Nutritional Requirements of Reef Systems

by J. Charles Delbeek M.Sc.

The nutritional requirements of the various organisms that can occur 
in reef systems is extremely varied and/or difficult to ascertain. We 
are no longer dealing only with the different feeding habits of the 
fish that we are keeping but also various orders of invertebrates, each 
with their 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. That is why I find the type of 
advertising used by some manufacturers of "reef foods" 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 will not be the purpose of this article to tell you what to feed 
each organism but more to outline the various feeding modes that one 
will encounter and to 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 a few references that I will give at the end of this 
article that will help you get started. Read as much as you can, ask 
questions of your dealer and friends. 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 to 
duplicate this 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 which are 
fed on by a multitude of fish and invertebrates. Another problem comes 
with corals where there are few studies published on the diets of 
corals and indeed, much debate on how much and even if corals need to 
feed.

There are various feeding mechanisms used by the inhabitants of our 
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 is. This is the case with many coral species and for 
this reason, coral feeding strategies will be dealt with separately. 

Many 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 
ranging in size from copepods and other zooplankton to detritus and 
feces. Those that feed on detritus either digest the bacteria living 
on them or the particulate organics that coat such particles (Wotton 
1988). The method of feeding that has probably elicited the most 
debate is the one involving the relationship between organisms such as 
corals, bivalves, 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-coloured 
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). The final method of feeding that we 
will examine in this article 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. These organisms may be omnivorous, herbivorous or 
carnivorous in nature and depending on their size and appetite may be 
able to fend for themselves in a reef tank. More common though is the 
situation where additional feedings are required. For those organisms 
that are carnivorous or omnivorous good 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 
goes for frozen foods. The problem with any prepared food is the issue 
of vitamins, pigments and other micronutrients. Many of these are 
destroyed during the manufacturing process and need to be added to the 
feed later. The problem is that these substances have a limited shelf 
life. By the time the food has gone through the hands of the 
manufacturer, wholesaler, retailer and finally to you, the majority of 
these substances may have been lost through oxidization. As a result, 
your organisms will not be getting the full nutritional value that you 
may think. 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 main vitamin 
available in this form is vitamin C and there are a number of foods 
and vitamin supplements now advertising its use. 

There are a number of precautions you should take when purchasing and 
using prepared foods. First of all only buy food from a dealer who 
regularly rotates their stock and who buys direct from the 
manufacturer or from a wholesaler who rotates their stock. Most flake 
foods have a shelf live of about 6 months if not refrigerated. 
Secondly, as soon as you open your food make sure you do not leave it 
close to the tank or in other warm, humid areas. Keep the container 
tightly sealed and in the frige. Some hobbyists soak their foods in 
vitamin supplements before feeding. This is a good idea only if the 
vitamins are fresh of course. Baby vitamins have been used with some 
success and have the added benefit of having expiry dates on the jars. 
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 
such as 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 shrimp etc. 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 then one day it will 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 will supplement their 
diet with the algae found in the system. 

Herbivores, in some systems, can get enough to eat from the various 
micro- and macroalgae in the aquarium. In some cases though, such as 
with tangs, surgeons and sea urchins, they will quickly strip the tank 
of algae and will 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 them 
before will devour the same greens when blanched. Additional feedings 
of vitamin enriched prepared foods are recommended too.

Organisms that feed on smaller particles such as detritus and its 
associated components, as well as zooplankton, can either be fed or, 
in some cases, ignored entirely. It has been my experience that many 
forms of fanworm will exist quite well in aquariums, where the fish 
are fed regularly, without any additional feedings. If you find that 
the crowns of these fanworms decrease in size, then this may be a sign 
that they are not getting enough to eat and supplemental feeding would 
then be required. Liquid foods work well for the smaller species while 
live baby brine shrimp will be accepted by the larger species i.e. 
Sabellastarte magnifica. Other such feeders include Sea Cucumbers, 
clams (other than Tridacna spp.), 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, zoanthids, anemones and gorgonians, can 
utilize a wide variety of feeding techniques. Not only can they 
utilize the photosynthetic products of their algal symbionts but they 
can also feed directly on plankton, bacteria, detritus and fish feces. 
Some corals have even been shown to be able to directly absorb glucose 
from the water (Stephens, 1962). Other zooxanthellae bearing organisms 
include some sponges and Tridacna spp. clams.

As mentioned above, most corals contain symbiotic algae in their 
tissues which can supply some of their nutritive needs. I say some 
because the degree to which zooxanthellae contribute to a coral's 
nutrition has been the subject of much research over the past 40 
years. It seems that the amount varies between species. In some 
species of zoanthids over 90% of their nutrition can be met by the 
zooxanthellae while in others this figure is much lower (60%) (Steen 
and Muscatine, 1984). However, 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
spp.) can actually feed on shrimp-sized prey which 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 own personal 
opinion many coral species do not need direct feeding. Many get more 
than enough from natural sources in the tank. Every time you feed your 
fish, particles of food and nutrients are added to the water. The 
presence of live rock and their associated algae and bacteria produce 
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, despite our best efforts.

Those polyps that are large enough to be fed 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, indicates to me 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 i.e. 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 then 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 and feed them. Smooth 
surfaced forms, however, generally obtain enough nutrition from their 
zooxanthellae. However, Elliott and Cook (1989) showed that the 
Caribbean corallimorpharian Discosoma sanctithomae relied both on 
their 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) then 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. 
while 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 as most of it ends up in the filter, in the gravel or under 
rocks.

The final method of feeding in reef tanks is through the direct uptake 
of organic compounds through the body walls of various marine worms, 
ascidians (i.e. Tunicates), bryozoans, etc. (Sepers, 1977). The 
mechanisms, importance and role of such feeding in marine ecosystems 
is not well understood and certainly bears more extensive research. 
Whether or not such feeding is an important component of our aquariums 
is certainly an unknown area but I feel that the majority of the 
nutrient needs of these organisms can be met by other means. 
Certainly, the addition of extra amounts of nutrients would be both 
unfounded and unnecessary given the lack of knowledge in this area. It 
would first have to ascertained exactly what substances were required 
by each organism and if these were depleted in an aquarium before any 
consideration could be given to additions.

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 out there than there are 
people actively researching this area. 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, spread your 
knowledge and experience, we will be all better off in the long run. 
The more we can demonstrate the value of our hobby, the less likely 
that it will be shut down.

References

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 
behaviour 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.

Wotton, R.S. 1988. Dissolved organic material and trophic dynamics: 
What is the food of filtering collectors in aquatic ecosystems? 
BioScience 38: 172-178.


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.

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.