Placement Of Corals In Reef Tank

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  1. jhnrb

    jhnrb

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    Aquascaping is more than simply placing the live rock and corals in a tank in a random fashion. Careful planning is crucial in that not only is it necessary to provide for an aesthetically pleasing tank, but it also determines in many instances the long-term health of the corals. One of the most frustrating aspects of keeping a reef tank is to lose a coral unnecessarily. One of the biggest reasons for this loss is improper placement of the coral. Improper placement can result in one coral falling into another if it is not positioned properly. In addition, if inadequate space is provided when the corals are introduced, they will eventually grow into each other; which can result in aggression between them and the eventual demise of one or both. For these and several other reasons outlined below, placing corals in the proper spot in a reef tank is crucial to the long-term success of the inhabitants.

    With the dramatic improvement and widespread use of metal halide lighting and the increasing availability of new coral species, extra consideration needs to be taken as to where corals should be placed. In addition, coral placement to reduce aggression needs to be taken into consideration because many of the corals that we are now keeping are much more aggressive than species we used to keep. This aggression has also become more of a problem as a result of our being more successful at keeping corals in general because now corals grow to much greater sizes than they did in the past. As the size of these corals increases, so too does their proximity to each other and as a result more of their aggressive nature manifests itself. While their aggressiveness was hardly observable and not a problem when many of these corals were small, their effect on neighboring corals becomes dramatically noticeable when they grow to their normal size.

    Before describing the placement of corals, a brief description of coral aggression will illustrate the need for adequate spacing between corals. This spacing of corals is important as corals will grow to their full potential only when sufficient room is provided. When looking at pictures of a reef, the first thing noticed is the diversity of life. All spaces on the reef are filled with different organisms suited to fill every niche. This diversity is the result of the organisms evolving through competition. The factor underlying this competition is aggression; both subtle and overt.

    This subtle aggression takes the form of gradual conflicts that occur slowly on a continuous basis. Subtle aggression is particularly prevalent among corals as a result of their constant battle for survival. Corals have to cope with currents, predation by fish and other invertebrates, as well as competition from neighboring corals for light, nutrients and food. Consequently, corals have developed several specialized mechanisms for protection and competition with other corals. These include sweeper tentacles, mesenteric filaments, and terpenoid compounds (Ates, 1989).

    Sweeper tentacles are the most common of these defense mechanisms in hard corals. These mouthless elongated tentacles form the outermost portion of the coral colony and act as a "patrol" along the periphery. When these tentacles encounter a competing coral, it may attack the competing coral and literally "burn" the offending coral to the point of either killing it or severely damaging it. This "burning" is the result of specialized stinging cells called nematocysts being present in these tentacles. The chemical present in these nematocysts is an alkali toxin similar to bee venom. This may be why corals are said to "sting one another". The relative toxicity of these tentacles differs among various species of corals, as does the length to which these tentacles can elongate (Sheppard, 1982). This degree of toxicity is important in that if two corals fall into each other the relative toxicity of their nematocysts will determine how long they can be in contact before serious injury results. Some corals will be killed after falling into another coral after just fifteen minutes of contact. When two corals have relatively equal toxicity in their nematocysts it is possible for them to kill each other when they fall together. This is why it is important to take precautions to prevent corals from falling into one another.

    The length of these sweeper tentacles is not correlated to the length of the normal coral polyp and may in fact be many times longer. One of the most dramatic illustrations of this is in the sweeper tentacles present the coral Pavona cactus, a small-polyped stony coral who's branches are thick and resemble potato chips. Despite the fact that the branches of this coral are only several millimeters thick, the sweeper tentacles that come out may be four or five centimeters long. Another example of dramatic elongation of sweeper tentacles is seen in the sweeper tentacles of the Crystal Coral (Galaxea fascicularis). In this coral, normal polyps are only one to two centimeters long while sweeper tentacles have been seen that are 30cm. long. These sweeper tentacles are especially toxic and often clear the area completely of any other invertebrates within the colony's proximity. Therefore, when selecting this coral, care should be taken to provide abnormally wide spacing between it and other corals.

    These sweeper tentacles not only appear when a coral is placed in close proximity to another, but they also seem to result when a coral senses the presence of another aggressive coral. I have observed this phenomenon first hand with my Crystal coral. When first placed in my tank no sweeper tentacles appeared during the first three months. However, a Hammer coral (Euphyllia species) was placed in this same tank, the sweeper tentacles appeared within three days. These tentacles also appear to be able to sense where the competitor is located in that regardless of where a Crystal coral is placed relative to the Hammer coral, the sweeper tentacles always develop toward the Hammer coral. Interestingly, the Euphyllia also developed sweeper tentacles, but they were present only on the side nearest the Crystal coral.

    In addition to these tentacles, several hard coral species can produce mesenteric filaments from their stomachs. Corals of the genera Favia, Favites, Scolymia, Pavona, and Cynarina all have this capacity (Chadwich, 1987). These filaments can also kill or devour other coral polyps through a process similar to digestion. Some corals even have the capacity to produce both of these defensive structures, enabling them to fight a battle on several fronts (Wallace, 1984).

    These two defense mechanisms are utilized predominantly by hard corals. It is only until recently that sweeper tentacles have been observed in the soft corals. I had never read or heard about soft corals producing these tentacles, but in my friend Dr. Michael Fontana's 1000 gallon reef aquarium I observed a Leather coral (Sarcophyton sp.) producing very fine sweeper tentacles that were irritating a nearby zooanthid colony. This tentacle did not appear to be as well defined as the sweeper tentacles produced by the stony corals, but they appeared to produce the same result.

    The soft corals generally compete with the hard corals by releasing terpenoid or sarcophine compounds into the water to injure or impede the growth or neighboring corals and then overgrow these impeded individuals in a process called allelopathy (Delbeek and Sprung, 1994). Like their name implies, these compounds are similar to turpentine in chemical structure and in most instances are just as toxic. By releasing these compounds, the soft coral injures neighboring stony corals and can thus grow above them and eventually block out the light that they are both dependent upon and thus kill the underlying hard coral.

    In a reef aquarium, this aggressiveness can have severe and dire consequences. If it is allowed to progress to its conclusion, a large and expensive coral head could die. If these sweeper tentacles or mesenteric filaments are observed, the reef keeper should make sure to move all invertebrates in their proximity out of the way. However, if these tentacles have reached their target they should immediately be removed from the injured animal. If a portion of tentacle remains attached it should be physically removed, otherwise the toxic substances these tentacles secrete will continue to work and will kill the colony to which they have become attached. Fortunately, if the reef tank is well maintained and no microalgae are allowed to grow on the skeleton, the coral colony may recover and grow back over the damaged area.

    (CONT)
     
    jhnrb, May 19, 2006
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  2. jhnrb

    jhnrb

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    (cont)

    While a reef tank does not contain the great diversity of life that an actual reef does, provisions should still be made to try and minimize the aggression among corals. As noted previously, the relative aggressiveness among coral species varies. Consequently, when setting up a miniature reef, adequate space that is invertebrate free should be given around each coral head. This zone should be at least 15cm in all directions for large polyped stony corals as sweeper tentacles have been reported to be at least this long (Sheppard, 1982). The distance between small polyped stony corals does not need to be as long. A distance of 5-8cm is usually sufficient. However, it should be noted that these are the fastest growing of all corals so extra space should be allowed for this. For this reason, I suggest that a buffer zone of 30% of the coral colony's size be set in place when originally placing the corals in order to allow for growth. This may seem extreme and may initially make the tank look sparsely decorated. However, in a well-designed and maintained reef tank, this space will be almost completely filled within the first year simply from growth. If growth space is not allowed for, what eventually results is the constant need to prune corals lest they burn and kill one another. For the most part, the space between soft corals does not need to be as great initially, as in most cases; soft corals do not burn each other to the same degree as do hard corals when they come in contact with one another. Consideration does need to be given for growth of these corals; otherwise, a faster growing coral will overshadow a slower growing coral and eventually starve it out for light. Another consideration when placing soft corals is that these corals should be positioned so that the mucous and terpenoids produced are not allowed to come into direct contact with their neighbors. These corals will do the least harm to other corals if the water movement in the tank is such that after the water moves across them it flows down an overflow and into a sump where the harmful compounds can be removed with either protein skimming or carbon.

    One last aspect of coral placement as it relates to aggression is preventing a colony from falling over. That is, how likely is it that one coral will tip over and land on another coral and as a result burn or be burned when it comes into contact with this other coral. This may seem trivial, but whole colonies of coral have been lost due to a small part of the colony being burnt as a result of it falling into another colony. This burnt area then becomes infected and consequently the whole colony dies as a result. This is particularly troublesome for small polyped stony corals that usually arrive unattached to anything. Therefore, when placing these corals on live rock a dab of waterproof epoxy or gel type super glue should be used to hold them in place until they encrust over the area themselves. An alternative is to use rubber bands or plastic cable ties to anchor the colonies in a less permanent manner. As mentioned above, when colonies do fall into one another, they should be separated as quickly as possible and any damage washed off in the current to try and rid the animal of nematocysts.

    Taking aggressiveness into consideration is only one factor in determining coral placement. Other factors that are crucial are light preference and water movement. These latter two factors are hard to quantify in that there are dramatic differences in lighting systems as well as methods and force of water movement. However, there are some generalities regarding coral placement that have been observed. When placing a coral, consideration of its lighting and current requirements should be made long before it is placed in a particular location. This is because moving a coral even a small distance once it has adapted to conditions at one spot cause the coral to "re-adapt" to these new conditions. It has been reported that it takes at least four weeks for a coral to adapt to new conditions and start to grow. Therefore placement should be planned so as to not inhibit the coral's growth by constantly moving it from place to place.

    In terms of lighting a reef aquarium there are now many choices, but fluorescent lighting is still the method of choice for most reef enthusiasts, particularly those predominantly keeping soft and large polyped stony corals. Fortunately, even in fluorescent lighting there are now many choices. In addition to standard bulbs, high output (HO) and very high output (VHO) bulbs are also available. Other smaller fluorescent bulbs such as power compact high output bulbs and T-5 lamps have now become a standard in the hobby. As their names imply these bulbs differ by the amount of light that they produce. For the majority of tanks housing soft corals and large polyped stony corals, fluorescent lighting (either VHO or power compact or T-5s) will provide all the light necessary to meet the animals' needs and allow them to thrive and grow.

    However, with VHO fluorescent lighting and more so with power compact and especially metal halide lighting, care needs to be exercised when placing the animals under an artificial lighting regimen. The lighting is artificial because no matter what lighting is used it is inferior to the natural lighting that the coral's are exposed to on the reef. Because of this difference, it is advantageous to slowly acclimate new corals to artificial light. This is because many of the lighting sources that are used contain more ultraviolet (UV) light than the corals are accustomed to. If the corals are not acclimated slowly to this light, it may cause them to bleach or burn. This is particularly the case with the VHO bulbs or metal halide lamps.

    For this reason, all new corals should be placed at the bottom third of the tank for at least one month after they are acquired. After this one-month acclimation period, the coral is gradually moved up to its desired final location over another one-month period. This may seem extreme, however tanks should be looked at as long term, five year plus, projects, so there is really no need to hurry.

    The above-described method is good for small polyped stony corals that show the least tolerance for being light shocked owing to the thin veneer of living tissue that is actually present on the colony. It is also useful for large polyped stony corals like Brain corals (Symphyllia, Favia), Open Brain corals (Trachyphyllia), Elegance (Catalyphyllia) as well as any other corals that contain a lot of bright green pigment in their tissues. Many of these green-pigmented corals show very severe shock responses if they are placed high up in a tank using metal halide lighting, as evidenced by their bleaching and dying very quickly. Because of this a very gradual system of acclimation to light should be adhered to and deaths due to light shock should be reduced.

    (CONT)
     
    jhnrb, May 19, 2006
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  3. jhnrb

    jhnrb

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    (cont)

    Under metal halide lighting, many corals can remain at the bottom of the tank. Mushroom anemones (Actinodiscus sp.), Plate corals (Fungia sp.), Tongue corals (Herpolitha sp.), Brain corals (Favia, Favites, Symphyllia, etc.) all do quite well in the lower depths of these tanks. In addition, Elegance (Catalyphyllia) and Bubble (Plerogyra) corals seem to do better under metal halide lighting when placed lower and to the far sides of the tank. In terms of placing the corals, once they have acclimated the general rule is the brighter the color of the coral the closer to the lights it should reside. Thus bright pink Bird's Nest (Seriatopora hystrix) or Cactus (Pocillopora verrucosa) corals usually should be placed higher in the tank than their brown counterparts. The reason for this is that the brighter color indicates pigments in the tissue have been produced to protect the coral from UV light that is present in the shallower depths (Delbeek and Sprung, 1994). Also once a coral has been acclimated to this bright light and begins to grow, the growth tips will usually be of a brighter color than the original colony itself.

    This same pattern also holds for soft corals. Brightly colored soft coral colonies like Yellow Tonga Leather corals (Sarcophyton sp.), bright green Finger Leather corals (Sinularia sp.) and white Xenia colonies all seem to do better with brighter lighting than their brown or beige counterparts. If the lighting is inadequate for these brightly colored corals, the bright colors will gradually fade over time. A good indicator of whether a coral is in the proper place and under adequate lighting is how does its color compare with what it looked like when it originally arrived. If the lighting is better and the coral is acclimated properly, it is even possible to bring out the colors of a coral so that over time it may be more green or pink than when it was originally collected. This may be the result of more UV light being present in our reef tanks than the coral was exposed to in the wild. Thus to compensate for this, brighter pigmentation occurs.

    The last factor to be concerned with in terms of a coral's placement is water movement. Most corals have very little means for cleansing themselves. This is because, for the most part, the current moving around the reef constantly performs this function. That is why powerheads or some other sources of water movements are so essential in a reef tank. Otherwise, detritus will settle on the corals and cause decay that quickly leads to algae formation and the demise of the animal. However, not all animals require the same amount of water movement. Corals that do best with strong water movement usually come from areas where wave action is greatest. These corals usually have small polyps and are either bulky or encrusting in form (Veron, 1986). Corals such as Porites, Turbinaria, and some Acroporas fall into this category. These corals can take the strongest water movement in a reef tank as they live on the outermost slopes of the reef.

    The next group of corals require moderate current as they come from the lagoons and back reefs where the current is not as great and in fact may be limited to the changing of the tides. Nevertheless if adequate water movement is not present these corals will not thrive. Most of these corals have either large polyps or are fairly large polyped encrusting corals. Corals such as Star polyps (Clavularia sp.) Flowerpot (Goniopora sp.), Leather and Finger Leather Sarcophyton and Lobophyton corals fall into this category. The next group requires even more water movement and includes Soft Finger Leather corals (Sinularia, Nepthea), Colt coral (Cladiella), Polyp rock (Zooanthid sp.) and Euphyllia and Elegance corals. The last group still requires water movement, but it is only a trickle relative to what the first groups of corals received. This group includes Mushroom anemones (Actinodiscus sp.), Elephant Ear Anemones (Rhodactis sp.), and Bubble Corals (Plerogyra).

    Despite the brevity given to water movement and its relation to coral placement, in many instances the difference between success and failure with a particular coral specimen has often been the result of moving an animal several centimeters in relation to the water movement. Also, when I have had an animal that was not thriving, it was generally due to inadequate water movement rather than too strong a current.

    Besides the three aspects of coral placement described above there are many other factors that need to be considered. My goal, however, was to provide general guidelines and factors to be considered rather than the actual requirements for every coral species. I would like to point out two rules that are truer than not: corals hate to be moved and require time to acclimate to new conditions, and if a coral is not thriving in a location after two weeks than chances are it will die unless you move it. Only by observing your tank regularly will you get a feel for how the corals are doing and whether a particular animal needs to be moved or not.

    END
     
    jhnrb, May 19, 2006
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