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Food Availability For Reef Animals

It has been known for some time that corals and other coral reef animals must feed. Eric Borneman has been discussing the various aspects of feeding in corals for the last several months in his column in this magazine, and some of those data are applicable to reefs in general. Nonetheless, it is hard to get a good handle on the feeding dynamics of many reef animals. Interestingly enough, however, what we do know about feeding on a reef, as a whole, indicates that aquarists, generally, go about feeding their animals in the wrong way and with the wrong foods. By examining the types of available food, and the processes of feeding on a reef, I think it will become apparent that many of the problems we have with reef aquaria, such as excess nutrients, excessive growth of undesirable algae, and the inability to keep some animals alive and healthy is simply due to the feeding of inappropriate foods, compounded by feeding in the wrong manner.

Before I can discuss the major points of this proposition, however, I need to "set the stage" and discuss feeding in general, and explore a bit of what we know of reef food webs. Some of this information is the result of basic biological investigations, while other data come from some very nicely done ecological studies on the Great Barrier Reef.

First Point: Animals Need To Feed.

All organisms need some sort of food; in fact, the ability to either feed or generate one's own food is probably a pretty decent criterion for describing life. Many organisms are photosynthetic; in other words, they absorb light and use that light energy to make chemicals. This production of chemicals using light energy is limited to those organisms that have chlorophyll in their bodies: photosynthetic bacteria (cyanobacteria), many types of algae, and plants. Photosynthetically-derived chemicals, which are mostly sugars, may be broken down, thereby releasing a portion of that absorbed light energy as chemical energy.

This secondary production of chemical energy, either in the initial organism that produced the sugar or in some other organism, is the basis for all basic energy utilization in all organisms. It is done by essentially the same chemical processes in all organisms, and this is one of the major reasons to consider all life as related. This process is called "respiration" and unlike the process that we normally associate with the term respiration, it really has nothing directly to do with breathing or gas exchange. Instead, it refers to the chemical breakdown and oxidation of sugar to release useable chemical energy in a cell.

The fact that this production of useable energy from the sugar is independent of the production of the sugar has some rather profound consequences. Because of this independence, the sugar may be broken down and used some distance away from where it is made and some time after it is made. This means that the organism that uses the sugar doesn't have to be the one that makes it. In other words, the development of the cellular biochemistry that allowed for the indirect utilization of light energy, allowed for the development of animals. Animals are organisms that don't have the photosynthetic machinery which is characteristic of the plants, algae, or cyanobacteria that comprise the primary producer trophic level in ecosystems. They must consume their food, and because of this they must either eat plants, plant byproducts, or other animals.

Figure 1. Most reef animals are eating small zooplankton that are gelatinous in nature. This is a photo of a larvacean tunicate called Oikopleura. Unlike benthic tunicates, or sea squirts, larvaceans spend their entire lives as planktonic animals. They construct a transparent "house" (the edges of which are indicated by the arrows) out of mucus secreted by the epidermis of the animal's head. This house is about ¼ inch in diameter. In the house are the filters they use to collect food. The animal pumps water through the filters, and when they are full, the animal eats them, abandons the house and secretes a new house. It may do this several times an hour. Larvaceans, and old larvacean houses, are some of the most common, and nutritious of the gelatinous zooplankton (See Alldredge, 1972).

Second Point: Animals Need To Feed On Organisms; They Don't Just Live On Photosynthetic Byproducts.

Photosynthesis produces only sugars. It is the process of using light energy to fuse six carbon dioxide molecules and six water molecules together to make a simple sugar. In doing so, it gives off six oxygen molecules as a waste byproduct. Sugar is called a carbohydrate, because it contains only carbon, hydrogen and oxygen. Carbohydrates are useful and necessary chemicals; they may be burned for fuel, converted to fats or starches to store fuel, or fashioned into long chains as structural molecules such as cellulose and chitin. What they can't do, however, is be used to directly make a protein.

Proteins are the building blocks of all animal tissues, and the major components of all cells in all organisms. They are made of subunits called amino acids, often hundreds of them, fastened together in long chains. Amino acids, as their name implies, are molecules that have both an acid and an ammonia residue attached to them. Over 150 amino acids are found in nature, but the vast majority of proteins are made from only about 20 of them. This large number of amino acids may be hooked together in an almost endless variety of ways. And animal chemistry can build, remodel, and modify proteins wonderfully well. What animal chemistry cannot do is synthesize an ammonia group from nitrogen and hydrogen, nor can animals utilize nitrate or nitrite to form ammonia. This synthesis is largely done by bacteria or photosynthetic organisms.

Animals cannot manufacture amino acids from such basic chemical constituents as an ammonia or amine group and an organic acid, consequently, they must get them from some other source. Coral reef animals have one or two options for obtaining their amino acids. If they have zooxanthellae, they may get some amino acids from the zooxanthellae. Unfortunately, this is a zero-sum situation. As the zooxanthellae live within their host, any ammonia that they can utilize must come from their hosts' tissues as a waste product. If such ammonia is a waste product of the host, it is largely a byproduct of the host's metabolism or digestion. This means that the hosts always will require more amino acids, by a very large margin, than the zooxanthellae can provide. What the zooxanthella may do, however, is provide particular types of nitrogenous products unavailable elsewhere. However, even so, zooxanthellate animals must be getting their nitrogenous chemicals from another source, and that source is from feeding of one sort or another. Animals without zooxanthellae will not, of course, have this option. They simply must fulfill all of their needs from feeding.

Marine animals typically require that between five percent and 60 percent of the dry weight of the diet must be protein. For optimal growth of fish, the diet must be from 30 percent to 60 percent, depending on the fish. The absolute requirement from most inactive invertebrates is toward the lower end of the range, but for highly active invertebrates such as squids, it is likely as high as fish. All of this protein must come from either eating some other animal, alga, or plant; direct absorption from the water around the animal, or from a zooxanthellate symbiont. Direct absorption of dissolved amino acids is typically efficiently done in most marine invertebrates, however, there really is very little of this material available in natural systems. In a coral reef aquarium, however, this may be major source of amino acid accumulation by many animals. Production of amino acids by a zooxanthellate symbiont is of limited value, as most animals require a far larger amount of amino acids than may be available from this source. However, this latter source may provide some essential materials. Most amino acids, however, probably come from the assimilation of foods, including bacteria. Bacteria, in fact, are an important food for most benthic or bottom-dwelling marine animals. This is because bacteria have higher nitrogen to carbon ratios in their cells than do either typical animals, plants or algae. As a consequence many marine animals are specialized to eat bacteria, either directly out of the water column or indirectly as a frosting on sediment or detritus particles.

Protein is often a critical resource for animals. Farmers and aquaculturists have long known that one way to get maximum growth in captive animals is to make sure that they have access to a high protein diet. Such diets promote rapid growth and seem to foster generally good health in animals. Unfortunately, such diets are quite unnatural in coral reef areas.

Figure 2. One other common member of the gelatinous zooplankton food category of coral reef animals is yet another type of pelagic tunicate called a "salp." Salps are much like the benthic sea squirts, except that they live totally in the plankton as mobile colonial animals. They form long chains of individuals fastened together at the sides of their tunics. They move through the water propelled by the water they suck through themselves to filter it. This is actually a type of jet propulsion. Salp colonies may be huge. I have seen salp chains, colonies really, consisting of many thousands of individuals, in excess of 70 feet long, moving through the water. In this photo, the guts of the animals are clearly evident and the edges of the bodies are faintly visible. Each individual here is about an inch wide. Both living and dead salp individuals and chains are also common foods of reef animals, including fishes (see Figure 4).

Some Data From Real Reefs

Good data about feeding in corals is relatively sparse and surprisingly diffuse. There are a few studies from the Caribbean, a few from the Red Sea, a few from here and a few from there. There is no good coherent body of knowledge regarding the types of foods corals and coral reef animals actually eat, and in what quantities. And, we really have few data on what foods and in what quantities are necessary to keep corals and coral reef animals in good health. There is a real good reason for this lack of information. Although it is conceptually easy to visualize how to collect such feeding data, in practice collecting such data becomes very difficult. The food is often small and fragile; the animals tend to regurgitate when handled. Additionally, these animals often digest the food very swiftly. This means the only way to collect the animals without losing a lot of information is to preserve the animal in place as you are collecting them. Often it involves injecting the coral, for example, with a preservative and then having to chisel or break that part of the colony free, after which it is put into a labeled container, and more preservative is added. Now, this all sounds easy enough, but then conceive of doing it while floating weightless in the water column with wave surge or currents tossing you around, all the while trying not to destroy the adjacent animals or inject yourself or your diving partner with the preservative, a potential outcome that can really ruin your whole day. Then when you get back to a laboratory, you are faced with the rather daunting, and severely boring, task of microscopically examining the gut cavities of all the preserved animals for food. Finally, if perchance you do find some gut contents, you have to try to recognize and count them. This may sound relatively easy, but speaking from the experience of having to identify gut contents, there are few tasks in biology harder than trying to identify half digested parts of, for example, gelatinous animals, or perhaps the feces of a fish that ate a gelatinous animal. So… the bottom line is that data from corals and many of the bottom-dwelling animals are few and far between.

Figure 3. Other members of the "gelatinous zooplankton" are larvae such as this small snail. The larval snail's shell is to the left, and the two lobes of an extendable feeding apparatus called the "velum" are extended from the shell. These small larvae are both favored foods of a lot of fishes, and surprisingly abundant in the plankton. This animal was about 1/250th of an inch across.

However, there are some data about what coral reef fishes are eating, and those data may provide some very useful insight in the feeding dynamics of this ecosystem and our captive counterparts. One of the inherent difficulties of the data that are available about feeding on a coral reef is that the whole story is seldom told. For example, to be able to assess how animals feed, one needs to know what is available for them to feed upon. There are lots of data, most of which are poorly taken, about what is available in the plankton around a coral reef. And there are some data about the feeding of coral reef animals on plankton. Unfortunately, there are very few studies of what the animals are feeding upon that are done with concurrent samples of the available food.

One quite good study discussing zooplankton availability and concurrent feeding by planktivorous reef fishes has been published (Hamner, et al., 1988), and the findings of that research are well-worth examining as they provide one of the few records of the amount of food available to reef animals. Additionally, and possibly more importantly, the data shed some light on the fates of the various potential foods that are moved over a reef crest by water currents.

These researchers examined a reef where they were able to sample water flow from a depth of 25m (82.5 ft) up over the reef crest. The water flow through this area was very nearly linear, so the water moved toward the reef crest, swept upward and over the crest with little mixing or turbulence. During a 12 hour period, 6,000 m3 (= 1,585,200 gallons or 132,100 gallons/hour) flows over that 1 m wide portion of reef crest. This was estimated to bring 1,098,000 potential food items, about 70 percent of which are copepods and larvacean tunicates. These items are the preferred prey of the plankton-feeding fishes living on the reef front. Over the space of that swath of reef front from 25m depth to the crest, were about 500 fishes of 13 different species. Those fishes consumed, over a 12 hour day, 1,180,000 items of food. This was a wet weight of about 0.4 kg per day per meter of reef front. This is roughly a pound of zooplankton food per day. This all works out to 236 items of food per fish, per 12 hour period, or approximately 2.0 grams of food eaten per fish per 12 hour period.

From these data (Hamner, et al., 1988), it is apparent that during the daylight, most relatively large (0.250 mm or roughly 0.01 inch diameter) zooplankton do not make it to the reef at all. The authors note specifically that the fish effectively feed constantly and the food transit time in their guts is often short, sometimes only a few minutes. These feces contain relatively large amounts of undigested foods and are often eaten by other fish. Eventually, however, a large amount of the zooplankton food that would have impinged upon the reef does make it to the reef, albeit modified into the form of fish feces. This is rapidly ingested by corals and other benthic animals. At night, of course, most of the zooplankton approaching the reef does encounter the benthic reef animals. The number and volume of animals during the night may be significantly higher than during the day as many zooplankton animals will migrate to shallower depths during the night, and these animals would be caught in the currents and swept over the reef.

A couple of points are worth noting here. The calculation of both feeding rates and plankton numbers are approximate, and probably accurate within about plus or minus ten percent. The fact that the number of potential food items, and the number of eaten foods agrees as well as it does, also within about plus or minus ten percent, is really quite remarkable. Also, what is apparent is that the fish eat ALL the plankton approaching the reef. NONE of it will reach the reef during the day when the fish are feeding.

Additionally, the researchers looked at the gut contents of 11 species of planktivorous fishes found in their one meter wide swath. These species included three species of fusiliers: Caesio cuning, Cuning's fusilier, C. caerulaurea, the scissor tail fusilier and Pterocaesio diagramma, the two-lined fusilier, as well as eight species of damsel fishes: Chromis atripectoralis, the black axil chromis, Neopomacentrus azysron, the yellowtail demoiselle, Abudefduf whitleyi, Whitley's sergeant fish, Amblyglyphidodon leucogaster, the whitebelly damsel, A. curacao, the staghorn damsel, Pomacentrus lepidogenys, the scaly-cheek damsel, Pomacentrus coelestis, the neon damsel, and Pomacentrus molluccensis, the lemon damsel. They examined the number of dietary items in the guts of the fish, as well the percentage of fish which had the items. The dietary categories they examined were: algal fragments, copepods, larvaceans, salps, foraminiferans, eggs, miscellaneous crustaceans, amphipods, gastropods, planulae (from corals), chaetognaths, pteropods, and scales (from fish). I modified and condensed these data and those data are summarized in Figure 4, which shows the average number of fishes of a given species containing the various dietary items. I condensed the data from the two months, September and December, sampled in the study, to give the single average values of Figure 4.

click here for full size picture
Figure 4. The average number of fish sampled of the given species containing the various items as gut contents. Data modified from September and December data, of Hamner, et al. 1988. Fusiliers forage some distance from the reef, and to differentiate them from the various damsels, they are color coded with various shades of pink. The various species of Pomacentrid (damsel) fishes are coded with yellows, greens, and blues.
Click on image for larger version.

Some things are immediately apparent. All of these fishes eat large amounts of crustacean prey, particularly copepods. Larvacean tunicates were also a major source of food, for all species, as were, to a lesser extent, pelagic gastropods. There are some interesting, and obvious differences in diets as well. Fusiliers do not eat algae, whereas all of the Pomacentrids had some algae in their guts and some had quite considerable amounts.

These data were the results of examining the gut contents of 240 individual fishes, and the number of prey items per fish was also enumerated. The number of individual prey items in the gut contents was quite considerable. The average number of food items in the guts of black-axil Chromis in December was 665, and for the average yellow tail demoiselle had 1,036 during the same sampling period. The average number of copepods in the guts varied from two to over 200, and the fusiliers typically had far more than the pomacentrids. The diversity of dietary items per fish species was generally high with most species eating at least items from ten of the 13 enumerated food categories.

Fish Diets

From this study, it is apparent that these fish are feeding continuously throughout the daylight hours. They are eating small items, but on the average they eat an item of food every three minutes all day during a twelve hour day. During that period they eat an average of two grams of food per day. As a comparison, during my food and additive study (Shimek, 2001) Ocean Nutrition Products, such as Formula 1, had 70 cubes per 7 ounce package of food. That meant each cube weighed 2.8 g. On the average, if you wish your fish to have the same mass of food that they are likely to eat in nature, presuming the data of Hamner et al., 1988, is applicable to other fishes, you should feed each fish in your aquarium that is the average size of a damsel fish, the equivalent of about 70% of a cube of this food per day. Large fishes would get proportionally more.

Coral Reef Invertebrate Diets

During the day on a natural reef, it appears that virtually no moderately large zooplankter would reach the coral on the reef's face. Nonetheless, this area would be bathed in a diffuse rain of particulate organic material derived from fish feces, dissolved material and microzooplankton. Much of this is generated in the habitat immediately adjacent to the reef by the action of the planktivorous fishes, but a lot of it comes from more distant regions. A lot of it consists of remnants of larvacean tunicate houses, (Alldredge, 1972), but it has other components as well (Alldredge and Silver 1988). Larvacean tunicate houses are gelatinous or mucoid constructions comprised primarily of the mucoid protein, chondroitin sulfate, and embedded or covered with coccolithophores, cyanobacteria and occasionally diatoms. Additionally, of course there are fish and copepod feces and other materials of rather dubious parentage and food value.

The particulate organic material that reaches the reef face proper to be ingested by reef animals in the daytime is not particularly high quality food. The average dry weight Carbon to Nitrogen ratio of the particulate material is about 7.8:1, which is equivalent to the Nitrogen/Carbon proportion of 0.127 (Alldredge and Silver, 1988). This is a food high in carbohydrates and low in proteins. Such a food ratio is very different in content from the average aquarium food, Figure 5. If the weight of protein in a food is divided by the weight of the carbohydrates and hydrocarbons in a food, a rough estimate of Nitrogen/Carbon may be obtained, and I did this manipulation for some of the foods from my earlier study of foods and additives. This estimate of Nitrogen to Carbon is likely to be a somewhat too high as there is carbon in the proteins. Additionally, the presence of oxygen and hydrogen would make the results a bit fuzzy, but these latter elements may largely cancel themselves out on both sides of the ratio. Aquarium food is much higher in nitrogen to carbon than is the typical food item that hits the reef during daylight hours. During the night, the quality of food impacting on the reef animals may be very much higher, consisting of the actual (" = unpreprocessed") zooplankton.

The amount of protein in virtually all aquarium foods is well in excess of what is found in natural reef foods (Figure 5). In the calculations used to construct Figure 5, the moisture contents of the various foods were removed, so these are data compared on a dry weight basis. Most of the wet or frozen foods listed are 75 percent to 85 percent water, so to compare them with regard to weight of the normal sample, see Shimek, 2001. It is apparent from Figure 5 that most marine aquarium foods are very highly enriched in protein with regard to natural Particulate Organic Material.

Figure 5. The amount of Protein divided by the sum the Carbohydrates and Fats from some common aquarium foods (Data from Shimek, 2001); based on a dry weight approximation. The value for natural particulate organic material is from Alldredge and Silver (1988) and is indicated on all foods below it.

Marine Aquarium Ecosystems Foods Differ Significantly From Natural Ones

Several major differences between marine aquaria and natural reef ecosystems with regard to food should be apparent. In this analysis, I am largely ignoring the bacterial component of the foods. There is not much an aquarist can do about this component of the food one way or another. However, all aquarists may significantly control the amount of particulate food in their aquarium. This food will mimic either the zooplankton or the particulate organic material components of coral reef feeding dynamics. For the animals in a system to be healthy, those animals must be fed foods that more-or-less duplicate the qualities of their natural foods, and they must be fed in a more-or-less normal matter. Reef aquarium foods and feeding regimes tend to fail rather spectacularly on both accounts.

The standard reef aquarium is probably fed once about once a day (Shimek, 2002), and the average daily feeding ration weighs 15.39 ± 15.90 grams, or roughly a half of an ounce, wet weight, of food. On a natural reef, this would be enough to provide roughly eight damsel fish with their normal daily allotment of food. Unfortunately, this amount of food all occurs effectively at once or over a very short period in an aquarium whereas on a natural reef it would occur over a 12 hour period. Additionally, aquarium food is a relatively high-protein material. When most reef fishes encounter planktonic patches of food, they eat voraciously, and material gets passed through their guts in a rapid manner resulting in incomplete digestion. This is precisely what happens to many fish in an aquarium when it is fed. If you watch some of your plankton feeding fishes, such as clown fish or damsels, you will see that shortly after the initiation of feeding they start defecating food at an increased rate. In effect, they are pumping food through their guts. The faster the passage of the food through the gut, the less the fish get from it. Perhaps in nature this doesn't matter, as the food is always coming at them. In the aquarium, this effect could be quite deleterious.

In aquaria, fish that naturally feed consistently on small particulate material throughout the day are being forced to exist on bulk feedings once a day or with less frequency. Under such conditions, the animal is going through continuous cycles of near starvation followed by satiation followed by near starvation… This cyclic feeding simply must have a deleterious effect on the fish. Under such situations one could expect lower than normal growth rates, higher stress, increased susceptibility to disease and possibly problems with nitrogen metabolism. During the periods of low food availability the fish would potentially metabolize excessive amounts of protein, resulting in excessive ammonia production. Something similar would be seen with the sessile animals on the rocks of a reef aquarium. Here the food intake is likely intensively periodic with significant periods of non-nutrient input. Growth will also likely be reduced and the animals stressed.

The amount of food impacting on the reef over the course of a day is substantial. Over a section of a natural reef about three feet on side, flows a continuous flood of water carrying with it about 2,000,000 food items with an aggregate weight of about two pounds in a 24 hour period. These tiny food items would be like a rain of diffuse nutrition on the reef and reef animals, particularly the fish.


It is apparent that coral reef planktivorous fishes, and this is most of those kept in aquaria, would benefit from changes to the normal aquarium feeding regimen.
They should be fed by some sort of continuous feeding apparatus.
The food dispensed by such an apparatus should be particulate in nature, and very small. The largest sizes should probably be on the size of a brine shrimp or smaller.
Such food need not be specifically formulated to be highly nutritious: rather it should be of low to moderate nutritional value. If aquarium fish are able to eat more continuously and slowly, they will get much more nutrition out of each food item than they do now. Feeding a low quality food should result in significantly less nutrient accumulation than is now commonly seen in tanks.

In effect, we need to turn our feeding regime on its head. Rather than feeding a small amount of highly nutritious food once a day, we should be feeding a large amount of low nutrient value food frequently. Such a feeding regime as this should reduce significantly the amount of pollution effects in reef aquaria. Additionally, there would not be a daily pulse of nutrients to temporarily overwhelm the biological filter. In turn, there would less potential growth of problem algae and the development of a more balanced and easily controlled assemblage of animals within the tank.

If you have any questions about this article, please visit my author forum on Reef Central.

References Cited:

Alldredge, A. L. 1972. Abandoned Larvacean Houses, A Unique Source of Food in the Pelagic Environment. Science. 177: 885-887

Alldredge, A. L. and M. W. Silver. 1988. Characteristics, Dynamics and Significance of Marine Snow. Prog. Oceanog. 20:41-82.

Hamner, W. M., M. S. Jones, J. H. Carleton, I. R. Hauri, and D. McB. Williams. 1988. Zooplankton, planktivorous fish, and water currents on a windward reef face, Great Barrier Reef, Australia. Bulletin of Marine Science. 42: 459-479.

Shimek, R. L. 2001. Necessary Nutrition, Foods and Supplements, A Preliminary Investigation. Aquarium Fish Magazine. 13: 42-53. Available online at:

Shimek, R L. 2002. What We Put In The Water. Volume 1. Number 3. April, 2002.

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Feeding The Reef Aquarium, A New Paradigm - by Ronald L. Shimek -