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Feed Your Corals Aquarium.Net Jan 97

This month Ron tells us why corals need the extra nutrition from feeding, Aquarium Net has numerous articles written by the leading authors for the advanced aquarist




By Ronald L. Shimek


Feeding of corals, and other cnidarians as well, has been a rather contentious issue in the reef aquarium literature within the past several years. Similar arguments have been found in the scientific literature.

A Caribbean brain coral with the tenacles extended at night to catch plankton. Nematocyst clusters are visible .

Consequently, in writing this article, I wished to document reasonably well the points I made. The reader may be tempted to recall the old saying, "If you can't bedazzle them with your brilliance, baffle them with your bs." However, such was not the intent; rather the excessive number of references is meant to be illustrative of the available data and an indication of how much work has been done. I have not - honestly - tried to list every reference, literally thousands of papers have been published that bear in some regard on the topic. Rather for those readers who wish to pursue this topic further, I have tried to provide some important benchmarks.


Aquarists that keep marine tanks generally either have fish-only tanks or tanks containing some inhabitants of coral reefs. Generally, the latter types of tanks contain some corals. These are animals regarded by biologists as belonging in the phylum Cnidaria. Over the years, a significant mythology and body of misinformation has built up in the aquarium hobby about the nutritional requirements of such animals.

In my discussions of animals in this medium, and elsewhere, I have always taken the tack that "nature's way is the best way." In other words, animals will always do better under the best of natural conditions. In this article, I would like to discuss the nutritional requirements of corals based on their natural foods and food sources "as we understand them". The latter phrase is a rather critical one as much research in this field is ongoing, and in some instances the data are "subject to change without notice." In addition, there is a second problem with discussing nutrition in the Cnidaria, and that is the unwarranted assumption of uniformity throughout the group.

The phylum Cnidaria is truly a very successful and very diverse group. However, there is an underlying simplicity of body form and structure, and as a result there is a tendency to regard all members of any given group as similar. In other words, one often hears the statements like, "Corals need..." implying that all corals are alike. Such statements often have a grain of truth in them - along with a significant amount of untruth, and it is often very hard to separate the two.

Nutritional Morphology

The basic morphology of any stony coral is pretty simple. They have a bag or sack-like body with a single internal opening into the gut. Most of them have tentacles around the mouth, and they possess nematocysts which are used to catch, subdue, and kill prey. Nematocysts are often called "stinging cells," but this is an incorrect name, as they are not cells, but are non-living structures produced by cells.

Some additional information about coral anatomy may be obtained from these links.


Nematocysts are typically small structures consisting of a capsule enclosing an internal space. In this space is a fluid, and a thread-like tube which is turned inside-out projecting into the fluid-filled space in the capsule. In some way that is not totally understood, there is a significant internal pressure contained within the nematocyst capsule. This pressure has been estimated to be about the equivalent of 2200 pounds per square inch or about 155 kilograms per square centimeter. This is an immense pressure for containment in a biologically produced structure and it appears to be created and maintained by a difference in osmotic or ionic balances inside and outside the nematocyst capsule.

Nematocysts can be considered to be biologically-secreted explosive devices specialized to project their internal thread out at exceptionally high velocity. In most nematocysts the thread is hollow, and the cellular contents, which often include some toxins, are ejected from it. Nematocysts are fired by the application of an appropriate stimulus to the capsule. That stimulus seems to vary, from species to species, time to time, and probably from nematocyst type to nematocyst type. Some nematocysts discharge in response to physical contact, some in response to chemical stimulus, some in response to nervous stimulation, and many in response to combinations of other stimuli (Ruppert and Barnes, 1994).

Over 25 different kinds of nematocysts have been described. Generally each major cnidarian group, such as corals, contains several different kinds, and often some distinctive varieties. These nematocysts are distributed differently in the various species, and are often used as a taxonomic character to differentiate groups. The quantity of nematocysts found in the tentacles of cnidarians can be exceptionally large; in the tentacles of some sea anemones or jellyfishes, it is not unusual to find over 10,000 nematocysts per square millimeter (an area about 1/25th of an inch on a side). After nematocysts are used in capturing or killing prey, or after they get too old, they are discarded and new ones are secreted. Given that even a small cnidarian may require millions of nematocysts to capture even a small prey animal, it is obvious that the production of nematocysts is an important part of the metabolism of any cnidarian. It also requires a lot of energy.

A simple nematocyst is illustrated in the first of the following links, and more information about nematocysts may be found in both of them.


The Gut


Although the basic cnidarian gut is a simple bag, the gut in stony corals is a complex structure with internal divisions formed by tissue walls or septa dividing the basic bag-like gut into sections. These septa extend from the body wall into the bag dividing it like slices of a pie.

This type of gut is found in corals and sea anemones. A few simple corals and sea anemones may have only a few sections, but most have many more and in complex corals and large sea anemones there can be from several hundred to several thousand "pieces of the pie."

An individual of the solitary coral, Fungia , photographed during the day in shallow water. The tentacles are with drawn and the tissue is contracted tight to the skeleton. Note the many radiating skeletal septa (the linear ridges). The tissue septa cover these and give the animal its color.

The coral skeleton is deposited on by the outside tissues of the body, but these follow the inner folds quite closely, and an indication of the complexity of the coral gut structure can be seen in the examination of coral skeletal cups (See Veron, 1986, for many good examples).

Coral septal patterns can be seen by following the following link.


In most corals and sea anemones there are internal tentacles called gastric filaments arising from the edges of these septa. These tentacles are often loaded with nematocysts to subdue prey, and other nematocysts which fasten the filaments to the prey. Digestion occurs in the tiny spaces between the filaments and the prey's body surface.

Caribbean star-coral, Eusmilia fastigata ,   photographed during the day with the tenacles withdrawn. The septa are visible as radiating ridges around the polyp mouths .

So, although the basic cnidarian gut is simple in concept, it is decidedly more complex in reality. Such complex structures are also metabolically expensive to produce and maintain.


The next structures related to nutrition in corals are not really part of the animals in which they are found. These structures are zooxanthellae. Zooxanthellae are single-celled algae which live inside of some other organism. Many different types of organisms possess zooxanthellae, including some sponges, some protozoans (so there are single-celled organisms which have other single-celled organisms living in them!), some flatworms, and a few other animals.

Caribbean star-coral, Eusmilia fastigata , photographed during the night with the tenacles extended. Nematocyst clusters are visible as dots on the tentacles.

However, zooxanthellae are found living in rather large number of cnidarians including all reef-forming or hermatypic corals.

Some additional information about zooxanthellae in corals can be found in:

The relationships with zooxanthellae are presumed to be symbiotic. The anemone or coral that hosts the zooxanthellae in its tissues supposedly benefits from the photosynthetic production of the algae, and the algal cells presumably benefit by receiving protection and possibly some organic nutrients such as amino acids or ammonia from the cnidarian. The algal cells are typically found living in their hosts gastrodermal tissue, close to gut lining, and typically the algal populations are quite dense.

[an error occurred while processing this directive] Zooxanthellae are dinoflagellate algae, and like all good algae they are photosynthetic, capable of making organic nutrients from light, water, and carbon dioxide. The process of photosynthesis creates only sugars, but the algal cell can use those sugars to build other materials such as fats (Al-Moghrabi et al., 1995). If the alga has access to nitrogen containing compounds such as ammonia or amino acids, it can also build proteins and other structural molecules, using the sugars as fuel for this process (Szmant-Froelich, and Pilson, 1984).

Algal cells are notoriously "leaky" and up to about 40 percent of the photosynthate produced by algae leak out into the surrounding medium (Lewis and Smith 1971). If the algal cells are living in an coral, the coral tissues can utilize this material as food, or as an organic nutrient for some other function such as building their skeleton. Coral skeletons are often thought of as being composed only of calcium carbonate, but actually, they have an organic matrix and the calcium carbonate mineral is deposited on this. So, if the organic matrix is lacking or reduced, skeleton production cannot occur.

The relationship between the algae and their hosts has been known since the late nineteenth century, but many of the fundamental processes involved with remain unresolved. By the mid-1930's the algal cells were presumed to be either parasites or commensals, providing no benefit to the host; and that hosts, such as the corals, were presumed to be wholly carnivorous. Such views persisted for some time thereafter (Yonge, 1958, 1968). With the advent of studies on the photosynthetic nature of the algae, it became evident that the coral or anemone host gained significant nutrition from the symbiont (Lewis and Smith, 1971; Johannes, 1974, Goreau et al. 1976). By the late 1970's, most everyone was convinced that corals while corals did feed, they did not need to feed (see, for example, Franzisket, 1969; Muscatine, 1973; 1990; Davies, 1984).

From the aspect of a reef aquarist, this view of the symbiosis is beautiful and compelling. Unfortunately, it is fundamentally flawed and wrong.

Logically an obvious question one needs to ask is a simple one. "If corals get all of their nutritional needs from zooxanthellae and don't need to feed, why is so much metabolic energy wasted on the production of nematocysts and gut tissues? Why aren't animals that harbor these algae simply large zooxanthellae gardens? And, why do all examinations of wild corals show that the animals feed (See, for examples, Johannes, and Tepley, 1974; Porter, 1974; Lasker, 1976; Lewis, 1977; Clayton, and Lasker, 1982; Sebens and Johnson, 1991)?


That feeding structures remain important to most cnidarians can be examined by looking at an example of a cnidarian that does all of its nutrition from zooxanthellae. Coral atolls in reef areas of the Indo-Pacific bounded by Palau, New Guinea, and Indonesia, often contain marine lakes. These are lakes which are filled not with fresh-water, but with sea water which percolates through the porous reef structures to fill the lake. In these lakes are commonly found a particular jellyfish, Mastigias papua .

The stingless and mouthless jellyfish, Mastigias papua . This animal gets all of its nutrition from its zooxanthellae.

These medusa at first glance appear quite similar to their relatives living outside the lakes in the open ocean. On close examination, however, it becomes clear that the lake jellyfishes lack mouths, and nematocysts. Indeed the gut appears to be modified to support massive internal populations of zooxanthellae. The metabolically expensive feeding structures such as gastric tentacles, mouths, and nematocysts have been lost. If corals could obtain all the nutrition that they needed from zooxanthellae, similar modifications in their morphology would also be expected, with perhaps some retention of nematocysts for defense. Nothing like this is seen in any stony coral.

In fact, just the opposite occurs. Recently researchers found that one deeper water zooxanthellae-containing coral has additional adaptations for maximizing its feeding input (Schlichter, 1992). This species, Leptoseris fragilis , has secondary openings to the gut. In all other feeding cnidarians undigested food, or feces, has to be expelled from the mouth at the completion of the digestive cycle. Such a process means that the animals food intake is limited as it cannot feed continuously. In Leptoseris fragilis , with its secondary gut openings, the animal can continually eat, maximizing its food intake per unit time and its symbiotic algae still can utilize the dim deeper-water light to produce the necessary photosynthate for skeletal production.

Since about the mid-1980's experimental research has shown that the cnidarian-algal symbiosis is much more complicated than either of the preceding simplistic views (Sorokin, Yu. I. 1990a, b). The coral-algal symbiosis is neither wholly predatory, nor wholly photosynthetic. It has now been shown that many corals feed not only on macroscopic plankton, but also on microscopic animal plankton such as rotifers and larvae. In addition, many of them feed as well on bacteria, either those microbes living free in the water column or those found in many reef environments on living on suspended sediment. In the latter cases, the sediment is ingested and bacteria on it are digested. Additionally, most corals can absorb dissolved organic material directly from the water (Bythell, J. C. 1990, Lesser et al., 1994; Al-Moghrabi et al., 1995). Indeed, it appears that many types of corals that are considered difficult to maintain, such as species of the genus Goniopora , may get most of their nutrition from these three sources, rather than from photosynthesis.

However, there are also a many good studies that show that corals can receive enough photosynthetic materials from their algae that their basic energy needs can be met by photosynthesis (see, for example, Muscatine, 1973).

It is becoming evident that the coral symbiosis of animal and alga is one that provides the maximum flexibility in obtaining nutrition in environments that might otherwise be marginal for either the alga or the cnidarian. In areas where there is significant runoff from terrestrial or island sources, and this includes lagoons around a central island, corals particularly appear to gain significant amounts of their nutrient from this runoff (Risk and Sammarco, 1991; Risk et al. 1994). This nutrient is either in the form of bacteria or in the form of microplankton that eat the bacteria. In these environments, the zooxanthellae appear to provide supplemental nutrition and some critical materials necessary for coral skeletal production.

On the other hand, on oceanic reefs far from sources of terrestrial material, the corals appear to gain significant amounts or most of their nutrition from their zooxanthellae. Here the animals appear to be able to collect plankton to supplement their algal sources with the nitrogenous food sources necessary for protein production.

Most corals appear to be able to get about 100 percent of their basic energy or caloric needs from EITHER zooxanthellae photosynthesis OR by feeding and absorption. However, animals need MORE than 100 percent of their basic needs; the excess goes for the production of additional tissues (growth or repair of injuries) or the production of gametes (reproduction). Additionally, they need more than basic energy, they need to obtain structural material that cannot be produced by photosynthesis. To grow, the coral-algal symbiosis needs to obtain significant material from non-symbiont sources (Szmant-Froelich, and Pilson, 1984)

Some links to information about coral feeding and nutrition.

Conclusions and Recommendations

Somewhere about the time that the view that corals and sea anemones didn't need food was prevalent in some of the popular environmental literature, many reef aquarium guides and "how-to" books were written. And they repeated the idea that corals didn't need food over and over in new editions. So now one often reads that corals don't need to be fed, but can exist just on photosynthetic products produced by their zooxanthellae. This is simply not the case. And following such instructions to the letter will result in dead corals. In a subsequent article, I will discuss some of the options for feeding reef aquarium corals, as they do need to be fed for optimal health or in some cases, for basic survival.




References Cited:

Al-Moghrabi, S., D. Allemand, J. M. Couret, and J. Jaubert. 1995. Fatty acids of the scleractinian coral Galaxea fascicularis : Effect of light and feeding. Journal of Comparative Physiology B: Biochemical Systemic and Environmental Physiology 165(3), 183-192.

Al-Moghrabi, S., D. Allemand, and J. Jaubert. 1993. Valine uptake by the scleractinian coral Galaxea fascicularis : Characterization and effect of light and nutritional status. Journal of Comparative Physiology B: Biochemical Systemic and Environmental Physiology 163, 355-362.

Bythell, J. C. 1990. Nutrient uptake in the reef-building coral Acropora palmata at natural environmental concentrations. Mar. Ecol. Prog. Ser. 68, 65-69.

Clayton, W. S., Jr, and H. R. Lasker. 1982. Effects of light and dark treatments on feeding by the reef coral Pocillopora damicornis (Linnaeus). Journal of Experimental Marine Biology and Ecology 63, 269-279.

Davies, P. S. 1984. The role of zooxanthellae in the nutritional energy requirements of Pocillopora eydouxi . Coral Reefs 2, 181-186.

Franzisket, L. 1969. The ratio of photosynthesis to respiration of reef building corals during a 24 hour period. Forma et Functio 1, 153-158.

Goreau, T. F., N. I. Goreau, and C. M. Yonge. 1976. Reef corals: Autotrophs or heterotrophs. Biological Bulletin 141, 247-260.

Johannes, R. E. 1974. Sources of nutritional energy for reef corals. Proceedings of the Second International Coral Reef Symposium 1, 133-137.

Johannes, R. E and L. Tepley. 1974. Examination of the feeding of the reef coral Porites lobata in situ using time lapse photography. Proceedings of the Second International Coral Reef Symposium 1, 127-131.

Lasker, H. R. 1976. Intraspecific variability of zooplankton feeding in the hermatypic coral Montastrea cavernosa . In: Coelenterate Ecology and Behavior. (Ed: G. O. Mackie) Plenum Press, New York, 101-109.

Lesser, M. P., V. M. Weis, M. R. Patterson, and P. L. Jokeil. 1994. Effects of morphology and water motion on carbon delivery and productivity in the reef coral Pocillopora damicornis (Linnaeus): diffusion barriers, inorganic carbon limitation, and biochemical plasticity. Journal of Experimental Marine Biology and Ecology 178, 153-179.

Lewis, J. B. 1977. Suspension feeding in Atlantic reef corals and the importance of suspended particulate matter as a food source. Proceedings of the Third International Coral Reef Symposium 1, 405-408.

Lewis, D. H. and D. C. Smith. 1971. The autotrophic nutrition of symbiotic marine coelenterates with special reference to hermatypic corals. I. Movement of photosynthetic products between the symbionts. Proc. Roy. Soc., Lond., B, Biol. 178, 111-129.

Muscatine, L. 1973. Nutrition of corals. In: Biology 1. Vol. 2. (Eds: Jones, O. A. and R. Endean) (Biology and Geology of Coral Reefs.) Academic Press, New York, 77-115.

Muscatine, L. 1990. The role of symbiotic algae in carbon and energy flux in reef corals. In: Coral reefs. Vol. 25. (Ed: Dubinsky, Z) (Ecosystems of the world.) Elsevier, Amsterdam, 75-87.

Risk, M. J. and P. W. Sammarco. 1991. Cross-shelf trends in skeletal density of the massive coral Porites lobata from the Great Barrier Reef. Mar. Ecol. Prog. Ser. 69, 195-200.

Risk, M. J., P. W. Sammarco, and H. P. Schwarcz. 1994. Cross-continental shelf trends in delta-13C in coral on the Great Barrier Reef. Mar. Ecol. Prog. Ser. 106(1-2), 121-130.

Porter, J. W. 1974. Zooplankton feeding by the Caribbean reef-building coral Montastrea cavernosa . Proceedings of the Second International Coral Reef Symposium 1, 111-125.

Ruppert, E. E. and R. D. Barnes. 1994. Invertebrate Zoology. Saunders College Publishing. Philadelphia. 1056 pp.

Schlichter, D. 1992. A perforated gastrovascular cavity in the symbiotic deep-water coral Leptoseris fragilis : A new strategy to optimize heterotrophic nutrition. Helgoländer Meeresuntersuchungen 45, 423-443.

Sebens, K. P., and A. S. Johnson. 1991. Effects of water movement on prey capture and distribution of reef corals. Hydrobiologia 226, 91-102.

Sorokin, Yu. I. 1990a. Plankton in the reef ecosystems. In: Coral reefs. (Ed: Dubinsky, Z.) (Ecosystems of the world, 25.) Elsevier, Amsterdam, 291-327.

Sorokin, Yu. I. 1990b. Aspects of trophic relations, productivity, and energy balance in coral-reef ecosystems. In: Coral reefs. (Ed: Dubinsky, Z.) (Ecosystems of the world, 25.) Elsevier, Amsterdam, 401-410.

Szmant-Froelich, A., and M. E. Q. Pilson. 1984. Effects of feeding frequency and symbiosis with zooxanthellae on nitrogen metabolism and respiration of the coral Astrangia danae. Marine Biology 81, 153-162.

Veron, J. E. N. 1986. Corals of Australia and the Indo-Pacific. Australian Institute of Marine Science. Published by University of Hawaii Press (1993), Honolulu. 644 pp.

Yonge, C. M. 1958. Ecology and physiology of reef-building corals. In: Perspectives in Marine Biology. (Ed: A. A. Buzzati-Traverso) University of California Press, Berkeley, 117-135.

Yonge, C. M. 1968. Living Corals. Proc. Roy. Soc., Lond., B, Biol. 169, 329-344.


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