisms to utilize this strategy, as clams, sponges, hydroids, foraminiferans, nudibranchs, and many other organisms also host photosynthetic algal or bacterial cells in their tissues for a similar purpose. As it turns out, sunlight is such a valuable commodity that means to attain as much of it as possible are built into the life history strategies and behaviors of organisms harboring such symbionts. For corals, regulation of the zooxanthellae population is possible, they expand or contract their tissues to expose more or less zooxanthellae to sunlight, and they modify their growth forms to those ideally suited to their “place in the sun.†Accessory animal pigments are also produced to further modify the light environment to which corals are exposed.

-Zooxanthellate and Azooxanthellate Corals
Corals far outside tropical areas, or those in very deep water, do not contain zooxanthellae. Oddly enough, perhaps, is that there are a great many azooxanthellate corals existing on coral reefs alongside or nearby their brethren with symbionts. If harboring zooxanthellae is such a successful strategy, why don’t all corals have these symbionts? Part of the answer lies in evolution. Perhaps it has not been advantageous for some species to adopt them, or perhaps not all species have recently invaded the shallow water zones and have not had enough evolutionary time to do so.

In fact, there are corals that are facultatively zooxanthellate; these corals, some from the tropics and some from sub-tropical regions can exist either with, or without, zooxanthellae. Commonly researched corals of this type include some species of Madracis, Astrangia, and Oculina. In fact, one of the nemeses of aquarists, Aiptasia pallida, the glass anemone, is also facultatively zooxanthellate. As it happens, these corals tend to exist with zooxanthellae in warm, clear, shallow waters and without them in turbid, cold, or deep waters.

What this means, among other things, is that if it doesn’t provide much of an advantage to host zooxanthellae in certain areas, why have them present at all? It could be argued that some photosynthesis is better than none at all, even in deep or temperate water. But, perhaps this is not the case… if there is a cost involved. And, indeed there is a cost to the organism to maintain zooxanthellae within their cells. Apparently, there is no such thing as a “free lunch†for corals, either. These facultative hosts must make a metabolic choice as to whether the benefits of hosting zooxanthellae outweigh the costs. In most coral reef environments, the symbiosis is not so optional, and is usually considered to be nearly an obligate association. I say nearly, because bleaching is a prime example of when the costs of maintaining the symbiosis outweigh the benefits, although the bleaching response is complex and such a statement represents something of a simplification.

This somewhat answers the question of why some corals maintain zooxanthellae: they retain benefits of photosynthesis where prevailing conditions are advantageous over the costs of maintaining them, such as on coral reefs. This group includes most of the hermatypic stony corals, most of the shallow water Caribbean gorgonians, a couple of shallow water Pacific gorgonians, more than half of the soft corals, about half of the number of species of zoanthids (although the vast majority in terms of numbers of organisms), most of the corallimorphs, and a single genus of hydrocoral (although this single genus, Millepora, contains the majority in terms of numbers of organisms on coral reefs).

It may seem logical that zooxanthellate species must exist in shallow water to take advantage of sunlight, but we are still left with one pressing question. How do some azooxanthellate species seem to compete so well amongst zooxanthellate species, in particular, some Pacific soft corals like Dendronephthya species and Tubastraea micranthus? The answer lies in the fact that most probably compete well with their rapid growth, prolific asexual reproduction, and other behaviors. For the most part, zooxanthellate corals do indeed compete their azooxanthellate kin for sunlight-drenched areas, often resigning those without algal partners to recesses, caves, nooks, crannies, and seemingly less desirable real estate. Lest it be thought that azooxanthellate corals are “inferior,†it is more accurately stated that, like many specialists that exist on reefs because of their specialization, they compete well where others cannot. In other words, every organism finds its place where it tends to be successful. As testimony to the fact that zooxanthellae are not necessarily the penultimate adaptation to shallow coral reef waters, at one time in history, all corals were azooxanthellate and did not compete for the space to catch sunlight at all.

(CONT)

 

[jhnrb]

Reef Food Part-2 (cont)

-Types of Zooxanthellae
So successful is the symbiosis between corals and their zooxanthellae that multiple relationships have developed. At one time, and not long ago at all, coral researchers were convinced that all corals held but one type of symbiont within them. These single celled algae were called, despite numerous synonymous names, Symbiodinium microadriaticum. Eventually, several other dinoflagellate zooxanthellae were found in the fire coral, Millepora, and in some zoanthids. However, it was still largely assumed that all other corals harbored a single species of algae. About twenty years ago, the walls around such a notion began to crumble, and it is now recognized that there are many clades (groups of biological taxa that includes all descendants of a common ancestor), such as species, types, and subtypes of zooxanthellae that inhabit coral tissue. In fact, so widespread is the diversity beginning to appear that a complete rewriting of coral symbiosis is beginning, with only a few introductory chapters written as of today. The diversity and nature of the various relationships is now hardly known. What is known is that not only may there be a variety of one coral/one symbiont relationships, but that various corals may harbor more than one symbiont, may potentially be able to harbor more than one symbiont even if it is not usually found to do so, and that even single corals may harbor more than one symbiont at the same time. I would refer the reader to more information in my article here, even though much of that information has already changed, so rapid are the advances in this field. There is a very large body of science regarding this subject, and to cover it in much more depth without many additional pages, I am afraid, would be doing the subject an injustice.

-What The Symbiosis Provides And How Much
Given the general background above, I can now delve into the crux of this relationship and describe just what it means to house autotrophs in a heterotrophic body. Zooxanthellae are initially acquired either from the water column (in broadcast spawning corals), or are given a starter culture from the parent polyp (in brooding corals). Over the course of their lives, coral polyps maintain various densities of zooxanthellae in their tissues according to environmental and metabolic conditions. Polyps periodically release or lose some, require more from the water column, and control their growth and reproduction within their tissues quite effectively. For a description of when the symbiosis does not go quite as smoothly, a process known as coral bleaching, see this article. The algae are maintained mostly in the underlying tissue layer, the gastrodermis, and within the tentacles of some species, in small containment vesicles called vacuoles. These vacuoles are formed within the gut cavity of corals after the dinoflagellates have been swallowed, and they can even migrate across tissue layers.

Once in place, the zooxanthellae reproduce until they form a mostly single layer within the tissue; an arrangement that maximizes light capture as a photosynthetic umbrella, or antenna, while minimizing shading of adjacent algal cells.

The upper layer of the Acropora sp. is the epidermis. The lower layer is the gastrodermis. Within the cells are round to oval golden spheres. These are the zooxanthellae.

The zooxanthellae are then carefully controlled by their coral host by being subjected to nitrogen limitation. As mentioned in last month’s article, nitrogen levels in coral reef waters are typically extraordinarily low, with most being found as ammonia. This is in contrast to aquaria where the dominant nitrogen species is usually nitrate. Nitrogen is the end all-be all for zooxanthellae growth and reproduction. By limiting nitrogen in the form of excretion products, the polyp keeps the zooxanthellae in the numbers and density that maximize photosynthetic efficiency for its own use. Using several released compounds, most of which are still unidentified, the polyp stimulates the zooxanthellae to release virtually all of the products of its photosynthesis, and these are then used by the polyp for its own needs. If nitrogen was made readily available to the zooxanthellae (for example, if high levels were present in the water and the dissolved nitrogen “diffused†into the coral tissue), it could then be accessed by the algae without limitation by the polyp, and zooxanthellae could begin to grow and reproduce like a “phytoplankton culture.†In this case, the symbiosis becomes less advantageous to the coral, and it will expel some of the symbionts to try and re-establish maximal benefit from its algal partners. As a practical note, when very high densities of zooxanthellae exist in coral tissue, the resultant coloration of the coral is usually a rich or dark brown color.

This relationship may not sound altogether “symbiotic.†It may even sound parasitic, since the coral is clearly taking advantage of the zooxanthellae, and seemingly without much “giving.†Yet, nitrogen is so limiting on coral reefs that even the limited excretion of the coral provides a relatively stable supply, as well as a protected stable environment, to the zooxanthellae.

Given that corals are “squeezing†their symbionts for all they are worth, what exactly are they worth? As it turns out, the symbionts provide a constant “sugar fix.†The high carbon products of photosynthesis are mostly sugars, and the coral squeezes out almost 100% of the algal production, allowing just enough to maintain the algae’s carbon needs for its survival. In shallow clear water, efficient corals can get over 100% of their daily carbon needs from their zooxanthellae. These photosynthetically-derived sugars are then used by the coral for metabolic functions that require energy, and much of them are lost in the copious production of mucus. Coral mucus, in turn, and as was shown in the previous article, is itself a food source to the reef. The production of mucus by corals is also very important for their protection, food acquisition, competition, and other functions.

Unfortunately, zooxanthellae don’t make much else besides sugar. The coral squeezes out what it can, but not much more ever results. In particular, nitrogen, once again, is a problem. It seems everyone on the reef is always scrambling for nitrogen, the substance needed to produce protein; proteins required for nematocysts, vitamins, tissue maintenance, injury repair, cell division, growth, gamete production, even the very toxins used to paralyze prey. Proteins are the ticket to growth and reproduction in zooxanthellae, as well as for coral polyps. Thus, it may come as little surprise that this great sugar fix provided by symbiotic algae comes up rather nutritionally short in the course of coral nutrition. To survive and, hopefully, thrive, corals need more than light. They need to swallow more than their symbiotic zooxanthellae. And this will be the subject of next month’s article.

END PART-2