Feeding Begets Food, 1

or…

Food Production By Design, How A Deep Sand Bed Can Produce Food For Reef Inhabitants.
By Ronald L. Shimek, Ph. D.

Introduction

Eons ago, about 1994 or 1995, when the reef aquarium hobby was in the dark days of its infancy, many aquarists did not bother to feed either their corals or any other animals with zooxanthellae. This rather silly (well – I could think of better adjectives, but "silly" works – and is not too impolite) practice was based on the idea that since these animals had photosynthetic endosymbiotic algae, they got all their necessary nutrition from those algae and did not need food. Food was considered a necessary evil; while necessary for fish, excess food polluted the tank and caused all sorts of problems. This was a time when the prevailing methodology for keeping reef aquaria advised a "bare bottom tank" which could be vacuumed clean of excess food and detritus on a daily basis.

About this time, a new way of keeping marine tanks, called the "Berliner system" became known to American aquarists. This methodology was more-or-less developed in Germany, hence the name "Berliner system," and involved keeping systems with an inch or two of fine sediment or sand on the bottom. One of the immediate results of using this system was that for the first time, it became easy for all aquarists to keep small-mouthed stony corals, such as Acropora, alive for extended periods. This success was generally attributed to the ability of a Berliner system to keep the mineral nutrients at low levels approximating those levels found on a natural reef. What most aquarists didn’t stop to realize was that this system for the first time provided a way for the reef aquarium to continually produce a source of food for those corals, and that this was why corals survived in such aquaria.

Coral Feeding – A Necessity

Until recently, most aquarists didn’t think they had to provide food for their corals and soft corals. The myth that corals get all of their necessary nutrition is really well entrenched in much of reef-aquarium reference "literature." That myth developed from a misunderstanding of the role of zooxanthellae, and a misinterpretation of coral physiological literature.

A bit of history is in order here. Corals have been known as being alive since antiquity, but just what kind of organism they were was not clear until fairly recently. Zooxanthellae were noticed in corals in the nineteenth century, but their role in coral physiology and nutrition was not clarified until the middle 1980’s. Initially, these algae were considered to be parasitic, and corals were presumed to be totally predatory. This was a view first elucidated clearly – and loudly - by the eminent coral research C. M. Yonge, in the 1930’s. Not much in the way of coral physiological research was done until the late 1960’s when a series of researchers began investigating zooxanthellae. A lot of these investigations were basic fundamental research asking questions on the order of, "What is this stuff?" and "What is doing inside the coral?"

These researchers rapidly came to the conclusion that corals were not totally predatory, but rather were symbiotic organisms, and that the zooxanthellae provided some essential nutrition to the coral animal. Subsequent work showed that for short periods, the zooxanthellae could provide all of the respiratory energy requirements for at least some corals, for periods up to a day. This conclusion, of course, got published in the scientific press, and made it into textbooks and, almost immediately, it was misinterpreted.

The problem comes from the phrase "respiratory energy needs." All animals respire, and on a cellular level, this means that they use oxygen, in this case oxygen dissolved in sea water, to break down sugar to carbon dioxide and water. The sum of this reaction can be written as C6H12O6 (= sugar) + 602 (= oxygen) à 6CO2 (= carbon dioxide) + 6H2O (= water) + Energy. Or, phrased differently, one sugar molecule, burned completely using six oxygen molecules, yields six carbon dioxide molecules and six water molecules, plus a whole lot of energy. In the coral’s cells, the sugar comes from the zooxanthellae, as they produce sugar by photosynthesis. Photosynthesis is basically the reverse of the reaction shown above; where light energy is used to fuse carbon dioxide and water to form sugar and oxygen. The coral cell uses the energy produced by the process of respiration to perform all of the work necessary to just stay alive. This work is called the basic metabolism of the coral cell.

Consequently, the bottom line is that corals don’t need any food to simply exist, provided the zooxanthellae are producing sugars by photosynthesis. Many early coral reef aquarium "authorities" seized on this fact like a pit bull on a postman, and said that corals don’t need to feed as their zooxanthellae do "it" all. Whatever "it" is...

And, they couldn’t have been more wrong.

While zooxanthellae provide nutrient to corals, they provide it only in the form of sugar, and while sugar can provide the energy for coral growth, it cannot provide the raw materials for coral growth. All animal tissue, and coral tissue is no exception, is made mostly of proteins, and proteins are made of amino acids. Each amino acid has, as its core, an amine or ammonia group built around a nitrogen atom.

All organisms can use sugars to provide the energy to utilize amino acids and to make or breakdown proteins, but the nitrogen for this process must come from some other source than photosynthesis. Zooxanthellae live inside coral cells, and are bathed in the cytoplasm or internal fluid of that cell. This fluid is rich in amino acids, and zooxanthellae can absorb, eat or incorporate those animal amino acids into themselves and use them to make proteins. These, in turn, may be used by the coral in part of its growth.

Aquarists often think that their corals produce a skeleton made of pure calcium carbonate, in the form of aragonite. This is not actually the case; the skeleton is actually formed of an organic matrix and calcium carbonate crystals deposited in two closely-linked phases. Initially, a fibrous proteinaceous matrix is laid down. Subsequently, the calcium carbonate is precipitated onto this matrix. There is some evidence that the precursor molecules of the organic matrix that is the basis for the coral skeleton are produced by the zooxanthellae. However, do note that the nitrogen for them must have come initially from the coral cell.

Thus, the question arises as to where the nitrogen in the coral cells comes from. There are two sources of this nitrogen. First, nitrogen may be available, dissolved in the water, as nitrate or ammonium ions. Although sea water is saturated with nitrogen gas from the atmosphere, only a few species of nitrogen fixing bacteria can convert it to useable form. Animals, such as corals, can not do this, and they either must rely on dissolved nitrates or ammonium or get their nitrogen from some other source. All corals appear to be able to absorb very small amounts of nitrogen containing compounds from the water, but these are insufficient to fulfill most metabolic requirements.

The second, and major, source of metabolic nitrogen for corals, sea anemones, and other zooxanthellate animals, comes from feeding, either on other animals or on particulate organic matter floating in the water. In either of these cases, the ingested food is rich in nitrogen. As I have already noted, animal tissues are mostly protein, and when an animal is killed and eaten, its tissues are reduced to their useable chemical constituents by the digestive process. These constituents are incorporated into the cells of the predator and used by that cell to produce more proteins. Only this time, they are components of the coral cell.

Particulate organic material is found in all natural sea water and is common in our aquarium waters as well. Often the particles are so small that they are not visible to the unaided eye. Such particles are small blobs of organic material, and they are covered with bacteria which are digesting them. Other such particles may simply be bacterial in origin. Bacteria are a very good source of useable nitrogen, as the bacterial cells have a higher ratio of nitrogen to carbon than do either plant or animal cells. The particles forming particulate organic material are often very small and most bacterioplankton are even smaller, so a couple of questions need to be asked. First, do corals actively feed on this material, and second is there enough of it in the normal water flowing over a reef to actually provide much nutrition on the reef. The answer to both questions is a resounding, "Yes."

Figure 1. The tiny polyps of this Porites, and their even tinier mouths are indications that much of the nitrogen necessary for this animal’s tissues come from microplankton.

Interestingly enough, much of the early work showing the importance of this material was done by Russian marine ecologists, particularly Yuri Sorokin. The work was published in Russian-language journals starting in the early 1980’s. Because this literature was not published in English, and because of the provinciality of many American researchers, particularly in the Cold-War years, it appeared simply to be ignored until about 1990 when translations became available. The disappearance of the Soviet Union in the early 1990’s also gave a boost to the acceptance of a lot of the research by Russian ecologists. Russia could no longer afford to support the research these scientists were doing and many of them went elsewhere to work, and started publishing in English language journals. Anyway, for whatever reason, the importance of small plankton started to become apparent and obvious to American ecologists around the middle part of the 1990s.

We now recognize several different categories of what may loosely be called microplankton. This is material that, by definition, is too small to be collected in the normal plankton collection devices used for either zooplankton or phytoplankton. It was simply missed by earlier workers; however, as more work is being done it has become apparent that microplankton provides a tremendous amount of food to coral reefs.

An old, but true, generalization of ecology, is that the size of the food gathering apparatus is correlated with the size of the food eaten. This is evident in the so-called small-polyped-stony corals. These animals are colonial and are made of many small individuals each with a tiny mouth. They form the dominant animals on reef faces and crests, the places that receive an almost continual bathing in this bath of microplankton. And they eat it. It is very difficult to actually get a handle on coral feeding, much of this microplankton is difficult to identify and rapidly digested, so that if a coral is collected for the purpose of examining what is in the gut, the gut contents may be digested beyond recognition between the time of collection and the time of preservation of the sample, even if this is only a few minutes. Nonetheless, research indicates that most of the microplankton hitting the reef corals is effectively removed from the water and ingested by these corals.

The fact that many of these corals grow very rapidly is testament to the availability of the appropriate food source. Without this food, they simply would not have the raw materials to build the tissues necessary for growth.

So… How can we provide such materials in our reef aquaria? Given the title of this essay, I will bet any reader will suspect the answer has something to do with sand beds.

Sand Beds As Producers Of Microplankton

That sand beds would be good producers of microplankton was alluded to earlier when I noted that even the shallow sand bed of the so-called "Berliner System" helped hobbyists keep previously unkeepable alive. Our reef aquaria are what a biologist might call "microcosms," or small copies of the real habitat. These contain the appropriate substrata, animals, and algae to be analogues of the real world.

Probably the best copy of a part of the reef system is found in a deep sand bed that has a thriving and diverse population of animals living in it. Provided the bed was set up with care toward adding the proper organisms, such deep sand beds contain many different types of bacteria, as well protozoans, and a diverse array of animals. In fact, the quantitative sampling that I have done in my own aquaria, indicates that numbers and types of organisms found in the sediments fall within the ranges found in natural environments. Phrased another way, a deep sand bed with a thriving array of animals is really indistinguishable from what would be found in the shallow sand communities of sheltered parts of coral reefs. This is important, as it allows the use of data from investigations of these shallow sand communities to explain what is happening in our tanks. Not all animals in a sand bed feed in the same manner, nor do they all feed upon the same things.

- There are predators that feed on other animals in the sand bed.
- There are herbivores that feed upon either true plants or algae.
- There will be scavengers feeding on dead animal matter.
- There will be detritivores, feeding upon organic debris of various sorts.
- There will be suspension-feeding animals that feed on material floating in the water above the sand bed.

And there will be animals that actively ingest the mineral sediments themselves, and digest off organic material, such as bacteria or other animals that adhere to them.

This wide diversity of feeding types is matched by a wide taxonomic diversity as well. There will be protozoans, which although they may differ from animals in many ways, may be considered to be animal analogues. There will be flatworms, round worms, and segmented worms of many different types. Small crustaceans, also of several different types, are often very abundant.. Numerous snails and echinoderms may also be found, although their diversity is often limited in aquaria.

Most aquarists consider that the major function of the sand bed is to provide some sort of biological filtration. A well-established sand bed does this very well by providing multiple biological pathways for energy utilization. Or, put another way, a multitude of potential food webs for the processing of excess food. Animal physiology being what it is, we always have to feed more food that can be utilized by the animals in our aquaria. Some of this food makes it directly to the bottom as uneaten food, but the majority of added food probably gets eaten by some aquarium organism. Eaten food is always processed rather inefficiently, and a sizeable component exits the animals as fecal material. Fecal material is not waste material, but rather indigestible food, often with a sizeable component of digestive enzymes still active in it. As such it liberates a lot of dissolved nutrients. Dissolved nutrients are really the food for bacteria, and the fecal material is rapidly colonized by bacteria. Both the fecal material and the bacteria on it may be food for other organisms, primarily sediment-dwelling animals.

All of this excess food, processed food, and detritus drifts through the water of an aquarium for an indeterminate time and may, in fact, go to feed coral and other suspension-feeders directly, but most of it reaches the bottom and enters the realm of the deep sand bed organisms. Here it is may be mobilized up in to the water column for one more times before all the nutrient value is lost.

As aquarists, it is to our advantage, and to the advantage of our corals, if we can turn this material into coral food or into food for all suspension-feeding animals. Fortunately, it is rather easy to do this, all it takes is some planning and minimal oversight on the part of the aquarist. The aquarist needs to manage their sand bed a bit, and the sand bed organisms generate a large amount of coral food from what appears to be useless waste.

Figure 2. Suspension feeding animals such as this feather duster, may provide a lot of food for the aquarium, but only if they are provided with the appropriate food of their own. Virtually all feather duster worms feed mostly on phytoplankton.

Sand Bed Food Processing and Production

To ensure the production of food for suspension feeders from the sand bed, aquarists must first ensure that the excess food added when fish and other animals are fed makes its way to the sand bed, and once there that it gets processed properly. The first of these requirements is quite easy. If the sand bed covers the bottom of the aquarium, excess food will end up there. However, for proper processing of the food, the sand bed needs to be constructed properly. A properly constructed sand bed needs to be made of sediments with the correct particle size distribution, and it needs to be populated with the proper array of organisms. Once this is done, the bed really functions on its own with little care by the aquarist.

For information on deep sand bed construction follow this link, http://www.rshimek.com/reef/sediment.htm

The basic construction of the sand bed is explained at the site linked to above. The type of sediment utilized is really immaterial; virtually anything will work. What is more important is that the sediment has the proper grain size distribution. Using sediments of the proper grain size almost, by itself, ensures success of the sand bed. The next critical step in constructing the sand bed is to inoculate it with the appropriate animals. This may be done in one two ways. One may purchase "sediment fauna starter kits" consisting of several animals of the appropriate types. The animals in such kits reproduce rapidly in most tanks and soon there are thriving populations of them. Several online vendors offer such "kits." The main thing to look for in such kits are the presence of fire worms, which are probably the best marine scavengers, and burrowing worms of several types. A variety of the latter worms are sometimes sold by some online vendors, but the best way to get a good variety of them is to purchase some freshly collected live sand containing many small animals.

Figure 3. This terebellid worm feeds only on surface detritus. Much of the food of this worm in nature is what is termed "phytodetritus" or detritus formed from phytoplankton that settles to the bottom. Similar food in our tank will be generated from sufficient feeding with phytoplankton or phytoplankton based foods. This worm can generate a vast amount of particulate organic matter in the course of a day.

In most aquaria, these small worms soon grow and reproduce, and populations of them will become established. Two main types of worms are desirable. These are subsurface sediment deposit-feeders and small suspension feeding worms. Now, I suspect the reader is thinking, "Okay, but how do I know what the worm is doing?"

With a bit of observation, determination of the worm’s feeding type is relatively easy. To do this, the sediment must be observed through the aquarium wall where the sediment is visible. Burrowing worms, which are generally deposit feeders, will be seen in the sediment by the trails they form through it. Like earthworms, many species of deposit feeding worms literally eat their way through the sediments. A few deposit-feeding worms have tubes or burrows that may end in a tube to the surface, but the worm is always facing head down into the sediment,

In contrast, filter-feeding worms extend some sort of structure into the water. Water passes through it, and particulate materials are removed from the water. These worms feed on primarily on phytoplankton, and regular additions of phytoplankton to the system will help ensure that these animals thrive.

The First Source Of Food From The Sediments.

To provide a continual source of food for corals and other suspension-feeding animals takes very little work. Aside from ensuring the sand bed is constructed properly, inoculated correctly, and fed adequately, the aquarist needs to do nothing. The animals do the rest. Both the deposit- and suspension- feeding animals will eat material out of the aquarium and convert it into fine particulate organic material by digesting it and defecating the remains. Generally, the small deposit-feeders back up to the surface to defecate, and in doing so they spray a fine mist of semi-solid worm feces into the water column. This material is fine particulate organic material which is just about perfect "sps" coral food. Filter-feeders, which are generally constrained to be "head-up" in a tube often have "fecal pellet grooves" which convey their fecal pellets, from the anus at the posterior end of the animal deep in the tube, up and out of the tube. These pellets often pile up around the base of filter-feeder tubes and get broken up and dispersed by detritus feeding animals such as amphipods. In so doing, they also spread fine particulate material into the tank’s water.

Such fine particulate material is "homogenized" worm feces, and it is rich in organic nutrients and food. Bacteria from the worms’ guts and the water colonize it and start to digest it. In doing so, they make it even more desirable as food item, by increasing the relative nitrogen content. Small mouthed corals, such as Acropora, Montipora, Pocillopora, and Seriatopora are really adapted to eat the finely dispersed feces of other animals. Animals that feed on the feces of other animals are said to coprophagous, and those corals that aquarists refer to as "sps" corals are really fine examples of this type of animal. In nature, they are often eating the dispersed feces of the hordes of plankton-feeding fishes swimming over the reefs, and in our tanks they thrive on the dispersed feces of the sediment worms.

So the rule of thumb is this: The more bristle worms that are found in the sediment, and the more food they have, the more food will be available to the corals….. To facilitate this portion of the sediment to water column to coral food chain, the tank needs to be well fed with a wide variety of foods such as normal fish food, live phytoplankton, and if possible live zooplankton.

The Second Source of Food From the Sediments.

The production of fine particulate organic material from the sediments is the most continuously produced and available food in marine aquaria, and best of all it is added without any real work on the part of the aquarist. There is, however, another important food produced by the sediment fauna. This other food source is also rich in desirable nutrients, however it is not produced as continuously as the first one. This food consists of the reproductive products of the sediment fauna, their eggs, sperms, and planktonic larvae. Once the populations of the sediment animals, such as small crustaceans such as copepods, and the polychaete worms mentioned above get going well, they start to reproduce. While some of the worms may reproduce asexually by fission, all crustaceans, and most of the worms will primarily reproduce by spawning into the water.

Generally, animals spawn after dusk. This timing is critical as it prevents plankton-feeding fishes from feeding upon the eggs, and early embryos. Such spawning times are built into the animals genetic code, and generally they will not "learn" to spawn during the day. Occasionally, however, some animals will mis-time there spawning or the aquarist leaves the lights on, a bit later than usual and spawning is visible.

Often developing embryos and larvae from any given spawning event will be in the water for a couple of weeks and during that time they are potentially food for any suspension-feeding coral or other suspension-feeding animal. Provided the sediment community is well fed, such spawning events will occur regularly, and relatively continuously.

Figure 4. This nectochaete larva of a bristle worm is a couple of weeks old and has survived in the plankton of my lagoonal reef tank by feeding on phytoplankton. In turn, this animal is fine food for corals.

The Third Source Of Food From The Sediments

The final food source from an aquarium’s sediments, results from another process that also mimics the natural environment. Many small crustaceans and other animals that live on the sediment surface, or just below the surface in between the sediment grains, leave the sediment to swim in the water column at night. Generally, they stay close to the sediment, and because of this they are known as "demersal’ zooplankton.

Demersal zooplankton is a rich and important source of food for corals in the natural environment. If the sediment animal community is well-developed and well-fed in a marine aquarium, a similar nightly migration occurs there as well. And, not surprisingly, it may provide the corals with another important source of food.

Things Not To Do.

The aquarist may do two things to either partially or completely remove these food sources. First, they may underfeed their aquaria. Either intentionally or accidentally, aquarists may "turn off" these sources of food by not providing sufficient nutrition to the aquarium. Many aquarists are under the mistaken opinion that coral reefs have little in the way of planktonic food for their inhabitants. This is a patently false idea, but seems common enough in the aquarium hobby. In reality, the reefs are bathed in a continual source of very small and very nutritious food. This notwithstanding, aquarists often feed their animals less than is necessary.

Often underfeeding is done to restrict the dissolved nutrient content in aquarium water. This is an admirable goal, but it should be accomplished by other means, such as nutrient export. On a coral reef, food is NEVER in short supply to suspension-feeding animals. When those animals are put into a standard anorexic aquarium they are severely stressed and become disease prone. The animals need a continual supply of nutritious food.

Only in this way will the animals be healthy and vigorous. A healthy well-fed aquarium will result in a production of a significant amount of internally generated food

The second thing an aquarist may do that may completely negate the production of food from the sediment is to mechanically filter the aquarium. By putting in particle barriers such as screens, filter floss, and filter cartridges, the water is cleansed of all of those nasty food particles, and the aquarist is left with crystal clear, and sparkling water. This is a totally unnatural situation, and will result in the unnecessary deaths of many animals.

Foam fractionation, or protein-skimming, generally removes some of the particulate organic material and it may remove some of the phytoplankton. It will generally not remove many of the larvae or demersal plankton as they have means of avoiding capture or entrapment in the skimmer’s bubbles.

Conclusion:

With some foresight, and a little work a sand bed may be constructed and maintained that provides a significant amount of food for suspension-feeders in a marine aquarium. The key to such food production is the proper construction and maintenance of a deep sand bed, followed by the appropriate feeding of the tank with a variety of foods, but including specifically phytoplankton, and small particulate material. Such a system will provide sufficient food to provide much of the food necessary for the proper and balanced nutrition of coral reef animals.

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