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Foods, and feeding in Aquarium Coral Husbandry Shimek Aquarium.Net Feb 97

This month Ron continues his discussion on feeding corals, Aquarium Net has numerous articles written by the leading authors for the advanced aquarist

Why and What: Foods, and feeding in Aquarium Coral Husbandry

by Ronald L. Shimek

Last month I discussed a bit of the nutritional biology of corals in a natural environment, and spent some effort to show that stony corals not only can feed, but do feed. This month, I would like to discuss why they must feed.

A species of Goniopora photographed in turbid water in a lagoon. Goniopora may receive a significant amount of its nutrition by absorption of dissolved nutrient.

It is worth a bit of time to reflect a bit further on the nutritional needs of animals and to examine how those needs are met by corals. Of necessity, this will involve a brief sortie into the realm of metabolic physiology. Now, it is at about this stage that many readers' eyes will glaze over and they tune out. I don't blame such readers - physiology does that to me, too - I am a field ecologist by training. If you find yourself in this situation, grab yourself a hit of your favorite personal stimulant, (I like a nice strong cup of caffeine soup) and try to work your way through the following discussion. What I will try to explain is a bit of HOW nutrients are used and WHAT nutrients can be used for specific tasks in the animal. Finally, I will finish off with a discussion of potentially useful foods for the aquarist to try feeding their captives.

Fundamentally, all animals need to obtain several different kinds of nutrition. They need to obtain nutrient energy - this is the foremost and primary need. Without sufficient nutrient energy, the animals cannot do anything and they die. Additionally, they need to obtain structural nutrients. Structural nutrients are chemicals that can utilized by the organism to manufacture skeletons, or other structural materials such as muscles or connective tissue. Finally, there are nutrients that are necessary in very small amounts. These particular nutrients often are utilized with or as enzymes to facilitate other reactions. I will examine each of these nutrient types in turn, and discuss how aquarists can facilitate their uptake.

Energy Nutrients

Nutrient energy for corals, as well as in all other animals, basically comes from carbohydrates. Carbohydrates are composed of only three types of constituent atoms: carbon (C), hydrogen (H), and oxygen (O). Carbohydrates get their name from the ratio of carbon to hydrogen and oxygen. In carbohydrates hydrogen and oxygen are always found in the ratio two to one; for example, the chemical formula for a simple sugar, glucose, is C 6 H 12 O 6 . In each molecule of glucose, there are twelve atoms of hydrogen, but only six of oxygen.

Plants use a lot of light energy to build sugars like glucose from carbon dioxide and water, and some of that energy remains in each glucose molecule, "stored" in the chemical bonds that hold the molecule together. During aerobic metabolism, each glucose molecule can be broken down back to carbon dioxide and water. This process, referred to as respiration, requires oxygen and it liberates the "stored" energy. During this process, the net result is the same as if the sugar was burned producing a flame, but during respiration's "slow burn," some of the released energy is captured by the organism and secondarily stored by producing molecules of a chemical called ATP (Adenosine TriPhosphate). Molecules of ATP are used by cells to provide the energy for most metabolic activities, and consequently, ATP has been referred to as "the energy currency" of the cell. Fundamentally, most complex carbohydrates are broken down to simple sugars and then utilized to produce ATP which in turn is used to "power" the cellular processes.

A link to some information and maps of glucose metabolism:

If several hundred to several thousand sugar units are combined, they can form a starch, another carbohydrate energy material. Starches are used for energy by breaking off individual sugar molecules and processing those one by one to produce ATP. Typically plants produce "standard" starches, such as the corn starch used in cooking, while animals produce a slightly different starch called glycogen, or animal starch.

Fats or lipids are also utilized as energy sources. In effect, fats are concentrated sugars, chemically combined by removing excess hydrogen and oxygen from the basic sugars and combining these sugars together to make much larger molecules. It takes some extra energy to make lipids, but they have inherently much more energy per unit weight in them than do sugars. Most fats also tend to be relatively insoluble in water. These two properties, high value and low solubility, make them suitable as storage products. Lipids, therefore, are chemicals used by organisms to store their concentrated chemical energy. During fat metabolism, oxygen is again necessary for the breakdown, but a lot more ATP molecules can typically be produced from given weight of lipid than from the same weight of sugar.

In summary then, sugars and fats are burned in animal cells to produce ATP and releasing carbon dioxide and water as byproducts. So to get useful energy from carbohydrate as food, the organism needs to utilize that food to produce an energy carrying molecule such as ATP.

Animals such as corals can obtain carbohydrates from their zooxanthellae, or from eating animals which have eaten plants (a good fastidious predator should always eat its prey's gut contents to gain the full benefit of the prey; remember this the next time you have a fast-food fried mashed cow-meat pattie. You really should ask for a plate of guts to round out your meal). Animals also get a significant amount of carbohydrate from the animal starch in muscle tissue.

Structural Nutrients

There are two basic types of structural nutrients: inorganic minerals, and organic materials. In turn, there are fundamentally two types of organic structural nutrients, proteins and carbohydrates. Both proteins and structural carbohydrates are made of molecules called polymers. Polymers are simply long or large molecules made of many similar subunits connected together.

Sugars and starches are carbohydrates, of course, and can be used as energy molecules as described above. However, the carbohydrate molecules used for energy typically contain sugar molecules hooked together in a specific three-dimensional orientation which is easy to break apart. Sugars combined to produce structural materials are fastened together in a different orientation which is very difficult for organisms to break down. Probably the most important properties of structural carbohydrates is their resistance to chemical attack. Basically once secreted, they cannot be altered. Two common structural carbohydrate polymers are found. The first is cellulose, found mostly in plants, but also in sea squirts or tunicates. Cellulose is simply a long polymer of glucose. The second structural carbohydrate is chitin, which is found in arthropod exoskeletons (where it generally constitutes less than ten percent by weight), annelid worm bristles, and numerous other places in the animal kingdom. Like cellulose, chitin is a glucose polymer, but in this case each glucose subunit has an amine or ammonia group attached.

Most animals can digest neither cellulose nor chitin. In fact, these two common compounds, amongst the most common biologically generated compounds on Earth, are almost immune to animal digestive metabolism. They are broken down generally by protozoans and bacteria, although a few animals can digest one or the other.

The other types of organic structural molecule are proteins. Proteins are polymers of amino acids. Amino acids are organic acids with an ammonia group attached. Ammonia, an exceptionally toxic gas, is a compound made of one nitrogen and three hydrogen atoms. When dissolved in water it forms ammonium hydroxide and the ammonia subgroup of this (one nitrogen and two attached hydrogens) can be metabolically bound to a carbon atom. If this carbon-ammonia complex is, in turn, bound to an organic acid, the resulting molecule is an amino acid. Amino acids also have the capability for other atoms or group of atoms to be fastened to them. The secondary or subsidiary atomic complexes bound to the amino acids generally determine the properties of the amino acids.

Some further information on protein metabolism may be obtained by examining and exploring these links:

There are about 30 to 40 amino acids commonly found in animals and a much larger array of rarer ones. When these are connected in sequence they form proteins. Proteins may be comprised of anywhere from several dozen to several thousand amino acids. The diversity of these polymers is almost limitless and is dependent on the number and arrangement of the amino acids and these secondary subgroups.

Some of the common structural and other proteins may be familiar. Vertebrate tendons and hair are largely composed of the structural proteins, collagen and keratin, respectively. Another protein, hemoglobin, binds and carries oxygen in the blood of vertebrates and many invertebrates, and the contractile fibers in all muscle is composed of the proteins actin and myosin. In fact, the living tissues of most organisms are mostly made of proteins in a water suspension. Many thousands of specific proteins are found in all animals and for their production it is necessary that the animal eat or otherwise obtain either other proteins or amino acids.

Now, it should be obvious to all readers that a significant amount of nitrogen is necessary to build an animal. Simply put that nitrogen goes to build proteins. However, there are additional uses for nitrogen. The ATP molecules that are the energy carriers have a significant amount of nitrogen in their composition. Furthermore the chitin that is a basic animal structural molecule also contains large amounts of nitrogen.

Some more information on nitrogen metabolism, including searchable databases showing maps of overall nitrogen use, is available at:

NO nitrogen can't enter any animal by photosynthesis. None. Nada. Zero. Zip.

Photosynthesis provides food energy, but absolutely no nitrogen byproducts.

So where does the nitrogen enter into corals or coral reef animals?

Well, there are three potential pathways.

It is possible that the zooxanthellae in corals utilize nitrogenous compounds that they absorb through their surfaces. In fact, this has been shown numerous times to occur.

Ouphyllia (?) Corals such as this often feed well on diced fish and crustaceans.

They use some their sugars to change these nitrogen compounds into amino acids and proteins. The corals may be able to benefit from those compounds. However, this just bucks the question one step up the ladder.

Those nitrogenous compounds that reach the zooxanthellae have to come through coral tissues, so how do they get into coral tissues.

Corals have been demonstrated to absorb small amounts of nitrogenous compounds out of the water surrounding them. These compounds could be transported by the coral tissue to the zooxanthellae or they could passively move through the corals to zooxanthellae. In any case some of the zooxanthellate nitrogen use could come from these sources.

However, all animal tissue produces nitrogenous waste as a byproduct of protein metabolism. This waste nitrogen, in the form of ammonia, is likely used by the zooxanthellae to satisfy their nitrogen needs. However, if you think about this process, it becomes evident that a relatively considerable amount of nitrogen has to enter the coral from some other source. Consider if some of the coral's waste nitrogen is used by zooxanthellae, and some of that product leaks out and is used by the coral, then the coral must be getting nitrogen from some other source to start the chain of events. Then consider as well, as the coral grows, much of its dry tissue weight is proteinaceous and contains nitrogen. This is the excess nitrogen that has to come from somewhere else.

[an error occurred while processing this directive] Certainly a small amount comes from absorption of ammonia and nitrate from the water surrounding the animal. There can't be too much of this happening though, as even the ammonia dissolved in water is quite toxic. So...

The majority of the nitrogen budget for these animals comes from two sources, feeding on flesh and feeding on bacteria. Of the two sources, weight for weight, bacterial tissue contains more nitrogen than animal flesh; however flesh has the advantage of coming in bigger lumps. As I indicated last month most corals can feed on bacterioplankton and obviously get some of their structural nutrient through this pathway. Most can also eat plankton and they get some of their structural nutrient this way. The moral of this long nitrogenous trail is that corals must feed either on bacteria or on plankton or on both to maintain enough amino acid input to live and grow.

They get a lot of caloric energy from the byproducts of their zooxanthellae, but to grow they need to feed to obtain the raw materials for building proteins.

This brings us to the other class of structural materials that corals need, inorganic or mineral structural nutrients. Primarily in the case of corals, this structural material is the calcium used to make the calcium carbonate component of their skeletons. There is a protein component to the coral skeletons as well, by the way, and that, of course, has to come from normal cellular protein metabolism, either from the coral or from its zooxanthellae.

The calcium that the coral can use can come from the dissolved calcium in the sea water surrounding them or it can come from their food. Calcium is a major component of both skeletal and muscular systems in all animals, and so animal plankton food is an excellent good source of this material. Small-polyped corals such as Acropora and Seriatopora probably can't get much of their calcium needs from this pathway, as they can't eat larger animals. However, some of the larger-polyped animals certainly can get the majority of their calcium from feeding. I once fed a small Fungia , which was about 3 cm in diameter, a number of small fish chunks daily. Over the course of two months, it more than doubled in diameter and thickness; during this period, I never once saw it defecate bones from the fishes it ate; they were turned into coral skeleton.

Feeding provides necessary protein and calcium to corals. Regardless of their ability to absorb these materials from the surrounding medium, feeding is probably the primary natural source of metabolic nitrogen and an important source of metabolic calcium.

Trace Nutrients

Trace nutrients are those necessary for the organism's growth or well-being, but only necessary in very small amounts. They can include metal ions, inorganic mineral nutrients or organic materials. The actual trace nutrients necessary for coral metabolism are unknown. Simply put, no scientific research has been done on this topic. Aquarists have built up quite a mythology around additives to marine reef tanks, but in most cases there is no scientific rationale for adding most of these additives.

Amongst the known necessary additives are some small levels of metal ions, such as cobalt, copper, iron, and magnesium. These materials are necessary in all animals in very small amounts to facilitate enzyme functions. The concentrations of these are regulated very well by the animals, and the animals conserve these materials; almost none are lost in wastes. However, amounts only slightly over natural sea water concentrations of many of these metals may be lethal. Aquarists can add these, of course, but it is at the risk of killing their animals. In many cases the corals appear to sequester these materials directly from sea water, but they also get them from the foods they eat.

Other necessary additives are organic materials such as including vitamin B 12 . These materials are biogenic in origin, and in most cases they have to be gotten from food; bacteria for example are an excellent source of vitamin B 12 . Vitamins, by and large, are defined by their action on humans, and most have absolutely NO known effect, either positive or negative, on other animals, particularly animals such as corals whose metabolism is significantly unlike that of humans. Addition of vitamin C or other human vitamins probably makes the aquarist feel better, but probably has no real effect on their animals.

A third type of material is often added by aquarists. These are materials that have anecdotal benefits or good sales reps... Probably the most ubiquitous of these is strontium, which has no demonstrated positive effect in the metabolism of any animal except some larval opisthobranch snails, and then only for a brief period. Strontium has been demonstrated to reduce the transport of calcium ions across coral epithelial tissues, so concentrations above those found in natural sea water probably have an inhibitory effect on coral growth and well-being. Chemically strontium acts very much like calcium and substitutes for calcium in many biochemical pathways, but where calcium is specifically necessary, strontium would be an inhibitory poison. Whatever, the additive, however, these are generally added in solution, and the animals may absorb them across their body surfaces.


I hope by now you are convinced that feeding of your animals is a "good thing." If so, then all we have to do is decide on the proper food. And this is where the problems often really start to arise. It is patently impossible to offer the same array of foods that are found on a reef. Corals are planktivores, that is animals that eat plankton, either bacterial plankton, microplankton, or larger more "normal" plankton.

Small polyped corals such as these obtain nitrogen from fish wastes and bacteria.

It can be said, by the way, that the term "plankton" is rather loosely defined as "animals found in the water column." Generally these are considered to be small organisms, but size is relative; a blue whale feeding on adult Antarctic krill, Euphausia superba , is eating 10-15 cm long shrimp-like crustaceans, which are 10,000 times larger in length, and 10,000,000 times the volume of the bacteria being eaten by a small-polyped coral.

Corals can either be fed directly or indirectly. Direct feeding is the placing of food directly on the coral's oral disk or in the tentacles. The animals then ingest the food. Indirect feeding is where the corals feed on materials in the aquarium water, but are not directly fed by the aquarist. The latter type of feeding can range from feeding on "left-over" debris or food from intentional feeding of other animals such as fishes, or it can be feeding on micro- or bacterio- plankton that are maintaining populations in the system.

Direct Feeding

Direct feeding can be exceptionally productive at maintaining coral health, but only for certain corals. Basically in the cnidarians, prey size is correlated with tentacle and or mouth size. What this means, is that if your coral has large tentacles and a visible mouth, it probably eats macroscopic or larger prey. Lobophyllia , open-brain, elegance, and plate corals are examples of corals that do best on larger food. The problem is finding food that the corals will accept.

In nature the animals may naturally feed on an array of planktonic food. Some species will specialized to eat only crustacean prey, others may eat fish larvae, still others small squids. To maintain these specialists the hobbyist must be prepared to offer a variety of foods to their charges until they find one or more that are accepted. Unacceptable foods will be sloughed from the disk or not captured at all. Good foods to try testing with are diced small fish (salt water fish, not fresh water ones), thawed frozen plankton (generally this is North Pacific Krill, Euphausia pacifica ), pieces of shrimp, squid, or clams. It is important to realize that whole animals provide better and more natural nutrition than to parts of animals, so, for example, don't feed just shrimp muscles, but supplement the diet with plankton or shrimp bodies (including the guts).

In general, as I have noted above, I have found that Fungiid plate corals seem to like diced fish, but the hobbyist should try a variety of different fishes to find the most acceptable food. Elegance, hammer, and ridge corals seem to crustacean foods, but some species may be very difficult to provide for. I once tried to maintain a type of ridge coral that would not eat thawed frozen krill or plankton, but positively went wild over eggs from spot-prawns that I obtained from a super market that specialized in sushi makings. It didn't like the spot prawn flesh, by the way. .

A Fungiid "tongue" coral. Fungiids can often be fed diced fish.

Unfortunately, spot-prawn eggs are seasonal and without them, my coral would not feed and it died. In its natural situation, I would suppose that the coral ate some sort of larval crustacean whose flavor (chemical cue) I had managed to mimic with the eggs. Unfortunately, I wasn't smart enough to buy a large amount of the prawn eggs and freeze them. Finding the best food, is often a process of trial and error, and a LOT of importance has to be given to finding a balanced diet providing all of the essentials of nutrient energy, structural nutrients and trace elements.

Indirect Feeding

Indirect feeding is often a whole lot easier, and it is really the only way to be feeding corals such as Acropora , Seriatopora , and probably Goniopora . These species will do well in brightly lit aquaria to boost the output from their zooxanthellae, but their nitrogen intake is probably best met by bacterioplankton in the system. In the real world, Goniopora , are often found in areas of rather high turbidity, and probably feed on bacteria adherent to the sediments in the water. This genus is also exceptionally able to absorb dissolved organic nutrient.

Bacterioplankton are a natural byproduct of an aquarium maintained with a good sand bed, high circulation, and a reasonable amount of live rock. Basic nutrient energy has to be added to the system by feeding, but the corals in this system are not fed directly. Rather the other tank inhabitants, such as fishes, larger-polyped corals or sea anemones are fed and their waste products and uneaten food fuel the bacterial populations of the system. Where sediment bacteria are healthy and water currents are sufficient, there will be bacteria in the plankton for the small polyped corals to feed upon. Additionally, there will be some dissolved organic material in the water for them to absorb.


In conclusion, one simple fact that we learned when we had our first pets, holds true in aquaria as well: Animals need to be fed. I have tried explain some of the physiological processes that are occur in all animals, and how these process dictate some of the types of food that are necessary in a coral aquarium. Well-fed corals are healthy corals, and healthy corals are amongst the most rugged and tough-to-kill animals in any ecosystem. Undernourished corals are fragile, delicate creatures that will succumb to just about any stress.

A Note on Links:

I generally try to provide readers with the opportunity to find more information by the use of links to other WWW sites. There are many hundreds, or perhaps thousands, of sites that deal in some regard with metabolism, and the reader is urged to check some of these out. However, be aware which organisms are being discussed at each site. Metabolism and metabolic pathways can vary significantly between organisms. While the fundamental properties may remain the same over many animals, the specifics may be vastly different.


Many of the specific references to diets and nutrition in corals were listed in last month's article and I will not repeat them here.

For basic physiology, I would refer the readers to any of the myriad of introductory college texts in basic biology, cell biology, or biochemistry. Numerous encyclopedias also treat this issue very well.

Created by liquid
Last modified 2006-11-18 18:31