Feeding Begets Food, 2

or…

Food Production By Design, Using A Refugium To Produce Food For Reef Inhabitants.
By Ronald L. Shimek, Ph. D.

Introduction:

In the first article of this short series, I introduced the concept of producing food for one component of the reef aquarium system, the sessile suspension-feeders, by feeding the organisms living a second component, the sand bed. This indirect or "ecosystem" method of feeding is beneficial for three reasons.

- First, it produces live food for organisms that have evolved to eat live food.

- Second, it provides food continuously, and many coral reef organisms, including many corals are adapted to feed more-or-less continuously.

- Third, it "automatically" provides a way to utilize excess or uneaten food from the normal tank food, and in doing so, it maximizes efficiency and minimizes cost.

However, the sand bed is not the only reef aquarium component that may be utilized in this manner. Many aquarists have discovered, in one way or another, the concept of the refugium, a predator-free area that may function to produce animals that are food for other aquarium animals. Such refugia are not difficult to establish, and they may become a very important part of the aquarium system. They provide a way of producing a different type of food than does the sand bed. The food exported from the refugium to the main tank is typically larger and more mobile, and is more often eaten by fish rather than by sessile invertebrates such as corals.

The Concept and Construction:

It may be argued that coral reef aquaria are miniature examples, microcosms, of the real world. However, if they are, the view of a natural reef that they present is one seen through Alice’s looking glass. Such aquaria are typically full of small, immature, or stunted corals, and the density of all coral life is often much higher than in the natural world. Additionally, the concentrations of fishes and many other animals are also maintained at high levels that have no parallel in nature.

By and large, the animals maintained in coral reef aquaria are predatory, and in many systems they have the potential of literally eating themselves out of house and home. In general, aquarists seem to realize this and attempt to feed the animals, although often the amounts fed are unrealistically low for good animal welfare. In addition, the foods chosen to be fed are often inappropriate simply because many of the natural foods, or even reasonable analogs of the natural foods, are not available. Faced, first, with this starvation or semi-starvation regime, and second, with a normal feeding behavior that results in continuous feeding, the aquarium animals respond by eating anything that they can catch that looks, smells, or seems like food. The predation pressure in many tanks is often such that some of the common beneficial detritivore and herbivore populations are kept cropped to low densities. Generally, these prey populations are not destroyed, but rather kept small and the animals are often hidden. The animals become nocturnally active or they are evident only in those areas where the predators are excluded. This latter point was evident to several aquarists, who more-or-less simultaneously "invented" the concept of the refugium in the mid-nineteen nineties to ensure the survival of these beneficial animals.

Recognizing that most of the aesthetically desirable aquarium animals, such as corals and sea anemones were predatory would seem like an easy task, but it certainly didn’t seem to be. In the early stages of the aquarium hobby, misguided aquarium "authorities" spread the word that although some aquarium animals such as the fishes needed to be fed, virtually nothing else in the tank could, would, or should eat. The rationale for this point of view was based on the knowledge that many aquarium animals, such as corals, contained internal symbionts called zooxanthellae. Zooxanthellae are unicellular microalgae belonging to the group called the dinoflagellates. While most dinoflagellates are free living, water-borne, micro-organisms, a few types have developed the symbiotic life style where they live inside of their hosts. In such an environment, they are protected from environmental problems. In turn, when these small algae are found associated with animals, they may produce much of the daily sugar or metabolic energy requirement needed by their hosts.

Zooxanthellae may actually provide sufficient cheap carbohydrates that all of the hosts’ sugar requirements are met, and the host really doesn’t need to feed to stay alive. However, as sugars are only carbohydrates, and carbohydrates are not used to make tissues, the coral hosts cannot grow, repair injury, or reproduce using just the photosynthetic production of the zooxanthellae.

Around the middle of the last decade, many hobbyists finally realized that for their animals to grow, they needed to be provided with high protein foods. One way of doing this was to feed the corals and other zooxanthellate animals directly. A second way was to feed the tank, as described in the first article in this series, Food Production By Design, How A Deep Sand Bed Can Produce Food For Reef Inhabitants. A third method for providing protein-rich food exists. It is a way that is particularly useful when the aquarist is trying to maintain or raise animals that need food larger than either bacteria, or protozoans, or invertebrate larvae. This way is specifically useful when the necessary food must be either continually added or must be alive. That third way, of course, is to set up a refugium and to let the refugium do the feeding.

Figure 1. Gammarid amphipods, such as this animal, do very well in refugia and will disperse back to the main tank where they function as detritivores and fish food. Normally gammarids, such as this, only reach lengths of about a centimeter long (4/10 inch) in aquaria. However, larger ones may be occasionally seen, particularly in refugia.

For once in a hobby replete with garbage terms, such as "sps, lps, and protein skimming" which really mean nothing, the term "Refugium" is quite descriptive, and apt. A refugium is a separate tank or portion of the main tank that is a refuge from the many predators in the aquarium system. Most of the time, the predators that need to be excluded are fishes, shrimps, and crabs. These predatory animals will eat a lot of the smaller animals in the tank, and as reef tanks tend be over populated with the predators, the smaller animals are continually at risk. Although many of these smaller animals are quite beneficial to the reef aquarium system, the hobbyist often tends to see them much as their predatory pets do; as food for those pets.

If these animals, mostly small crustaceans such as amphipods, harpacticoid copepods, and mysid shrimp can be protected from excessive predation, and if they are provided with sufficient food, they reproduce very rapidly. The offspring from this reproduction are highly nutritious food for most reef predators, which, of course, is why these animals need to be protected in the first place.

Figure 2. This diagram shows the structures of a typical mysid. Most aquarium mysids are almost totally transparent, see Figure 3.

Figure 3. A mysid swarm (from a real reef). However mysid aggregations in refugia look much the same. Because they are planktonic, unprotected mysids in a reef tank are often eaten by fishes. In a tank with a refugium, the mysids will eat phytoplankton and particulate material in the water, and disperse into the main tank.

Given a place of safety from predation, all of these types of animals can reproduce plentifully and grow rapidly. As an example, many harpacticoid copepods produce eggs that hatch in a day or two, with the adult stage being reached within a couple of weeks. This means the time between successive generations is very short. The adults may live for several months to a year or more with each female producing a batch of 20 to 30 eggs per week. The offspring produced from those eggs can be reproductive in two weeks, followed by their offspring in two more weeks, and then two weeks later their offspring, and in two more weeks.…

Well, I think the reader will realize the number of copepods that may be produced in an aquarium tank may be astronomical. And this is definitely "a good thing." Harpacticoids are small bugs, and they and their larvae are a nutritious and very important food for such predators as small fishes, and small mouthed corals. Many fish are specialized to eat them, and may be very adept at picking them off, explaining the need for the refugium.

Figure 4. Harpacticoid copepods are the smallest common crustacea in reef systems. They thrive in refugia and during their nocturnal dispersions, are often consumed by corals and other predators.

Slightly larger, but still small, crustaceans, such as mysid shrimps and amphipods reproduce equally well in refugia. As they are larger than the harpacticoids, their generation time is correspondingly longer. Amphipods and mysids don’t release their eggs into the sea, or aquarium, in this case, to develop without parental care. Rather the females are good parents, caring for their offspring during early growth and development. The females carry the developing embryos in a pouch on their ventral surface. Here they protect the eggs, and care for them until they are fully developed, but sexually immature juveniles. Once the eggs have hatched, the juveniles tend to disperse or spread throughout the habitat. This dispersal behavior makes both mysids and amphipods ideal animals for refugia. The adults remain well protected and are, in effect, baby factories, producing offspring continuously as long as they have sufficient food. The offspring help maintain the refugium populations, and disperse out of the protected areas to become either food or detritivores for the main tank.

Construction of a Refugium

In its simplest concept, a refugium may be nothing more than an area in the reef tank from which predators are excluded. Such in-tank refugia are often accidental developments of rock or ornament placing. However, they may be specifically designed into the aquarium rockwork. With some planning, it is relatively easy to set up some rocks in such a way as to create a volume while excluding "large" predators. In my first in tank refugium, these large predators were a pair of mature maroon clowns, Premnas biaculeatus. After arranging the rocks to limit fish access, I further enhanced the refugium by providing it with various algal substrata, in the form of stiff algae such as Halimeda and Chaetomorpha. Halimeda is calcareous and well-growing clumps can effectively be used as a fence to exclude fishes. Chaetomorpha forms tough tangles of thin tubular structures, looking and feeling much like tough, green, monofilament fishing line. In addition to providing protection from predation, such tangled webs of tubes provide a LOT of habitat space and variability for the crustaceans to live on and in.

In tank refugia are, however, limited by their very nature to being relatively small and, consequently, their production of food exported to the rest of the tank is small. To increase that production, an external refugium is necessary. As these are limited only by the space available and the finances of the aquarists, the refugia may become quite elaborate and some cases very large indeed.

As with all do-it-yourself equipment, there is no single or standardized design for a refugium. Every aquarist has a unique vision of what "their" refugium should contain. I think, however that there are some consistent things that should be considered for all refugia to maximize their utility. First, as many microhabitats as possible should be included in the refugium, and to make this an exercise that is not trivial, that means that the refugium should be as large as possible. Small external refugia are simply not worth the trouble. The refugium must be "sized" to the main aquarium; the amount of food exported by a 10 gallon refugium to a 150 gallon tank is relatively small, and probably insignificant in the feeding economics of the large tank. The same 10 gallon refugium, however, might produce a significant amount of food relative to the volume of the main tank when attached to a 40 gallon tank.

The refugium should contain a sand bed that is at least a couple of inches deep, this will promote the growth of sediment worms and crustaceans whose larvae may help maintain these beneficial organisms in the main tank. Having a couple of pieces of live rock in the refugium is a good idea, as is the presences of some macroalgae. In addition to eating the algae, many amphipods and other organisms require it necessary as a substrate. The water flow through a refugium may be almost any velocity.. However, a relatively low velocity is best as slow flow rates often encourage swimming in the food organisms. Generally, a small power head will provide sufficient flow into and out of this auxiliary tank without inhibiting dispersal or swimming behavior. Such swimmers are more likely to be swept out of the refugium into the main tank, where at least some of them will be caught by predators. Additionally, some of them will disperse into the main tank, helping to maintain the detritivore populations there. Obviously, the plumbing connections between the main tank and the refugium must be constructed so that no predatory organisms may enter the refugium. On the other hand, it often is to the aquarist’s advantage to ensure that the connection allows the migration of larvae from the main aquarium to the refugium. Often a simple screen constructed out of some large meshed material will keep the predators where they belong and allow larvae to make the trip. .

Intensive lighting is not necessary, but there should be enough light to allow good algal growth in the refugium. The refugium may be used as a place for growing the algae to export nutrients and this is a good use of the refugium. If you desire to do this, the use of "plant" lights of relatively high intensity will facilitate algal growth. In any case, there should be a good growth of diatoms in the refugium, as these microalgae are the food for many of the animals growing in the refugium.

Maintenance of Animal Populations

Many aquarists set up a refugium in their sump. They stock this with algae for nutrient export, and sometimes put a deep sand bed in it for biological filtration. In some cases they expect this multipurpose portion of their system to process excess unused food, detritus, and feces in the biological filter, grow algae for export, and produce animals to feed their main tank. In most cases, such multipurpose refugia fail miserably at all three of these goals. It is next to impossible to move sufficient detritus and excess food out the main tank to keep the refugium well fed, and the refugium deep sand bed "foot print" is often so small as to be a negligible factor in the overall biological filter. With little food and nutrients available, the refugium animals starve and their populations slowly vanish.

Aquarists need to be proactive with this component of their systems. Generally, a refugium doesn’t replace the sand bed or other filtration in the main tank; at best, it will supplement it. A refugium may indeed require additional or extra filtration above what is available in the main system. This needs to be determined by observing animal and algal growth.

What the aquarist does NOT want to create in the refugium, is a site of active export of nuisance algae or animals to the main tank. This may be avoided by careful monitoring of the system, with the removal of undesirable organisms. Refugia need to be periodically monitored carefully and closely, at least on a weekly basis. This system component generally works quite well to disperse beneficial organisms through main aquarium, however, should the refugium become contaminated with a pest organism, for example, Aptasia, this can be just as efficiently spread throughout the system.

Refugium animals need good foods just like any other animal, and they will benefit from regular feeding. The feedings should be tailored to the refugium both in their size and their content. As many of the animals targeted for the refugium are detritivores, detritus from the main tank would be a good food. Unfortunately, there are detritivores in the main tank too, and they tend to get to the detritus and process it – as they should – before the material can be moved to the refugium. This means the refugium needs to be fed with other more "traditional" foods.

Of all the common animals in refugium, the sand bed animals, and suspension-feeders such as feather duster worms, will likely benefit directly from phytoplankton feedings.Mysids, and to some extent amphipods and copepods, will also derive significant nutrition from direct phytoplankton feedings. The mysids will take the phytoplankton from the water colum directly, the amphipods and copepods will eat aggregated phytoplankton particulate material as it settles out of the water. They will respond as was described the deep sand beds in the first article in this series Aquarists often find it difficult to determine how much food should be added to a tank containing no fish, but that shouldn’t be a problem. For example, the aquarist can roughly estimate the "mass" of all the visible animals in their refugia in "Chromis equivalent" units. This involves scanning the refugium periodically and carefully observing the visible animals. Roughly estimate their total cumulative volume in terms of the size of an average green or blue Chromis, rounding to the next larger number. Then double this value to account for those animals out of sight in the sediment. When feeding, add the amount of food that would normally be added for that number of fish, and disperse it through out the refugium. So, for example, if your refugium looks to have about one Chromis worth of bugs visible, then add one more Chromis equivalent for the buried or hidden animals and feed as if there were two Chromis in the refugium. Feed once or twice weekly. This will help maximize the production of live food for your main tank by providing enough food to keep the animals in the refugium reproducing at full speed.

Conclusion:

By the use of a little extra space, time and material, a refugium may be attached to the main reef tank, providing an aquarist may provide a means of continually providing live food and replenishing the detritivore populations in the main tank. Although a refugium cannot produce as much food as continually comes to a normal reef, at least some will be continually added, and this is the first step in providing proper nutrition to the reef. Supplemented with normal feedings of prepared foods, live phytoplankton, and cultured zooplankton, the captive reef should be well on its way to good health.