Corals in the Reef tank. What They Are and How 
They Live

by J. Charles Delbeek M.Sc.

One of the main components of most reef aquariums today are live 
corals. Unfortunately, for the average hobbyist, the availability of 
these animals is not matched by the information available concerning 
their biology and care. Too often hobbyists know more about who 
manufactures their aquarium equipment than about the basic biology or 
the proper identification of the animals they are striving to keep. 
Without this information it makes it difficult to provide the optimum 
conditions that will allow them to grow and reproduce.

Systematics
All corals belong to the phylum Cnidaria. Cnidarians are simple 
tentacle-bearing organisms that have a single body cavity, the 
digestive cavity, with only one opening that serves as both mouth and 
anus. The body plan is made up of two layers, the outer epidermis and 
the inner endodermis (gastrodermis). Between these two layers lies a 
jelly-like matrix called mesodermis. Cnidaria are also characterized 
by the presence of stinging cells; nematocysts. Many Cnidaria possess 
symbiotic algae cells (actually dinoflagellates, which are a type of 
protozoan) called zooxanthellae, in their tissues, which provide the 
animal with nutrition through their photosynthetic actions (see 
Delbeek, 1990; Delbeek and Sprung, 1994).

There are three main classes into which the Cnidaria can be divided:

Scyphozoa 
The Scyphozoa are the so-called true jellyfish and do not contain any 
specimens that are normally kept in home aquaria. The one exception 
occasionally offered for sale is the Caribbean jellyfish, 
Cassiopeia sp. This species is usually found lying on the sand 
on it's back in shallow water, exposing the tentacles to light. They 
feed on small prey items that may land on the tentacles, but they also 
have zooxanthellae and can do fine in aquaria without any supplemental 
feeding. They may, however, release nematocysts into the water as I 
have felt hundreds of tiny stings on my skin whenever I have swam over 
a shallow field of these jellyfish in the Florida Keys. These animals 
are not recommended for reef aquaria as they are not true reef animals 
and can harm other inhabitants with their sting. They can, however, 
make an interesting display for public aquaria especially as part of a 
mangrove or a seagrass flat exhibit.

Hydrozoa
The Hydrozoa contain colonial organisms generally known as hydroids. 
The polyp has a gastric cavity that does not contain septa (walls of 
tissue partially dividing the cavity into chambers), pharynx or 
internal tentacles. These animals usually have a two stage life cycle. 
Part of their life is spent attached to a substrate, often in a colony 
of polyps, and part of their life cycle is spent as a free-swimming 
reproductive, medusoid stage, similar in appearance to the familiar 
jellyfish. Some have only a medusoid stage such as the Portuguese Man-
of-War (Physalia sp.). Hydrozoans also have a variety of polyps 
within a colony, some for defense, some for feeding and some for 
reproduction. A few hydroids possess zooxanthellae e.g. 
Aglaeophenia cupressina (Alderslade, et al. 1984). 

Within this class there are two orders that are commonly called 
"corals" due to their formation of calcareous skeletons. The Fire 
Corals (Millepora spp.) belong to the order Milleporina, while 
the Hydro Corals belong to the order Stylasterina. Fire Corals, as 
their name implies, can produce a powerful sting that can easily be 
felt through human skin. Usually a mustard brown in colour, they grow 
in shallow reef areas and can form either encrusting, branching or 
upright, plate-like growths. They have two types of tiny hair-like 
polyps covering their surface and they possess zooxanthellae in their 
body tissues. The larger hairs, called dactylozooids, are mouthless 
and possess numerous small side branches filled with nematocysts. The 
smaller gastrozooids have a small mouth and have numerous small knobs 
filled with nematocysts (Hyman, 1940). The medusae stage are used 
strictly for dispersing eggs and sperm into the water column (Hyman, 
1940). Hydro Corals come in a variety of colours ranging from violet 
to pink and tend to have a branching pattern; they too are covered 
with numerous hair-like polyps.

Hobbyists sometimes encounter Fire Coral as by-catch on live rock but 
it is rarely imported as a collected specimen. This is unfortunate as 
they are extremely hardy and will grow rapidly under the proper 
conditions of strong water movement and lighting. Feeding is not 
required to maintain these animals but if you wish to do so, use very 
fine foods such as rotifers and liquid coral foods, targeted directly 
onto the colony; not simply dispersed into the water. They are also 
easily propagated by simply snapping off small pieces. Due to the 
encrusting growth form of some species, they can easily grow over 
glass, live rock and other corals, so they can be a bit of a problem. 
To the best of my knowledge Stylasterina have only been imported as 
dried skeletons for the curio trade so it is unknown how they would 
fare in a reef aquarium. However, since they are not photosynthetic 
they would need to be fed fine planktonic foods as outlined above.

Anthozoa
The third class, Anthozoa, or "flower animals" contain all the animals 
we commonly call "corals" as well as sea anemones and sea pens. 
Medusae are never formed and the polyps of anthozoans are much more 
complex than those of the hydrozoa, having several internal septa and 
a pharynx. The Anthozoa are also the largest Cnidarian order, with 
over 6000 species described so far. The Anthozoa can be subdivided 
into two subclasses, Octocorallia (Alcyonaria) and Hexacorallia 
(Zoantharia), based on the structure of their polyps.

Octocorallia
Octocorals contain all the corals we normally call soft corals and 
some you may be surprised to learn that are classified as soft corals. 
Octocorallia always have eight tentacles on each polyp and their 
polyps are internally divided by eight septa. Each tentacle also has 
numerous small, side branches called pinnules, that give them a 
feathered appearance. The polyps extend well into the tissue of the 
coral and the polyps are all interconnected by a tubule system. For 
example, if one cuts a leather coral in half, it looks just like a 
plant stem, with numerous conducting tubules visible. The thick, 
gelatinous tissue of a soft coral is impregnated with several 
calcareous particles known as spicules. In some octocorals the 
spicules are sparsely distributed throughout the tissue while in 
others they are very densely packed. The spicules can be separate or 
they can be fused together to form a strong skeleton. The shape of the 
spicules is used by coral taxonomists to classify the various 
octocorals. In some octocorals, a horny material is secreted 
internally which serves as a strengthening skeleton. Octocorals are  
at present divided into six orders: Alcyonacea, Coenothecalia, 
Gorgonacea, Pennatulacea, Stolonifera and Telestacea.

Alcyonacea contain all the genera that hobbyists are most familiar 
with as soft corals, such as Cladiella, Lobophytum, 
Sarcophyton, Sinularia, Anthelia, Xenia, 
Capnella, Dendronephthya, Lemnalia, 
Litophyton and Nephthea. All of these genera, aside from 
Dendronephthya, have zooxanthellae. Alcyonacea have fleshy or 
leathery colonies that tend to be irregular in shape with various 
lobes or finger-like projections. Some resemble large toadstools (e.g. 
Sarcophyton and Lobophytum) while others are more tree-
like in shape (e.g. Dendronephthya and Litophyton). 
Anthelia and Xenia are two genera that are often 
confused with each other. Anthelia tend to have longer, more 
slender polyp stalks than Xenia and they grow from an 
encrusting, fleshy, mat whereas Xenia usually has a stalk with 
polyps arising from the top of the stalk.

Coenothecalia contains only one species, Heliopora coerulea, 
the Indo-Pacific Blue Coral. This species grows in large brownish or 
greenish-grey mounds and has numerous delicate white polyps over it's 
surface. The colonies are heavily calcified and so appear similar to 
Millepora. When dead, or broken, one can see the blue colour of 
the skeleton caused by the infiltration of iron salts (Hyman, 1940). 
These photosynthetic corals are rarely available but they do very well 
in aquaria, even growing over pipes and aquarium walls.

Gorgonacea contain numerous tree-like corals such as sea whips, sea 
fans and sea feathers, some of which are photosynthetic, some that are 
not. Generally, if they have brown or light brown polyps, they contain 
zooxanthellae. Gorgonians have a strong, flexible, interior axial 
skeleton made of a horny material called gorgonin. This skeleton, in 
the form of rods, provides for greater flexibility and support. The 
skeleton is surrounded by a layer of tissue in which the polyps are 
embedded. This tissue also contains numerous conducting tubules and 
calcareous spicules. Although there are some encrusting and single-
stemmed species, the majority resemble trees in their branched 
appearance. The precious deepwater red and pink coral of the 
Mediterranean, Japan and Hawaii, are gorgonians too but they lack 
gorgonin, having instead fused calcareous spicules, which form the 
highly prized material from which jewelry is made. Photosynthetic 
Caribbean gorgonians have also gained a great deal of attention from 
the medical community as they are natural sources of some very 
interesting anti-inflammatory chemical compounds such as 
prostaglandins (see Faulkner, 1992).

The Pennatulacea contain the sea pens and the sea pansies. These 
large, fleshy, soft corals are found embedded in soft substrates by a 
fleshy projection called the peduncle on top of which sits the 
expanded portion of the primary polyp, the rachis. Sea pens and 
pansies are composed of a single large axial polyp (rachis) from which 
arise numerous, smaller secondary polyps. Although sometimes available 
commercially, unless one has a deep, soft substrate they should not be 
purchased. These animals are not photosynthetic and must therefore be 
fed fine planktonic foods regularly. 

Stolonifera are octocorals whose polyps arise from a creeping base 
that may consist of separate flat root-like structures called stolons 
or an encrusting mat. The polyps in most forms consist of two 
sections. The softer, thinner portion that possesses the tentacles and 
mouth is called the anthocodia and this portion can retract into the 
lower, stiffer non-retractile portion called the anthostele. Star 
Polyps (Cornularia spp. and Clavularia spp.) exhibit 
this structure nicely when retracted. In the Tubipora (Red 
Pipe-Organ Coral), the spicules are so dense they fuse together to 
form a calcareous skeleton. All Stolonifera kept in aquaria are 
photosynthetic and do very well under proper conditions without any 
additional feedings. I have never observed these corals to feed, and 
it is entirely possible that they absorb nutrients directly from the 
water or feed on bacteria trapped in their body slime.

The last order, Telestacea, also grow from creeping stolons but they 
give rise to large upright polyps from which smaller, lateral polyps 
can arise, giving the whole colony a tree-like appearance. The 
spicules are united through calcareous secretions forming a skeleton 
(Hyman, 1940). Although rarely imported due to their fragility, 
according to Wilkens and Birkholz (1986) these corals do well under 
the proper conditions and some species may be photosynthetic while 
others need to be feed planktonic foods.

Hexacorallia
In contrast to the Octocorallia, the Hexacorallia exhibit a great deal 
of anatomical variation, which makes them difficult to describe in 
general terms. Unlike the octocorals, hexacorals usually have 
tentacles and internal polyp septa in multiples of six, although 
exceptions do occur, but never eight. The oral disk has a prominent 
mouth that may be situated on a protruberance or have a protruding 
margin (Hyman, 1940). The tentacles of the polyps do not have pinnules 
as in the octocorals.

There are at present six recognized orders of Hexacorallia: 
Actiniaria, Antipatharia, Ceriantharia, Corallimorpharia, Scleractinia 
and Zoanthidae.

The order Actiniaria contains all the organisms we call sea anemones. 
Sea anemones are quite diverse in their appearance ranging in size 
from several centimetres to over 0.5 metre in diameter in the case of 
Stichodactyla gigantea. The bottom of the anemone is formed 
into a basal disc with which it attaches itself to the substrate. The 
other end contains the mouth and is situated in the middle of a broad 
oral disc surrounded by tentacles of varying length and shape 
depending on the species. Many species of tropical anemone contain 
zooxanthellae but most temperate species do not. Anemones will benefit 
from the occasional feeding of finely chopped foods such as shrimp but 
they can do very well without such feedings provided they receive 
adequate lighting.

Antipatharia contain the well known precious Black or Thorny corals 
usually found at depths greater than 20 metres. These tree-like corals 
have a thin axial skeleton with small thorns made of a material 
similar to gorgonin, out of which jewelry is made. There is a thin 
veneer of living tissue out of which the simple polyps arise. These 
corals are have never been kept in aquaria to the best of my knowledge 
and are not photosynthetic.

The Ceriantharia are non-photosynthetic anemone-like anthozoans that 
have a muscular, elongated, cylindrical body with a fleshy foot that 
extends deep into the sand such that the oral end bearing the 
tentacles extends outwards. The tentacles tend to be extremely fine 
and long, and some specimens of Cerianthus can sting quite 
powerfully. Cerianthus species from the Mediterranean and North 
Sea have been very popular in European temperate water aquaria for 
many years. The larger and more colourful tropical species are often 
seen for sale in North American pet stores. These do best when 
provided with a thick substrate and are best shown in species displays 
as opposed to mixed displays such as reef aquariums. Since these 
anemones are not photosynthetic they require weekly feedings of 
chopped shrimp or frozen marine foods.

Corallimorpharia contain the popular Mushroom Anemones, which are not 
really anemones; also known as "false-corals" they resemble stony 
corals but lack skeletons. Polyps can occur as solitary individuals or 
in colonies. The tentacles are usually reduced to knobs or small 
branched protruberances, arranged around one or more mouths. Various 
genera such as Actinodiscus, Amplexidiscus, 
Discosoma, Rhodactis and Ricordea are included in 
this order but these genera are under taxonomic revision and it is 
almost certain that they will be changed in the next few years. These 
anthozoans also contain zooxanthellae and can grow and reproduce in 
aquaria without supplemental feeding. Some of the larger species such 
as Amplexidiscus, can occasionally be fed chopped shrimp and 
are quite capable of trapping and eating small fish.

Scleractinia represent the stony or hard corals. Stony corals are 
basically anemones that are surrounded by a calcareous skeleton. The 
polyps can be solitary or they can exist in large colonies joined by a 
common tissue called coenenchyme. Due to the colonial nature of many 
stony corals, they can build massive structures that result in the 
development of entire coral reefs. The majority of stony corals 
harbour zooxanthellae and derive much of their nutritional 
requirements from the metabolic products they produce. Some of the 
larger polyped genera such as Plerogyra, Euphyllia, 
Catalaphyllia, Polyphyllia, Trachyphyllia and 
Cynarina will benefit from the occasional (once a week) feeding 
of finely chopped shrimp but this is not a requirement for success 
with these corals.

Zoanthidea are a small group of solitary, sometimes colonial, 
anemone-like anthozoans that lack a skeleton. They are unlike any 
other anthozoan internally, having a large number of paired and 
unparied septa (Hyman, 1940). Zoanthid polyps can occur as single 
individuals in large groups or they can be joined together by a thin 
stolon, a thin coenenchyme or a very thick coenenchyme from which only 
the mouths and tentacles visible e.g. Palythoa caribaeorum. The 
most common genera offered for sale are the photosynthetic members of 
Palythoa, Parazoanthus and Zoanthus. 

Keeping Corals
Great strides have been made by hobbyists in the last decade in the 
art and science of keeping corals in captivity. Unfortunately, it 
appears that many in the scientific community are unaware of these 
advances, which has lead to some uninformed comments by marine 
scientists such as those that have appeared in several national 
magazines (e.g. see Derr, M., April 1992 Audubon; Stein, J., 
Sept-Oct. 1993 Rodale's Scuba Diving). In truth many people are 
not only keeping corals alive in aquaria, but they are also growing 
and propagating them. Having said this, I would be remiss if I did not 
mention that many others also experience difficulty. This is for the 
most part due to the spreading of misinformation in the hobby and lack 
of proper instruction. Readers of this magazine and AFM generally get 
better information than most, but still there are pit falls that 
await, and dealing with those can tax the patience of most people. If 
you are not prepared for the sacrifice and expense that must be made 
to keep animals such as corals in captivity, then I strongly urge you 
not to attempt to do so.

Keeping corals basically involves three main criteria: light, 
filtration and water quality. Much has been written on these topics in 
the past and readers of AFM and Aquarium USA may recall the eight part 
series I wrote back in 1990 plus my two articles in 1993. Much of what 
I wrote then is still applicable today and I urge you to seek out the 
issues listed below for more details on the techniques required.

1) Lighting
First, the spectrum of light produced should not contain high amounts 
of red or yellow. Second, they should contain sufficient amounts of 
blue light. Third, they should not overheat the aquarium. Fourth, they 
must not emit ultraviolet light in large amounts (if they do, then the 
light fixture should have proper UV shielding). And fifth, they should 
provide adequate intensity for the depth and types of animals you will 
be keeping.

When dealing with fluorescent lamps, most meet the above criteria. 
However, beware of using so-called "plant lights" as they contain too 
much red in their spectrum and may promote the growth of undesirable 
microalgae. If you are using 8 or more lamps then you can use one such 
light to enhance colours if you wish, but they must not form the 
majority of the lighting. 

The use of metal halide lighting (specifically HQI, mercury quartz 
iodide) has increased greatly in the last few years. Again the same 
criteria mentioned above apply to these bulbs. To help ensure this, 
the colour temperature of the bulb should be over 5000 Kelvin and the 
colour rendition index (CRI) should be over 90. These values are 
usually printed in the literature or ad copy of the bulb. When used 
properly, these bulbs give excellent results but they tend to be more 
expensive than fluorescents. Recently 6500 K HQI lamps have appeared on 
the market and so far these appear to be giving superior results even 
without the use of additional actinic lighting. Metal halides have a 
tendency to overheat aquaria if placed too close to the water surface 
and/or when they are vented poorly. When using such bulbs the lamps 
should be at least eighteen inches above the water surface.

2) Filtration
In order to keep corals the water quality should be maintained 
relatively low in dissolved organic and inorganic substances. Many of 
the corals kept today come from inshore areas and as such, are less 
susceptible to high nutrient loads in the water. Unfortunately, with 
high nutrient loads, other problems arise that can threaten coral 
health such as excessive undesirable algae growth and pH depression. 
In order to keep these nutrients in check some sort of filtration 
mechanism is required. These usually take the form of biological and 
chemical means. Many of today's reef tanks are being installed with 
live rock as the only biological filter and protein skimmers (foam 
fractionators) as the sole means of filtration. This constitutes the 
so-called "Berlin" method first advocated by members of the Berlin 
Association for Marine Aquaristics. Activated carbon is also used for 
chemical filtration but some reef keepers prefer to limit its use to 
represent chronic shortages of essential trace elements, specifically 
iodide. In this case the carbon is added for a few days, once a month, 
to remove the yellowing matter that builds-up in the water. Although 
trickle filters are still useful filtration aids, experience has shown 
that they are not required to attain success in keeping corals when 
the methods mentioned above are used.

3) Water Quality
Of course irrespective of the filtration and lighting systems, if the 
proper water quality cannot be maintained, the animals will not 
survive. Water quality is a rather amorphous term and encompasses 
many different things some of which we can easily measure and some we 
cannot. Water chemistry is one portion of water quality that must be 
maintained within certain limits. Generally speaking the following 
parameters are of concern: temperature, pH, alkalinity, calcium, 
nitrate and phosphate.

a) Temperature
If you cannot provide the correct temperature range you will not be 
successful no matter what else you do. The temperature should be 
between 74 and 78 oF for the best results. If the temperature varies a 
few degrees during the course of the day this is usually not a 
problem. The most common problem is too high a temperature, that is 
80 oF+. There are a number of possible solutions to this and one or all 
of them may be necessary. First you should ensure that the top of the 
tank has adequate air circulation, either by having a hanging light 
fixture over the aquarium or by having a well-ventilated light hood. 
This will aid gas exchange and evaporative cooling, and will retard 
heat build-up in the hood. You could also design your sump so that it 
is open and provides a large surface area for evaporative cooling. 
Evaporative cooling can be further enhanced by placing a fan over the 
water surface. The most expensive but effective solution is to 
purchase an aquarium chiller designed for saltwater use. Somewhat less 
expensive is to purchase an air conditioner unit for the aquarium 
room. This has an added benefit for the aquarist of course, which may 
help you convince your spouse of the extra expense!

b) pH
The pH of aquarium water is a measure of the concentration of hydrogen 
and hydroxide ions. If the hydroxide ion is in greater abundance in a 
solution then the solution is said to be basic. If the hydrogen ion is 
more common the solution is acidic. Values of pH range from 0 to 14. 
If the number is less than 7 the solution is acidic, if it is greater 
than 7 it is basic and if it equals 7 it is neutral. Seawater is a 
basic solution with a pH of 8.2 to 8.4. In our aquariums, natural 
processes tend to lower the pH and we need to keep an eye on it. The 
pH is easy to measure with a saltwater pH test kit and should be 
maintained between 8.0 and 8.5. A sure sign that the pH is too low or 
high is the failure of your corals and clams to open completely. 
Lowered pH can also result in the spontaneous growth of undesirable 
algae, as well as a dinoflagellate growth that has the consistency of 
nasal mucus (i.e. snot). It traps air bubbles and in severe 
infestations can make your tank look like a field of tiny hot air 
balloons. Raising the pH to 8.3-8.5 through the use of a calcium 
hydroxide solution has cured this malady, however, if the pH falls 
below 8.2 it can quickly reappear.

c) Alkalinity
This is a term that has caused a great deal of confusion amongst both 
novice and advanced hobbyists. Stated simply the alkalinity of a 
solution is its ability to act as a pH buffer against drops in pH. The 
greater the alkalinity the greater its ability to prevent rapid pH 
swings. Once the alkalinity is exhausted the pH can fall rapidly. 
Alkalinity is provided in the aquarium by various negatively charged 
ionic compounds (anions) such as carbonates, bicarbonates, borates and 
hydroxides, to name just a few. The rather confusing term carbonate 
hardness has also been used to describe alkalinity, but this refers 
only to the carbonate and bicarbonate portions of alkalinity and does 
not take into consideration the other compounds involved. Therefore 
alkalinity is generally slightly higher than carbonate hardness 
(Spotte, 1979). 

To further add to the confusion, there are two different units of 
measurement used. There are numerous test kits available and they are 
all simple to use. Some test kits for alkalinity use the metric unit 
of milliequivalents per litre (meq/L), while other kits use the German 
unit, degrees of carbonate (German = karbonat) hardness (dKH). To 
convert meq/L to dKH simply multiply by 2.8. Natural seawater has an 
alkalinity of 2.1 to 2.5 (6-7 dKH). Alkalinity values in the aquarium 
should be maintained between 2.5 and 3.5 meq/L (7-10 dKH) through the 
use of commercial buffers. These buffers should also be used to 
maintain pH. There a numerous powdered buffers available that will 
maintain both alkalinity and pH, and the majority work very well by 
raising the alkalinity without causing rapid changes in pH. Beware of 
any buffers that cause rapid changes in pH.

d) Calcium
Calcium is the primary building block of the corals, clams, calcareous 
algae and many other organisms that we would like to grow in our 
aquariums. Without adequate supplies of calcium these organisms will 
not flourish and most will eventually waste away and die. Calcium 
levels are measured in parts per million (ppm) or milligrams per litre 
(mg/L). Calcium levels in natural seawater range from 380 to 480 mg/L
depending in location. In our aquariums calcium levels should be 
maintained between 380 and 450 mg/L to ensure proper growth and 
longevity of our animals. 

How one obtains these levels is a matter of choice and debate. Some 
advocate the use of powdered calcium chloride while others recommend 
powdered calcium hydroxide. There are also numerous calcium 
supplements on the market with wondrous claims as to their efficiency. 
It remains to be seen how well these products will perform over long 
term use. 

Calcium chloride is easy to use and does not directly affect the pH of 
the aquarium. The danger in its use is that one can easily overdose 
and quickly raise the calcium ion level in the aquarium. This has two 
effects. It raises the specific gravity rapidly and it can cause a 
rapid drop in alkalinity, and subsequently, pH. Calcium chloride can 
be added directly to the sump as a powder but it is best to dissolve 
it first in freshwater and slowly drip it into the aquarium over a 
period of several days

Calcium hydroxide is messy to use, has a high pH that can be hazardous 
as it can rapidly raise the pH if it is added too quickly, and it is 
caustic. It does add calcium slowly to the aquarium and does not 
deplete alkalinity. Generally sold as a powder, it is added to 
freshwater at the rate of 2 teaspoons per gallon (1.5 grams per 
litre). This solution, commonly called kalkwasser (German for calcium 
water) is then shaken and allowed to settle. The clear liquid that 
results is then slowly added to the aquarium as make-up water for 
evaporation. This is best done using a float valve or dosing pump. For 
more details on calcium additions see Sprung and Delbeek (1990), 
Sprung (1991) and Delbeek and Sprung (1994).