Aquarium Reefkeeping Coral Information
An in-depth overview of aquarium reefkeeping and coral information.
J. Charles Delbeek
They are mysterious guests from an exotic world. Unlike anything else we keep in an aquarium, they represent the essence of the coral reefs that we try to duplicate in our living rooms. Our success at this effort depends greatly on how well we understand these unusual life forms.
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Zoanthids (button polyps) are generally very easy to keep and propagate.
Unfortunately, for the average hobbyist, the increased availability of these animals is not matched by the information available concerning their biology and care. Too often, hobbyists know more about who manufactures their aquarium equipment than basic biological information, or proper identification of the animals they are striving to keep. And yet, without this information it is difficult to provide the optimum conditions that will allow them to grow and reproduce.
All corals belong to the phylum Cnidaria. Cnidarians are simple tentacle-bearing organisms that have a single body cavity — the digestive cavity — with only one opening that serves as both mouth and anus. The body plan is made up of two layers, the outer epidermis and the inner endodermis (gastrodermis). Between these two layers lies a jelly-like matrix called mesodermis. Cnidarians are also characterized by the presence of stinging cells: nematocysts. Many cnidarians possess symbiotic algae cells called zooxanthellae (actually dinoflagellates, a type of protozoan) in their tissues. These provide the animal with nutrition via their photosynthetic actions (see Delbeek 1990, Delbeek and Sprung 1994). There are three main classes into which the cnidarians can be divided.
The Scyphozoa are the so-called true jellyfish and do not contain any specimens that are normally kept in home aquariums. The one exception occasionally offered for sale is the Caribbean jellyfish, Cassiopeia sp. This species is usually found lying on the sand on its back in shallow water, exposing its tentacles to light. They feed on small prey items that may land on the tentacles, but they also have zooxanthellae and can do fine in aquariums without any supplemental feeding. They may, however, release nematocysts into the water — I have felt hundreds of tiny stings on my skin whenever I have swum over a shallow field of these jellyfish in the Florida Keys.
These animals are not recommended for reef aquariums because they are not true reef animals and can harm other inhabitants with their sting. They can, however, make an interesting display for public aquariums, especially as part of a mangrove, or a seagrass flat exhibit.
The Hydrozoa contain colonial organisms generally known as hydroids. The polyp has a gastric cavity that does not contain septa (walls of tissue partially dividing the cavity into chambers), a pharynx, or internal tentacles. These animals usually have a two-stage life cycle. Part of their life is spent attached to a substrate, often in a colony of polyps, and the other part of their life cycle is spent as a free-swimming reproductive medusoid stage, similar in appearance to the familiar jellyfish. Some have only a medusoid stage, such as the Portuguese Man-of-War (Physalia sp.).
Hydrozoans also have a variety of polyps within a colony — some for defense, some for feeding and some for reproduction. A few hydroids possess zooxanthellae (e.g., Aglaeophenia cupressina, Alderslade et al. 1984).
Within the Hydrozoa there are two orders that are commonly called "corals" due to their formation of calcareous skeletons. The fire corals (Millepora spp.) belong to the order Milleporina, while the hydro corals belong to the order Stylasterina.
Fire corals, as their name implies, can produce a powerful sting that can easily be felt through human skin. Usually mustard brown in color, they grow in shallow reef areas and can form encrusting, branching or upright, plate-like growths. They have two types of tiny hair-like polyps covering their surface, and they possess zooxanthellae in their body tissues.
The larger hairs, called dactylozooids, are mouthless and possess numerous small side branches filled with nematocysts. The smaller gastrozooids have a small mouth and have numerous small knobs filled with nematocysts (Hyman 1940). The medusae stage is strictly for dispersing eggs and sperm into the water column (Hyman 1940).
Hydro corals come in a variety of colors ranging from violet to pink and tend to have a branching pattern. They too are covered with numerous hair-like polyps.
Hobbyists sometimes encounter fire coral on live rock that they have purchased, but it is rarely imported as a collected specimen. This is unfortunate because they are extremely hardy and will grow rapidly under the proper conditions of strong water movement and lighting. Feeding is not required to maintain these animals, but if you wish to do so, use very fine foods, such as rotifers and liquid coral foods. These should be targeted directly onto the colony — not simply dispersed into the water.
These corals are also easily propagated by simply snapping off small pieces. Due to the encrusting growth form of some species, they can easily grow over glass, live rock and other corals, making them a bit of a problem. To the best of my knowledge Stylasterina have only been imported as dried skeletons for the curio trade, so it is unknown how they would fare in a reef aquarium. However, because they are not photosynthetic, they would need to be fed fine planktonic foods as outlined above.
The third class, Anthozoa, or "flower animals," contains all the animals we commonly call "corals," as well as sea anemones and sea pens. Medusae are never formed and the polyps of anthozoans are much more complex than those of the hydrozoa, having several internal septa and a pharynx. The Anthozoa are also the largest Cnidarian order, with more than 6000 species described so far. The Anthozoa can be subdivided into two subclasses: Octocorallia (Alcyonaria) and Hexacorallia (Zoantharia), based on the structure of their polyps.
Octocorals contain all the corals we normally call soft corals. Octocorallia always have eight tentacles on each polyp, and their polyps are internally divided by eight septa. Each tentacle also has numerous small, side branches called pinnules that give them a feathered appearance. The polyps extend well into the tissue of the coral and are all interconnected by a tubule system. For example, if you cut a leather coral in half, it looks just like a plant stem, with numerous conducting tubules visible.
The thick, gelatinous tissue of a soft coral is impregnated with several calcareous particles known as spicules. In some octocorals the spicules are sparsely distributed throughout the tissue, while in others they are very densely packed. The spicules can be separate or they can be fused together to form a strong skeleton. The shape of the spicules is used by coral taxonomists to classify the various octocorals. In some octocorals, a horny material that serves as a strengthening skeleton, is secreted internally. At present, octocorals are divided into six orders: Alcyonacea, Coenothecalia, Gorgonacea, Pennatulacea, Stolonifera and Telestacea.
Alcyonacea contains all the genera that hobbyists are most familiar with as soft corals, such as Cladiella, Lobophytum, Sarcophyton, Sinularia, Anthelia, Xenia, Capnella, Dendronephthya, Lemnalia, Litophyton and Nephthea. All of these genera, with the exception of Dendronephthya, have zooxanthellae.
Alcyonacea have fleshy or leathery colonies that tend to be irregular in shape, with various lobes or finger-like projections. Some resemble large toadstools (e.g., Sarcophyton and Lobophytum), while others are more tree-like in shape (e.g., Dendronephthya and Litophyton). Anthelia and Xenia are two genera that are often confused with each other. Anthelia tend to have longer, more slender polyp stalks than Xenia, and they grow from an encrusting, fleshy mat, whereas Xenia usually have a stalk with polyps arising from the top of the stalk.
Coenothecalia contains only one species, Heliopora coerulea, the Indo-Pacific blue coral. This species grows in large brownish or greenish-gray mounds and has numerous delicate white polyps over its surface. The colonies are heavily calcified and thus appear similar to Millepora. When dead or broken, one can see the blue color of the skeleton caused by the infiltration of iron salts (Hyman 1940). These photosynthetic corals are rarely available, but they do very well in aquariums, even growing over pipes and aquarium walls.
Gorgonacea contain numerous tree-like corals, such as sea whips, sea fans and sea feathers. Some are photosynthetic, some are not. Generally, if they have brown or light brown polyps, they contain zooxanthellae.
Gorgonians have a strong, flexible interior axial skeleton made of a horny material called gorgonin. This skeleton, in the form of rods, provides for greater flexibility and support. The skeleton is surrounded by a layer of tissue in which the polyps are embedded. This tissue also contains numerous conducting tubules and calcareous spicules.
Although there are some encrusting and single-stemmed species, the majority resemble trees in their branched appearance. The precious deep-water red and pink corals of the Mediterranean, Japan and Hawaii are gorgonians too but they lack gorgonin, having instead fused calcareous spicules that form the highly prized material from which jewelry is made. Photosynthetic Caribbean gorgonians have also gained a great deal of attention from the medical community because they are natural sources of some very interesting anti-inflammatory chemical compounds, such as the prostaglandins (see Faulkner 1992).
The Pennatulacea contain the sea pens and the sea pansies. These large, fleshy soft corals are found embedded in soft substrates by a fleshy projection called the peduncle, on top of which sits the expanded portion of the primary polyp, the rachis. Sea pens and pansies are composed of a single large axial polyp (rachis) from which arise numerous, smaller secondary polyps. Although sometimes available commercially, unless you have a deep, soft substrate they should not be purchased. These animals are not photosynthetic and must therefore be fed fine planktonic foods regularly.
Stolonifera are octocorals whose polyps arise from a creeping base that may consist of separate, flat, root-like structures called stolons, or an encrusting mat. The polyps in most forms consist of two sections. The softer, thinner portion that possesses the tentacles and mouth is called the anthocodia, and this portion can retract into the lower, stiffer non-retractile portion called the anthostele. When retracted, star polyps (Cornularia spp. and Clavularia spp.) will display this structure nicely. In the Tubipora (red pipe organ coral), the spicules are so dense they fuse together to form a calcareous skeleton.
All Stolonifera kept in aquariums are photosynthetic and do very well under proper conditions without any additional feedings. I have never observed these corals to feed, and it is entirely possible that they absorb nutrients directly from the water, or feed on bacteria trapped in their body slime.
The last order, Telestacea, also grow from creeping stolons but they give rise to large upright polyps from which smaller, lateral polyps can arise, giving the whole colony a tree-like appearance. The spicules are united through calcareous secretions forming a skeleton (Hyman 1940). Although rarely imported due to their fragility, according to Wilkens and Birkholz (1986) these corals do well under the proper conditions and some species may be photosynthetic, while others need to be fed planktonic foods.
In contrast to the Octocorallia, the Hexacorallia exhibit a great deal of anatomical variation, which makes them difficult to describe in general terms. Unlike the octocorals, hexacorals usually have tentacles and internal polyp septa in multiples of six — although exceptions do occur — but never eight. The oral disc has a prominent mouth that may be situated on a protuberance or have a protruding margin (Hyman 1940). The tentacles of the polyps do not have pinnules as in the octocorals.
There are at present six recognized orders of Hexacorallia: Actiniaria, Antipatharia, Ceriantharia, Corallimorpharia, Scleractinia and Zoanthidae.
The order Actiniaria contains all the organisms we call sea anemones. Sea anemones are quite diverse in their appearance, ranging in size from a few inches to more than 18 inches in diameter (in the case of Stichodactyla gigantea).
The bottom of the anemone is formed into a basal disc with which it attaches itself to the substrate. The other end contains the mouth and is situated in the middle of a broad oral disc surrounded by tentacles of varying length and shape, depending on the species. Many species of tropical anemone contain zooxanthellae, but most temperate species do not. Anemones will benefit from the occasional feeding of finely chopped foods, such as shrimp, but they can do very well without such feedings provided they receive adequate lighting.
Antipatharia contain the well-known precious black or thorny corals, usually found at depths greater than 65 feet. These tree-like corals have a thin axial skeleton with small thorns made of a material — similar to gorgonin — out of which jewelry is made. There is a thin veneer of living tissue from which the simple polyps arise. To the best of my knowledge, these corals have never been kept in aquariums and are not photosynthetic.
The Ceriantharia are non-photosynthetic, anemone-like anthozoans that have a muscular, elongated, cylindrical body with a fleshy foot that extends deep into the sand so that the oral end bearing the tentacles extends outward. The tentacles tend to be extremely fine and long, and some specimens of Cerianthus can sting quite powerfully.
Cerianthus species from the Mediterranean and North Sea have been very popular in European temperate water aquariums for many years. The larger and more colorful tropical species are often seen for sale in North American pet stores. These do best when provided with a thick substrate and are best shown in species displays, as opposed to mixed displays, such as reef aquariums. Because these anemones are not photosynthetic they require weekly feedings of chopped shrimp or frozen marine foods.
Corallimorpharia contain the popular mushroom anemones, which are not really anemones — they are also known as "false corals" and resemble stony corals, but lack skeletons. Polyps can occur as solitary individuals, or in colonies. The tentacles are usually reduced to knobs or small branched protuberances, arranged around one or more mouths.
Various genera, such as Actinodiscus, Amplexidiscus, Discosoma, Rhodactis and Ricordea, are included in this order, but these genera are under taxonomic revision and it is almost certain they will be reclassified in the next few years. These anthozoans also contain zooxanthellae and can grow and reproduce in aquariums without supplemental feeding. Some of the larger species, such as Amplexidiscus, can occasionally be fed chopped shrimp and are quite capable of trapping and eating small fish.
Scleractinia represent the stony or hard corals. Stony corals are basically anemones that are surrounded by a calcareous skeleton. The polyps can be solitary or they can exist in large colonies joined by a common tissue called coenenchyme. Due to the colonial nature of many stony corals, they can build massive structures that result in the development of entire coral reefs.
The majority of stony corals harbor zooxanthellae and derive much of their nutritional requirements from the metabolic products produced by the zooxanthellae. Some of the larger-polyped genera, such as Plerogyra, Euphyllia, Catalaphyllia, Polyphyllia, Trachyphyllia and Cynarina, will benefit from the occasional (once a week) feeding of finely chopped shrimp, but this is not a requirement for success with these corals.
Zoanthidae are a small group of solitary, sometimes colonial, anemone-like anthozoans that lack a skeleton. They are unlike any other anthozoan internally, having a large number of paired and unpaired septa (Hyman 1940). Zoanthid polyps can occur as single individuals in large groups or they can be joined together by a thin stolon, a thin coenenchyme or a very thick coenenchyme, from which only the mouths and tentacles are visible (e.g., Palythoa caribaeorum). The most common genera offered for sale are the photosynthetic members of Palythoa, Parazoanthus and Zoanthus.
Great strides in the art and science of keeping corals in captivity have been made by hobbyists in the last decade. Unfortunately, it appears that many in the scientific community are unaware of these advances, which has lead to some uninformed comments being made by marine scientists, such as those that have appeared in several national magazines (e.g., see Derr, M., April 1992 Audubon; Stein, J., Sept-Oct. 1993 Rodale's Scuba Diving).
In truth, many people are not only keeping corals alive in aquariums, but are also growing and propagating them. Having said this, I would be remiss if I did not mention that many other aquarists also experience difficulty.
For the most part, this is because of the spreading of misinformation in the hobby and the lack of proper instruction. Readers of this publication and Aquarium Fish Intl. generally receive better information than most, but there are still pitfalls that await, and dealing with those can tax the patience of most people. If you are not prepared for the sacrifice and expense that must be made to keep animals, such as corals, in captivity, then I strongly urge you not to attempt to do so.
Keeping corals basically involves three main criteria: light, filtration and water quality. Much has been written on these topics in the past, and readers of AFI (as well as Aquarium USA 1994) may recall the eight-part series I wrote back in 1990, plus two articles of mine in 1993. Much of what I wrote then is still applicable, and I urge you to seek out the issues listed in the sidebar for more details on the techniques required.
Alderslade, P.N., Lovell, E.R. and C.C. Wallace. 1984. Corals. In: Reader's Digest Book of the Great Barrier Reef. Mead and Beckett Publ., Sydney, Australia.
Barnes, R.D. 1974. Invertebrate Zoology. W.B. Saunders Co., Toronto.
Delbeek, J.C. 1989. Reef Aquariums Part 1: An Introduction. Aquarium Fish Intl. 2(2):24-33.
1990a. Reef Aquariums Part 2: Filtration. Aquarium Fish Intl. 2(3):28-37.
1990b. Reef Aquariums Part 3: Chemical Filtration. Aquarium Fish Intl. 2(4):16-28.
1990c. Reef Aquariums Part 4: Lighting. Aquarium Fish Intl. 2(5):26-37.
1990d. Reef Aquariums Part 5: Nutrition. Aquarium Fish Intl. 2(6):26-35.
1990e. Reef Aquariums Part 6: Coral Aggression. Aquarium Fish Intl. 2(7):26-32.
1990f. Reef Aquariums Part 7: Examples of the best. Aquarium Fish Intl. 2(8):20-31.
1990g. Reef Aquariums Part 8: More Representative Reef Systems. Aquarium Fish Intl. 2(9):20-31.
1993a Your first reef aquarium: Stocking for success. Aquarium Fish Intl. 5(9):18-31.
1993b. Your first reef aquarium: How to create a miniature coral reef system at home. Aquarium USA 1994: 18-32, 114-122.
J. Sprung. 1994. Reef Aquarium: A Comprehensive Guide to the Care and Maintenance of Tropical Marine Invertebrates, Stony Corals and Tridacnid Clams. Ricordea Publishing, Coconut Grove, FL, USA, 33133. (In Press)
Faulkner, D.J. 1992. Biomedical uses for natural marine chemicals. Oceanus 35:29-35.
Fautin, D.G. and G.R. Allen. 1992. Field Guide to Anemonefishes and their Host Sea Anemones. Western Australia Museum, Francis, St. Perth, WA 6000.
Hyman, L.H. 1940. The Invertebrates: Protozoa through Ctenophora. McGraw-Hill Book Co. Inc., New York, London, 726 pp.
Kaplan, E.H. 1982. A Field Guide to Coral Reefs of the Caribbean and Florida. Houghton Mifflin Co., Boston, 289.
Veron, J.E.N. 1986. Corals of Australia and the Indo-Pacific. Angus and Robertson, Publ., North Ryde, Australia, 644 Pp.
Vine, P. 1986. Red Sea Invertebrates. IMMEL Publishing, London, England, 224 Pp.
Wilkens, P. 1973. The Saltwater Aquarium for Tropical Marine Invertebrates. Wuppertal Elberfeld, 216 Pp.
--------- 1975. An experimental marine aquarium. Mar. Aquarist 6(5):49 - 55.
--------- 1976. More flower animals. Mar. Aquarist 7(10):31-44.
--------- 1990. Invertebrates: Stone and False Corals, Colonial Anemones. Engelbert Pfriem Verlag, Wuppertal, Germany, 136 Pp.
J. Birkholz. 1986. Invertebrates - Tube-, Soft- and Branching Corals. Engelbert Pfriem Verlag, Wuppertal, Germany, 134 Pp.
Williams, G.C. 1993. Coral Reef Octocorals: An Illustrated Guide to the Soft Corals, Sea Fans, and Sea Pens inhabiting the Corals Reefs of Northern Natal. Durban Natural Science Museum, 63 Pp.
Wood, E.M. 1983. Reef Corals of the World: Biology and Field Guide. T.F.H. Publications, Inc., Ltd., Neptune, NJ, U.S.A. 256 Pp.
The spectrum of light produced by the bulbs or tubes should not contain high amounts of red or yellow, but should contain sufficient amounts of blue light. The lights should not overheat the aquarium. They must not emit ultraviolet light in large amounts (if they do, then the light fixture should have proper UV shielding). Finally, they should provide adequate intensity for the depth and types of animals you will be keeping.
When dealing with fluorescent lamps, most meet the above criteria. However, beware of using so-called "plant lights," which contain too much red in their spectrum and may promote the growth of undesirable microalgae. If you are using eight or more lamps, then you can use one such light to enhance colors if you wish. The point to remember is that red spectrum lighting must be a minor component of the lighting system.
The use of metal halide lighting (specifically HQI — halogen quartz iodide) has increased greatly in the last few years. Again, the same criteria mentioned above apply to these bulbs. To help ensure this, the color temperature of the bulb should be more than 5000 Kelvin, and the color rendition index (CRI) should be above 90. These values are usually printed in the literature, or ad copy of the bulb.
When used properly, these bulbs give excellent results, but they tend to be more expensive than fluorescents. Recently, 6500 K HQI lamps have appeared on the market, and so far these appear to be giving superior results even without the use of additional actinic (blue) lighting. Metal halides have a tendency to overheat aquariums if placed too close to the water surface and/or when they are poorly vented. When using such bulbs the lamps should be at least 18 inches above the water surface.
In order to keep corals, the water quality should be maintained with relatively low dissolved organic and inorganic substances. Many of the corals kept today come from inshore areas and, as such, are less susceptible to high nutrient loads in the water. Unfortunately, with high nutrient loads, other problems arise that can threaten coral health, such as excessive undesirable algae growth and the downward drift of pH.
In order to keep these nutrients in check, some sort of filter system that provides biological and chemical filtration is required. Many of today's reef tanks are being installed with live rock as the only biological filter, and protein skimmers (foam fractionators) as the sole means of chemical filtration. This setup is referred to as the "Berlin" method, first advocated by members of the Berlin Association for Marine Aquaristics.
Activated carbon is also used for chemical filtration, but some reefkeepers prefer to limit its use to prevent chronic shortages of essential trace elements, specifically iodide. In this case, the carbon is added for a few days, each month, to remove the yellowing compounds (organics) that build up in the water.
Although trickle filters are still useful filtration aids, experience has shown that they are not required to attain success in keeping corals when the Berlin method is used. This, by the way, is only true for reef tanks with live rock. Fish-only setups will need a supplemental biological filter.
Of course, irrespective of the filtration and lighting systems, if the proper water quality cannot be maintained, the animals will not survive. Water quality is a rather amorphous term and encompasses many different things, some of which we can easily measure and some we cannot. Water chemistry is one aspect of water quality that must be maintained within certain limits. Generally speaking, the following parameters are of concern: temperature, pH, alkalinity, calcium, nitrate and phosphate.
If you cannot provide the correct temperature range, you will not be successful no matter what else you do. The temperature should be between 74 and 78 degrees Fahrenheit for the best results. If the temperature varies a few degrees during the course of the day, this is usually not a problem. The most common problem is too high a temperature — above 80 degrees Fahrenheit.
There are a number of possible solutions to this, and one or all of them may be necessary. First, you should ensure that the top of the tank has adequate air circulation, either by having a hanging light fixture over the aquarium, or by using a well-ventilated light hood. This will aid gas exchange and evaporative cooling, and will retard heat buildup in the hood.
You could also design your sump so that it is open and provides a large surface area for evaporative cooling. Evaporative cooling can be additionally enhanced by placing a fan over the water surface.
The most expensive, but effective, solution is to purchase an aquarium chiller designed for saltwater use. Somewhat less expensive is to purchase an air conditioner unit for the aquarium room. This has an added cooling benefit for the aquarist as well, which may help you convince your spouse of the extra expense!
The pH of aquarium water is a measure of the concentration of hydrogen and hydroxide ions. If the hydroxide ion is in greater abundance in a solution, the solution is said to be basic. If the hydrogen ion is more common, the solution is acidic. Values of pH range from 0 to 14.
If the number is less than 7, the solution is acidic. If it is greater than 7, the solution is basic. If it equals 7, the solution is neutral. Seawater is a basic solution, with a pH that is 8.2 to 8.4.
In our aquariums, natural processes tend to lower the pH, so we need to keep an eye on it. The pH is easy to measure with a saltwater pH test kit, and it should be maintained between 8.0 and 8.5. A sure sign that the pH is too low or high is the failure of your corals and clams to open completely.
A lowered pH can also result in the spontaneous growth of undesirable algae, as well as a dinoflagellate growth that has the consistency of nasal mucus. It traps air bubbles and, in severe infestations, can make your tank look like a field of tiny hot air balloons. Raising the pH to 8.3 to 8.5 through the use of a calcium hydroxide solution can cure this malady. However, if the pH falls below 8.2 this problem can quickly reappear.
This is a term that has caused a great deal of confusion among both novice and advanced hobbyists. Stated simply, the alkalinity of a solution is its ability to act as a pH buffer against drops in pH. The greater the alkalinity, the greater its ability to prevent rapid pH fluctuations. Once the alkalinity is exhausted the pH can fall rapidly.
Alkalinity is provided in the aquarium by various negatively charged ionic compounds (anions), such as carbonates, bicarbonates, borates and hydroxides, to name just a few. The rather confusing term carbonate hardness has also been used to describe alkalinity, but this refers only to the carbonate and bicarbonate portions of alkalinity and does not take into consideration the other compounds involved. Therefore alkalinity is generally slightly higher than carbonate hardness (Spotte 1979).
To add even more confusion, two different units of measurement are used among the numerous test kits available, all of which are simple to use. Some test kits for alkalinity use the metric unit of milliequivalents per liter (meq/L), while other kits use the German unit, degrees of carbonate (German = karbonat) hardness (dKH). To convert meq/L to dKH, simply multiply by 2.8.
Natural seawater has an alkalinity of 2.1 to 2.5 (6 to 7 dKH). Alkalinity values in the aquarium should be maintained between 2.5 and 3.5 meq/L (7 to 10 dKH) through the use of commercial buffers. These buffers should also be used to maintain pH. There a numerous powdered buffers available that will maintain both alkalinity and pH, and the majority work very well by raising the alkalinity without causing rapid changes in pH. Beware of any buffers that cause rapid adjustments of pH.
Calcium is the primary building block of the corals, clams, calcareous algae and many other organisms that we would like to grow in our aquariums. Without adequate supplies of calcium these organisms will not flourish and most will eventually waste away and die. Calcium levels are measured in parts per million (ppm) or milligrams per liter (mg/L). Calcium levels in natural seawater range from 380 to 480 mg/L depending on location. In our aquariums, calcium levels should be maintained between 380 and 450 mg/L to ensure proper growth and longevity of our animals.
How one obtains these levels is a matter of choice and debate. Some reef aquarists advocate the use of powdered calcium chloride, while others recommend powdered calcium hydroxide. There are also numerous calcium supplements on the market with wondrous claims as to their efficiency. It remains to be seen how well these products will perform over long-term use.
Calcium chloride is easy to use and does not directly affect the pH of the aquarium. The danger in its use is that you can easily overdose and quickly raise the calcium ion level in the aquarium. This has two effects. It raises the specific gravity rapidly and it can cause a rapid drop in alkalinity and, subsequently, pH. Calcium chloride can be added directly to the sump as a powder, but it is best to dissolve it first in freshwater and slowly drip it into the aquarium over a period of several days.
Calcium hydroxide is messy to use, has a high pH that can be hazardous because it can rapidly raise the tank's pH if it is added too quickly, and it is caustic. It does add calcium slowly to the aquarium and does not deplete alkalinity. Generally sold as a powder, it is added to freshwater at the rate of 1 teaspoon per gallon (1.5 grams per liter). This solution, commonly called kalkwasser (German for calcium water), is shaken and allowed to settle. The clear liquid that results is then slowly added to the aquarium as make-up water for evaporation. This is best done using a float valve or dosing pump. For more details on calcium additions see Sprung and Delbeek (1990), Sprung (1991) and Delbeek and Sprung (1994).
Nitrate has often been implicated as a causative agent for failure in reef aquariums. However, if the truth be known, there has never been any conclusive evidence for this. Nitrate levels of 40 ppm have been measured in aquariums with large stony coral growths. It would be safe to say that maintaining a low level of nitrate is still desirable, but you need not become fanatical about it. Where nitrate could still pose a problem though, is in undesirable algae growth.
Phosphates can cause problems in marine aquariums, especially reef aquariums, if they are allowed to build to levels above 0.2 ppm. Elevated phosphate levels will fuel unwanted algae growth and will interfere with the calcification processes of corals and coralline algae. Phosphates are present in many forms in the aquarium, not all of which can be easily measured. The real trick with maintaining low phosphate levels is to minimize their input and maximize their removal.
Minimizing input relates directly to the quality of the freshwater you are using for evaporative top-offs. This water should be as free of phosphate and nitrate as possible. If you are using this water to make kalkwasser, the high pH of this solution will result in the rapid precipitation of any remaining phosphate, in the form of calcium phosphate. Beware also of any additives and salt mixes that may contain unacceptable levels of phosphate. Avoid using any liquid food supplements that may contain phosphate, and check your activated carbon to make sure it does not release phosphate (see Delbeek and Sprung 1994).
To maximize the removal of phosphate, the use of adequate protein skimming is essential. Also, the frequent (every two to three days) cleaning or replacing of the prefilter is a must. In aquariums with poor water flow, the regular removal of detritus from beneath and between the rocks should be a part of your monthly routine.
The topic of trace elements has become quite popular again in the last few years. As a rule, very little is known about what elements corals require to grow and survive. At this point it is safe to say that strontium is a very important element in the growth of many calcareous organisms. Strontium supplements can now be purchased in almost any marine reef aquarium store. Iodide appears to be another important trace element for corals, clams and crustaceans. Again, potassium iodide and other iodide sources are readily available for purchase.
At the time of this writing, there is no substantial evidence for the importance of any other elements. That is not to say that in the future the importance of others will not come to light, but for now, these two are the only ones I can definitely say, in my experience, are necessary for optimum growth.
Although a series of books could be written on the care of individual coral species, space limits me to generalizations on the more common genera. See Wilkens (1990), Wilkens and Birkholz (1986) and Delbeek and Sprung (1994) for more detailed information on coral identification and care.
Octocorallia / Alcyonacea / Leather Corals
There are many different species of Alcyonacea that fall under the category of "leather" corals. Most belong to the genera Sarcophyton, Lobophytum and Sinularia. Generally speaking, they do well in moderate to strong lighting and require a good current with frequent strong bursts. No feeding is required. Be careful when placing them in the tank because some varieties of Sinularia can sting other corals badly.
Sinularia spp. can be easily propagated by cutting off small sections with sharp scissors and fastening the cuttings to a rock with thread, or rubber bands. Protect the cuttings from strong currents until they firmly attach — then treat them as adult colonies. Sarcophyton and Lobophytum spp. are more difficult to propagate. Cuttings can be taken from the capitulum, but this tends to leave the mother colony with a moth-eaten appearance.
Colt Corals (Cladiella spp.)
These soft corals include the common colt coral, as well as a number of other varieties. They do quite well in medium to strong lighting with a moderate current. No feeding is required. They can be propagated by gradual pinching off of branch tips or by cutting with scissors.
The other genera in this order tend to have a tree-like appearance, such as Litophyton, Lemnalia and Capnella. These corals tend to be more delicate than the other Alcyonacea and are not recommended for the beginner. They prefer medium to intense light and medium to weak water motion. Feeding is not required, but they will take fine planktonic food when offered.
Other Soft Corals
Another common soft coral is Anthelia spp. These have large polyps with long stalks (6 inches). They grow as an encrusting mat and require moderate to strong lighting and moderate current. Closely related to Anthelia is the genus Xenia. There are many species of Xenia available, some that rhythmically pulsate their polyps. Xenia tend to be rather delicate and are not recommended for the beginner. These two corals do not feed and most likely get their nutrition from their zooxanthellae and direct uptake from the water.
Gorgonacea / Photosynthetic Gorgonians
The majority of Caribbean gorgonians are photosynthetic and are therefore easy to keep without direct feeding, unlike their Pacific cousins, which are not photosynthetic. Most have brown, blue or purple stems, some are yellow, and all have brown polyps. The thicker branched varieties are the easiest to keep.
Some forms will grow very quickly (inches per month), while others grow more slowly. They can be easily propagated through cuttings and can be given supplementary feedings with live baby brine or adult brine shrimp. I have also fed them successfully with live blackworms. Not all species will feed, and it is not mandatory to feed photosynthetic gorgonians. Specimens belonging to the genera Pseudoplexaura, Pterogorgia, Pseudopterogorgia, Eunicea and Muricea are all easily maintained.
Gorgonians require moderate current with the occasional strong burst, and medium to strong lighting. Many photosynthetic gorgonians are sensitive to ultraviolet light. If your specimens do not open after a period of time, try placing a piece of ultraviolet-absorbing material under your light source, such as glass or plexiglass — the recovery is often dramatic.
If there are bare portions of the skeleton showing, these can regrow very quickly provided the specimen is healthy and no microalgae is growing on it. Sometimes there are numerous bare spots, especially at the tips, when a specimen first arrives from the collector. If you take a pair of scissors and cut these bare areas off, as close to the living tissue as possible, the ends will quickly (within a day) seal over and no algae can invade the colony.
Star polyps (family Clavulariidae) come in a variety of colors ranging from brown to iridescent green. They do well in medium to strong light, and, if given a strong current, will rapidly spread over rocks and glass. No feeding is required. These corals can be easily propagated by cutting off small pieces and placing them in other areas of the tank. If you have no substrate, try placing some sections of star polyp on the bottom of the tank. They should spread out over the glass making a nice "lawn" of soft coral.
Red pipe organ coral (Tubipora musica) requires medium to strong lighting and medium to weak water movement. Feeding is not required, and it is likely that they receive enough nutrition from their zooxanthellae and direct uptake from the water. Calcium, strontium and, perhaps, iron are the key elements to encourage this species to grow in the aquarium.
Hexacorallia / Corallimorpharia (Mushroom Anemones)
Various species are available in a variety of colors and forms. Some forms do well in lower light areas of the tank (i.e., brown varieties), while others require more light (blue varieties). In general, they require gentle current flows, and do not need to be fed. Keep them away from stony corals as they tend to damage them quite easily. See Delbeek (1987) for more detailed information on keeping mushroom anemones.
If, after six to 12 months, your tank is doing well and you have no microalgae problems, then you can try some stony (hard) corals. However, I hesitate to recommend them to beginners, and you can have a very nice and interesting tank without them. If you can concentrate on soft corals, you should have a very stable tank that will grow quite nicely. Eventually you can propagate many of the soft corals from cuttings and trade them for other species you may not have.
If you would like to try keeping some stony corals, the following is a listing of the common species offered for sale. First, however, there are a few points you should make note of before you purchase a specimen.
Number one is to make sure that there are no bare areas on the coral skeleton. These areas may recover, but more often than not they only become substrate for microalgae. Once microalgae takes hold you can pretty well say goodbye to that particular piece — eventually the algae will spread and destroy the rest of the coral. It is possible for such a piece to heal, but this requires that no microalgae be present in your tank and that none has begun to grow on the damaged areas.
Secondly, check to see that the tissue of the coral extends well over the edges and down the sides of the skeleton. This is not always easy to see, as the polyp(s) may be so large that they obscure the skeleton underneath (e.g., Euphyllia and Catalaphyllia corals). Usually, if all other conditions are optimum (i.e., calcium levels above 420 mg/L and adequate lighting), the coral should regrow these areas without too much difficulty. The exception is when microalgae has already impregnated the skeleton.
Ask the salesperson to gently agitate the piece so that the polyp begins to retract. This will give you a good view of the skeleton and the associated tissue.
The easiest stony corals to keep are Plerogyra sinuosa (bubble coral), the Euphyllia spp. corals (hammer coral, E. ancora, and octobubble coral, E. divisa, are the two hardiest), Favia spp., Favites spp., Herpolitha, Polyphyllia, Fungia and Catalaphyllia (elegance coral). Several of the so-called open brain corals (Trachyphyllia) are quite hardy, as are the Turbinaria (chalice/ plate) corals and Cynarina corals (meat polyp).
Most require only moderate lighting to do well, while Turbinaria does best under stronger lighting. They will also accept feedings of shrimp and clam, but these should be kept to a minimum (e.g., once every two weeks up to once a month), if at all.
For stony corals to do well you should maintain a calcium ion level of at least 420 mg/L. There should be no microalgae present in the aquarium, and a strontium solution should be added weekly. These corals do well in gentle to medium currents. The Euphyllia and Catalaphyllia do enjoy an occasional strong burst of current.
Plerogyra, Euphyllia and Catalaphyllia corals are capable of strongly stinging other corals. Make sure they are placed far enough away from other corals so that their long "sweeper" tentacles cannot touch them.
Some stony corals have been observed to release eggs and sperm in the aquarium, but no reports have been seen on the success of planula formation. However, asexual reproduction has been observed in many of the genera listed above. Asexual reproduction strategies in corals include asexually produced brooded planulae, the formation of polyp "balls," polyp "bail-out," fission and fragmentation, outgrowths and various means of "budding" (Delbeek and Sprung 1994).
A technique employed by faviid corals is the formation of a single polyp with a bit of skeletal material that, when heavy enough, separates from the parent colony and settles on the adjacent substrate. Furthermore, large single-polyped corals, such as Trachyphyllia, Euphyllia and Catalaphyllia, may produce unattached septae that may drag a bit of tissue with them as they separate from the parent by their weight, forming a new colony (Delbeek and Sprung 1994).
Environmental stress plays a role in the stimulation of other means of asexual reproduction. In the case of polyp "bail-out," water temperature, oxygen concentration, pollution, or other environmental stimuli cause a polyp to separate from its skeleton and drift free. This affords great opportunity for dispersal away from the site of stress, without formation of planula larvae.
In aquariums, polyp separation from the skeleton can occur slowly as a result of numerous circumstances. This polyp separation is quite distinct from the bail-out method that occurs in nature. It is much slower, and is more akin to slow death than a quick escape. Such separation can be healed if caught in time, and it is also possible for a separated polyp to form a new skeleton. Portions separated may form new skeleton and drop off as a new colony as in Catalaphyllia (see Delbeek and Sprung, 1994).
The other environmentally induced means of asexual reproduction is the formation of anthocauli in fungiid corals. The environmental stressor is typically an injury to the original polyp, such as burial, stinging by an adjacent anemone or coral, or predation by fish or invertebrates. The area of tissue loss soon produces tiny individual polyps called anthocauli. These form a skeleton of their own as they grow, and eventually they separate from the original injured fungiid, forming a complete new coral. The small point of attachment, once broken, forms a new anthocaulus on the original Fungia, so the reproduction is perpetual. Hobbyists have used such injured, reproductive Fungia and Herpolitha specimens to propagate new colonies for sale and trade.
Finally, Euphyllia, Catalaphyllia and Trachyphyllia species may also produce new polyps like anthocauli between the septae when injured, and Euphyllia spp. regularly bud new polyps along the walls of the skeleton when they are healthy. These new polyps break off at the constricted point of attachment when they are large enough, or fuse with the main colony to form a new branch (Delbeek and Sprung 1994).
Zoanthids (button polyps) are generally very easy to keep and propagate, and they come in numerous colors and shapes. They form colonies on live rock and spread as an encrusting growth. Requirements include moderate to strong light and medium to strong water motion. Some genera, such as Palythoa spp. will feed.
Parazoanthus gracilis is a species that is commonly available to hobbyists. These colonies are usually bright yellow, but in the aquarium they tend to darken to a yellowish-brown color due to the increase of zooxanthellae pigments. Some of the larger-tentacled species (i.e., Palythoa and Parazoanthus) will feed on adult brine shrimp and blackworms, but if fish are present in the aquarium, enough stray food particles should reach the zoanthids.
Keeping corals in captivity is not as difficult a task as many would have you believe, but it is not without its problems. Rapid changes in water quality, sudden outbreaks of undesirable algae growth and other unexpected maladies can tax the patience and resolve of even the most experienced hobbyist. However, the potential rewards and satisfaction that can be gained from a thriving captive reef ecosystem is a powerful lure for many aquarists.