The nutritional requirements of the various organisms that can occur in reef systems are extremely varied and sometimes difficult to determine. In a reef aquarium, we are no longer just dealing with the different feeding habits of fish but also various orders of invertebrates, each with its own peculiarities. Add to this mass of confusion the fact that very little is known about the nutritional requirements of these organisms and one can quickly see that the topic of nutrition in reef systems is a most intimidating one. For this reason, I find the advertising used by some manufacturers of “reef foods” to be rather distressing. Very few of these products are based on any form of original research, and some appear to contain additional chemical additives (e.g., glutamic acid, proline or alanine) that will elicit a feeding response in some invertebrates but have questionable nutritional value.
It is not the purpose of this article to tell you what to feed each organism. The goal here is to outline the various feeding modes that you are likely to encounter and help you decide how best to deal with them. To go into detail on the diet of each organism would require a complete book. There are some references that appear near the end of this article that will help you get started. Read as much as you can and ask questions of your dealer and other hobbyists. Don’t be afraid to experiment when it comes to feeding, but remember to always feed in moderation.
The general approach taken in feeding organisms in captivity is to closely study the diets of these organisms in the wild and duplicate them as much as possible. This approach works quite well with most fish and some invertebrates. However, many natural diets are almost impossible to duplicate (e.g., coral polyps and sponges that are food for a multitude of fish and invertebrates). Another problem is with corals themselves. There are few studies published on the diets of corals. Indeed, there is much debate on how much or even if corals need to be fed.
As noted above, there are various feeding mechanisms used by the inhabitants of reef aquariums. In some instances, the same organism may use more than one feeding strategy, which is probably an adaptation to ensure that as much nutrition as possible can be extracted from the nutrient-poor area that a reef represents. This is particularly the case with many coral species, and for this reason coral feeding strategies will be dealt with separately.
Many marine organisms actually trap and/or catch and digest other organisms, such as shrimp, copepods and marine worms. Some organisms, commonly known as filter feeders, feed by trapping much smaller particles, which range in size from copepods and other zooplankton to detritus and feces. Detritus feeders either digest the bacteria living on this material or the particulate organics that coat such particles (Wotton 1988).
The mode of feeding that has probably elicited the most debate is the one involving the relationship between organisms, such as corals, bivalves and sponges, and the symbiotic algae that live in their tissues. Contrary to popular belief, these algae (usually referred to as zooxanthellae) are not green but are in fact gold-colored dinoflagellates of the genus Symbiodinium. At one point it was thought that there was only a single species, Symbiodinium microadriaticum, but it has recently been shown that there are, in fact, several strains of zooxanthellae: some fast growing, some slow (Blank and Trench 1985; Trench 1979). One other method of feeding that we will examine is the direct uptake of nutrients, such as ammonia, phosphate, proteins and dissolved organics, from the water through the body wall.
The majority of the animals that we keep in our aquariums, such as fish, shrimp, starfish and urchins, fall into the first category of feeding and are consumers of other life forms. These organisms can be omnivorous, herbivorous or carnivorous in nature. Therefore, depending on their size and appetite, these animals may be able to fend for themselves in a reef tank, finding adequate amounts of food without supplemental feedings. More commonly, however, the aquarist will need to provide some or all of the food.
For those organisms that are carnivorous or omnivorous, high-quality flake foods with low amounts of carbohydrates, large amounts of protein and the proper lipids (fish oils as opposed to plant oils) should suffice. The same is true for frozen foods. One concern with any prepared food is the issue of vitamins, pigments and other micro-nutrients. Many of these are destroyed during the manufacturing process and need to be added to the food later. The problem is that these substances have a limited shelf life. By the time the food has moved from the manufacturer to the wholesaler, the retailer and, finally, the hobbyist, the majority of these substances may have been lost through oxidization. As a result, the reef tank inhabitants may not be getting the full nutritional value the food was originally formulated to deliver.
Fortunately, recent developments in aquaculture are making their way into the pet industry, and many foods are now available that contain vitamins stabilized against deterioration. The primary vitamin available in this form is vitamin C. There are a number of foods and vitamin supplements now containing this stabilized vitamin C.
There are some precautions you should take when purchasing and using prepared foods. First, buy food only from a store that appears to have a high turnover of stock. Dusty containers indicate that the food may already have been on the shelf too long. Most flake foods in sealed containers have a shelf life of about 6 months if not refrigerated.
Second, as soon as you open a container of food, store it properly. Primarily, this means not leaving the food close to the tank or in other warm, humid areas. It is best to keep the container tightly sealed and in the refrigerator. Some hobbyists soak their foods in vitamin supplements before feeding. This is a good idea, but only if the vitamins are fresh. Liquid baby vitamins have been used with some success and have the added benefit of an expiration date on the bottle.
Of course, the best foods are those that are as fresh as possible, such as raw shrimp, clams, mussels and marine fish. For large predators, including groupers and lionfish, it is extremely important to avoid feeding them freshwater fish as a staple food. Try and wean them from these foods and onto prepared marine foods or fresh raw foods. The reason for this is that freshwater fish lack certain essential fatty acids that are mandatory for marine fish. A lionfish may live for years on a diet of goldfish and then one day inexplicably die. When these fish are examined, they are usually found to have fatty livers caused by the abundance of improper fatty acids in their diet.
Most omnivorous invertebrates can be fed on prepared foods supplemented with pieces of shrimp, mussels or clams. Organisms that fall into this category include brittle stars, Fromia sp. starfish, sea urchins, shrimps and crabs. Some of these animals will supplement their diet with the algae found in the reef system.
In some reef systems, herbivores can get enough to eat from the various microalgae and macroalgae in the aquarium. Certain species, however, such as tangs, surgeons and sea urchins, will quickly strip the tank of algae and will therefore require additional feedings. These animals can be fed a variety of fresh greens such as green peas, spinach, romaine lettuce, bok choy and various marine algae that can be found in oriental food markets. I have found that blanching the greens for 30 seconds or so helps to soften them, and fish that would not touch these greens before will devour them when blanched. Additional feedings of vitamin-enriched prepared foods are recommended as well.
Organisms that feed on smaller particles such as detritus and its associated components, as well as zooplankton, may or may not need supplemental food. It has been my experience that many forms of fanworm, for example, will exist quite well in aquariums where the fish are fed regularly, without any additional feedings. If you find that the crowns of the fanworms are decreasing in size, possibly indicating that they are not getting enough to eat, supplemental feedings should be offered. Liquid foods work well for the smaller species, whereas live baby brine shrimp will be accepted by the larger species (i.e., Sabellastarte magnifica).
Other species with similar feeding habits include sea cucumbers, clams (other than Tridacna sp.), tunicates and crinoids. Given that large amounts of detritus are produced in reef systems, and that this detritus is often found as a fine suspension in the water due to the actions of burrowing worms in the rock, fish, bacterial action and water movement, most filter feeders receive more than enough to eat and do not need any additional feeding.
Zooxanthellae-bearing corals (hermatypic), both hard and soft, including mushroom anemones (which are not true anemones), zoanthids, anemones and gorgonians, can feed using a wide variety of techniques. They not only can utilize the photosynthetic products of their symbiotic algae, but they can also feed directly on plankton, bacteria, detritus and fish feces. Some corals have even demonstrated the ability to absorb glucose directly from the water (Stephens 1962). Other zooxanthellae-bearing organisms include some sponges and Tridacna sp. clams.
As mentioned above, most corals contain symbiotic algae in their tissues that can supply some of their nutritive needs. I say some because the degree to which zooxanthellae contribute to a coral’s nutrition requirements has been the subject of much research during the past 40 years. It seems that the amount varies depending on the species. In some species of zoanthids, more than 90 percent of their nutritional needs can be met by the zooxanthellae, whereas in others this figure is much lower (60 percent; Steen and Muscatine 1984). The general consensus is that zooplankton do not contribute a major portion of the caloric or carbon requirements of hermatypic corals (Muscatine and Porter 1977).
The general rule seems to be the smaller the polyps, the more important zooxanthellae are in the diet (Porter 1976). Corals feed in a variety of ways. The larger polyped forms (e.g., Euphyllia sp.) can actually feed on shrimp-sized prey that they capture with their tentacles. Other forms collect the slime that forms on the polyps and swallow the microorganisms and detritus trapped in it (Kuhlmann 1985). Still others can directly absorb nutrients (ammonium, nitrate and phosphate used by the zooxanthellae, as well as various amino acids) from the water (Franzisket 1974, Muscatine and Porter 1977, D’Elia 1978, Muscatine and D’Elia 1978).
The problem comes when deciding when and how much to feed. In my personal opinion, numerous coral species do not need direct feeding. Many get more than enough from natural sources in the tank. Every time the fish are fed, particles of food and nutrients are added to the water. The presence of live rock and their associated algae and bacteria produces copious amounts of nutrients, vitamins and other products. Even in the presence of a highly efficient skimmer, Wilkens (1987) found that the levels of amino acids in the aquarium were many times higher than on the reef. It would be safe to assume that many other “nutrients” are just as abundant.
Those polyps that are large enough to eat small pieces of shrimp can be fed once a week or so by directly placing pieces of food on some of the polyps. Zooxanthellae require phosphate, and although they may be able to absorb this from the water, it is generally felt that the main source is from the prey captured by the polyps. However, some soft corals (e.g., Xenia) have never been observed feeding. Lacking stinging cells in their tentacles, Xenia may absorb phosphate and other nutrients directly from the water. Judging from the large number of successful aquariums that I have seen in which the corals are never directly fed, I have to believe that most zooxanthellae-bearing corals do not require direct feeding to survive, grow and multiply.
If you do decide to try and feed your corals, be very careful about overfeeding. Feed sparingly. An occasional feeding of live baby brine shrimp or one of the better liquid foods may be appropriate for some specimens but not others. Pay careful attention when feeding. If it looks like the coral is not ingesting any food, perhaps it does not require additional feeding.
For example, some species of mushroom anemones will feed if food is placed on their discs, but others are never observed to feed. A general rule with mushroom anemones is that if they have large bumps or tentacle-shaped protrusions, you should try to feed them. Smooth-surfaced forms generally obtain enough nutrition from their zooxanthellae. However, Elliott and Cook (1989) showed that the Caribbean corallimorpharian Discosoma sanctithomae relied both on its zooxanthellae and on nocturnal morphological changes to capture prey. Even in those species that have short tentacles, the tentacles are non-retractile, non-motile, practically devoid of musculature and lack significant numbers of nematocysts (Den Hartog 1980). Those mushroom anemones (e.g., elephant ear mushrooms, Rhodactis howesii sp.) that do feed, do so primarily by envelopment, where the prey is trapped in the disc cavity and digested by mysenterial filaments extruded through the mouth.
When one is dealing with corals that do not contain zooxanthellae (ahermatypic), feeding takes on extreme importance. Examples of such organisms include certain gorgonian species, Dendronepthya sp. soft corals and orange cup coral (Tubastrea aurea). In these cases, live or prepared foods should be used. Live foods such as baby brine shrimp and rotifers are excellent for most gorgonians and Dendronepthya sp., whereas Tubastrea should be fed larger items such as live adult brine shrimp or small pieces of shrimp, scallop or fish. Prepared foods can be used as well. Dried or freeze-dried foods can be finely ground and soaked in a vitamin preparation. This sludge is then fed directly to the coral through a pipette or baster. You should not feed such food by simply placing it into the water. This only results in added pollution because most of it ends up in the filter, in the gravel or under rocks.
One last method of feeding seen in reef tank organisms is the direct uptake of organic compounds through the body walls of various marine worms, ascidians (i.e., Tunicates), bryozoans and so on (Sepers 1977). The mechanisms and the importance of this mode of feeding in marine ecosystems are not well understood and certainly needs more extensive research. As it now stands, whether supplementary feeding is necessary in a reef aquarium is essentially unknown, but I believe that the majority of nutrient needs of these organisms can be met by using other techniques.
Considering the lack of knowledge in this area, the addition of increased amounts of nutrients to a tank would appear to be both unfounded and unnecessary. It is always best to take a conservative approach in situations like this. Thus, until it is ascertained exactly what substances are required by each organism and whether these nutrients are depleted in an aquarium, it would be best for hobbyists to proceed with caution.
As I mentioned at the beginning of this article, the topic of nutrition in aquariums is poorly understood at best. This is an area where the experiences of hobbyists can be of utmost importance to scientific researchers. There are a lot more hobbyists than there are people actively researching this field of endeavor. It would be a shame if this tremendous pool of information and experience went unused. Share your information with others, write articles for club or national magazines, keep detailed notes on each of your specimens, disseminate your knowledge and experience, and we will be all better off in the long run.
More Light on Lighting
In a previous installment of this series, I discussed lighting. Because of the somewhat technical nature of the subject, there was confusion concerning a few aspects that need to be clarified. In particular, mention was made of quartz halogen lighting and metal halide lighting. Unfortunately, instead of quartz halogen, the text should have read halogen quartz iodide (HQI). HQI lighting is a form of metal halide lighting suitable for aquariums, whereas quartz halogens are used for automobile headlights. Because their spectrum is not suitable, quartz halogen lights should not be used on a reef aquarium.
There are numerous sources for HQI lights. Two that I am most familiar with are the Energy Saver 5500-Kelvin, 175-watt lamp and a model manufactured by Osram that offers 5200 Kelvin, 250 watts. Osram also manufactures 5600-Kelvin, 400-watt and 6000-Kelvin, 1000-watt lamps. Other HQI lamp manufacturers, such as Philips and General Electric, should also be explored.
Also in the article, the reference to actinic 03 high output (HO) tubes and very high output (VHO) tubes was not correct. There are only 40-, 60- and 120-watt HO actinics and 140-watt VHO actinic 03 tubes available from Philips. The other wattages mentioned were in reference to entirely different HO and VHO tubes, such as Sylvania daylights. These tubes require the use of special HO and VHO ballasts. If such ballasts are not used, then the tubes will not perform at their rated outputs, which can make a large difference in a reef system.
HO tubes require high output 800 milliampere ballasts, while VHO tubes require very high output 1500 milliampere ballasts. Many commercial light fixtures are designed for 40-watt fluorescents. In most cases, these are not sufficient for marine aquariums. The 4-foot, 60-watt HO actinic 03 should be the minimum tube used on most reef tanks deeper than 12 inches. Smaller aquariums (15 to 35 gallons) can use the 2- or 4-foot, 40-watt HO actinics.
Blank, R. and R. K. Trench. 1985. Speciation in symbiotic dinoflagellates. Science 229:656-658.
D’Elia, C. F. 1977. The uptake and release of dissolved phosphorus by reef corals. Limnol. Oceanogr. 22:301-315.
Den Hartog, J. C. 1980. Caribbean shallow water corallimorpharia. Zool. Verhand. 176:1-83.
Elliott, J. and C. B. Cook. 1989. Diel variation in prey capture behavior by the Corallimorpharian Discosoma sanctithomae: Mechanical and chemical activation of feeding. Biol. Bull. 176:218-228.
Franzisket, L. 1974. Nitrate uptake by reef corals. Int. Rev. Gesamten Hydrobiol. 59:1-7.
Kuhlmann, D. 1985. Living Coral Reefs of the World. Arco Publ.
Muscatine, L. and C. F. D’Elia. 1978. The uptake, retention, and release of ammonium by reef corals. Limnol. Oceanogr. 23:725-734.
Muscatine, L. and J. W. Porter. 1977. Reef Corals: Mutualistic symbioses adapted to nutrient-poor environments. BioScience 27:454-460.
Sepers, A. B. J. 1977. The utilization of dissolved organic compounds in aquatic environments. Hydrobiologia 52:39-54.
Steen, R. G. and L. Muscatine 1984. Daily budgets of photosynthetically fixed carbon in symbiotic zoanthids. Biol. Bull. 167:477-487.
Stephens, G. C. 1962. Uptake of organic material by aquatic invertebrates. I. Uptake of glucose by the solitary coral Fungia scutaria. Biol. Bull. 123:648-657.
Trench, R. K. 1979. The cell biology of plant-animal symbiosis. Annu. Rev. Plant Physiol. 30:485-532.
Wilkens, P. 1987. Niedere Tiere: Steinkorallen, Scheiben- und Krustenanemonen. Engelbert Pfriem Verlag, Wuppertal, Germany.
The following references contain information on feeding marine invertebrates:
Colin, P. L. 1978. Caribbean Reef Invertebrates and Plants. TFH Publ., Inc., Neptune City, NJ.
de Graaf, F. 1973. The Marine Aquarium Reference. The Pet Library, LTD., Harrison, NJ.
Freshwater and Marine Aquarium Magazine. R.C. Modeller Corp., Sierra Madre, CA.
Haywood, M. and S. Wells. 1989. The Manual of Marine Invertebates. Tetra Press, Morris Plains, NJ.
Kaplan, E. G. 1982. A Field Guide to Coral Reefs, Caribbean and Florida. Houghton Mifflin Co., Boston, MA.
Marine Fish Monthly. Publ. Concepts Corp., Luttrell, TN.
Marine Reef Newsletter, Aardvark Press, Bridgeport, CT.
Moe, M. A., Jr. 1989. The Marine Aquarium Reference: Systems and Invertebrates. Green Turtle Publ., Plantation, FL.
Teh, Y. F. 1974. Keeping live corals. Marine Aquarist 5(1):19-24.
Wilkens, P. 1973. The Saltwater Aquarium for Tropical Marine Invertebates. 2nd. Extended Ed. Engelbert Pfriem, Wuppertal-Elberfeld, Germany.
Wilkens, P. 1976a. Mini-Reef. Marine Aquarist 7(5):37-42.
Wilkens, P. 1976b. Flower animals. Marine Aquarist 7(9):32-43.
Wilkens, P. 1976c. More flower animals. Marine Aquarist 7(10):31-44.
Wilkens, P. and J. Birkholz 1986. Niedere Tiere: Rohren-, Leder- und Hornkorallen. Engelbert Pfriem Verlag, Wuppertal, Germany.
Wilkens, P. 1987. Niedere Tiere: Steinkorallen, Scheiben- und Krustenanemonen. Engelbert Pfriem Verlag, Wuppertal, Germany.