Corals of the genus Porites (pronounced por-eye-tees) are some of the most widespread corals in the world, with their geographical range spanning both the Atlantic and Pacific oceans. Porites corals can be interesting additions to reef aquaria and are quite hardy if aquarists are willing to meet their requirements.
A successful reef aquarium depends on many factors, including water motion, lighting, water temperature and chemistry, and more. This article will detail how to maintain Porites corals in captivity.
As reef hobbyists, we often think of intense lighting as a prerequisite for success with corals. While it is true that attention must be paid to lighting requirements, water motion is just as important (perhaps even more so).
Estimating the water motion requirements of Porites is relatively easy. Boulderlike (massive) Porites specimens (Porites lobata, P. lutea and others) are often found in relatively shallow waters and can be subjected to intense wave surge, while fingerlike (P. cylindrica, P. compressa) or delicate plating Porites (e.g., P. rus) are usually found in areas sheltered from wave action or in deeper waters. Hence, any of the newer propeller pumps are recommended, though the number used and the size of the pump(s) will vary. However, do not blast the animals with a continuous stream directly from the pumps. Instead, an indirect current will be appreciated by your Porites. If the coral polyps do not expand a day or so after introduction to the aquarium (and the coral is not in the direct path of the pump discharge), increase the amount of indirect water movement. Corals can sense water motion and will not expend the energy to keep polyps expanded if there is little chance of catching suspended food particles. Once the polyps have expanded, look for current-induced movement of the polyps (they should appear as a field of wheat gently nodding in a breeze).
Providing too much indirect flow is not likely to occur in aquaria. During research efforts in even calm water (such as Children’s Beach at Kahalu’u here in Hawaii), I have had sensors attached to 5-pound lead dive weights picked up by wave surge and tossed about as if they weighed only a few ounces.
Since all healthy Porites corals should contain symbiotic algae (dinoflagellates of the genus Symbiodinium, or “zooxanthellae”), lighting is one of the primary concerns for these animals. Most recommendations for “proper” lighting of corals are based on subjective observations. Although these can lead to success, there is an objective method for making such recommendations. First, we should consider the physiology of the coral animal. The Latin word “Porites” is based on the root word “pore,” meaning its calcium carbonate skeleton perforated with small holes, and these pores are filled with living coral tissue and zooxanthellae.
Obviously, light cannot penetrate very far into the skeleton. This provides a clue — the zooxanthellae within Porites’ tissues are adaptable to low light intensity, which makes for a paradoxical situation. The coral and its zooxanthellae are often found in shallow water (sometimes only a few inches deep) and often are subjected to intense sunlight, but they do not require a great deal of light.
Enter the second set of evidence — genetic fingerprinting of zooxanthellae. At one time, we thought there was only one species of zooxanthellae (Symbiodinium microadriaticum). Today, through the efforts of genetic scientists and their work with algae from different corals, we know there are many species and hundreds of “subspecies” called clades. These clades can be “specialists” (found in only one or two coral species) or “generalists” (highly adaptable to a number of environmental conditions, such as light, temperature or both). Researchers have determined that many Porites specimens contain a zooxanthella clade that is remarkably adaptable to changes in light intensity and is tolerant of slightly elevated temperatures (clade C15). In fact, black corals containing C15 zooxanthellae have been found at depths of about 1,500 feet.
So, we are faced with another paradox — if these zooxanthellae require light, and there is none to speak of at a depth of 1,500 feet, how do they manage to survive? The answer seems to be this: At extreme depths, the relationship between the coral and zooxanthellae shifts from one of symbiosis (where both partners benefit) to one where zooxanthellae become parasitic and mooch off the coral host in order to survive. The coral feeds on organisms in the water column, such as zooplankton, in order to obtain nutrition and survive.
These examples demonstrate the flexibility of the relationship between the host and symbiont but should not be used as an excuse to use insufficient lighting. Luckily, almost any of the lighting systems currently commercially available should suffice. I prefer the use of multiple LED lamps, but I have had success with Porites while using metal halide lamps and fluorescent lamps (including the use of several very high output tubes as well as four to six T5 fluorescent lamps).
Many Porites corals are brightly colored, ranging from deep brown (almost black) to gaudy purples, greens, yellows and other colors. Many hobbyists are disappointed when their corals’ colors shift or fade after a period in captivity. However, an understanding of the nature of these colors (along with a little luck) will aid the hobbyist wishing to maintain these brilliant colors.
Entire books could be (and have been) written on the subject of coral coloration. For the sake of brevity, I’ll stick to some basics. First, we should realize that there are two general types of pigments: those that glow under a black light or a blue actinic lamp (these are “fluorescent”) and those that do not (nonfluorescent “chromoproteins” that merely reflect some of the light falling on them). A very common pigment is the green fluorescent protein. This colorant is made by the coral animal (not zooxanthellae) and may be produced as long as there are adequate environmental conditions. Showcase these colors using blue actinic lamps or blue LEDs. The other pigments — the chromoproteins — are usually a response to light intensity, especially violet and blue light, so intense light weighted toward the blue end of the spectrum will help induce their production. Greenish-yellow light can cause shifts in some of the red pigments to those that appear even redder. Exposure to light heavily skewed to the red portion of the spectrum will likely cause the coral to expel its symbiotic zooxanthellae in an event called bleaching.
There is some evidence that trace elements (mostly transition metals, including iron, manganese, etc.) may play a part in making corals colorful. It is (in my opinion) best to avoid use of products claiming to promote coral pigments.
Porites corals will occasionally withdraw all polyps, and their tissue surfaces will acquire a waxy appearance. This is normal and is thought to be a mechanism for the animal to shed excess carbon (obtained through feeding or translocated to the coral from zooxanthellae). This covering can coat captive corals for a week or two, but it rarely lasts this long on corals found on natural reefs. Water motion plays a large part in sweeping this coating away.
Porites corals are among some of the world’s longest-lived invertebrates, and it is not uncommon to see underwater photographs of massive Porites colonies that are purported to be hundreds of years in age. It may be true that these corals live a long time, but this should not be construed to mean they are slow growers. Most Porites corals can grow rapidly (for a coral) if aquarium conditions are correct. Since coral skeletons are a form of calcium carbonate (aragonite, to be specific), it is critical that efforts are made to supply calcium and alkalinity supplements. Calcium levels should be about 400 parts per million, and alkalinity must be about 2.5 degrees carbonate hardness (or 10 dKH, or about 180 parts per million as CaCO3). These substances will be incorporated into the coral skeleton as well as help buffer against swings in pH (which should be from 8.1 to 8.3 standard pH units).
Magnesium is an important component of seawater, with levels often reaching 1,500 parts per million. Magnesium is also used to build the coral skeleton and helps to maintain calcium levels through a complex chemical pathway.
Strontium supplements are often recommended but are probably not necessary. Analyses of mineral and metal content of coral skeletons indicated that strontium is accumulated to the same levels as the water column. This suggests that strontium is not truly required, however more research is needed.
A visit to any well-stocked pet store will showcase a variety of supplements for reef aquaria, including iodine, iron, manganese, molybdenum and so forth. Generally, I do not add anything to the aquarium that I cannot (or do not) test for, and I recommend that you don’t either.
Filtration — Get Rid of the Bad Stuff!
Successfully keeping Porites corals in captivity depends on managing “good things” as well as keeping “bad stuff” out of the water, hence good filtration is a must. This should include a protein skimmer at a minimum. Almost any skimmer is better than none at all. Note that good water motion keeps particles from settling, hence their removal can be accomplished by skimming or filtration by a filter sock or canister filter.
The element phosphorus is known to inhibit the calcification process (calcium and phosphorus can bind together, creating the mineral known as apatite). Phosphorus is added to the aquarium in the forms of food, dust particles, poor-quality activated carbon, poor-quality make-up water, etc. Control phosphorus levels to 0.05 parts per million through use of any of the commercially available phosphorus-removing compounds, regular water changes, fish foods low in phosphorus content, intentional macroalgae growths, protein skimming, etc.
Nitrate, an inorganic form of nitrogen, tends to accumulate in aquarium water if no means are taken to control it. Keep nitrate levels as low as possible; however, nitrate control is not as critical as that of phosphorus. Nitrate levels are elevated in the waters surrounding the Big Island of Hawaii and are due to leaching of this compound from lava rock. Testing has revealed high levels of nitrate, but coral growths are fine.
Use activated carbon if you wish to remove dissolved organic substances. Make sure the brand you use does not add phosphorus to the aquarium water. Dissolved organics tend to turn the aquarium water an unattractive yellow color. Furthermore, these yellow substances tend to absorb blue light. This is a good example of one parameter directly (though it may not be obvious) affecting another factor; in fact, almost everything we do in an aquarium has a domino effect (whether good, bad or indifferent).
As we just discussed, some zooxanthellae found in Porites are tolerant of slightly elevated temperatures. I have personally measured temperatures as high as 88 degrees Fahrenheit in some tide pools containing healthy, thriving Porites colonies, but I strongly recommend against maintaining a coral reef aquarium at that high of a temperature. The successful aquarist will strive to keep the aquarium temperature around 78 to 80 degrees to reduce stress on the corals, their zooxanthellae and the artificial biotope in general.
Porites corals do not generally need to be fed in captivity. The mouths of their polyps are tiny, indicating they require even tinier prey. Food likely consists of plankton (probably in the form of zooplankton), bacteria, food scraps from fish feeding and other similarly sized particles.
Porites corals are often home to a number of attractive symbionts. Perhaps the most well-known inhabitant of Porites corals are Christmas tree worms (Spirobranchus spp.). These colorful worms live within the corals’ skeletons, and only their colorful gills are exposed (this publication’s cover shows polychaetes doing just this, as they emerge from a large Porites colony near Fiji — Eds.). Any hint of danger causes these gills to be withdrawn in an instant back into the safety of their coral home. The gills expand again after a few minutes.
Another natural inhabitant is a small alpheid shrimp. These animals burrow shallow depressions into the surface of the coral, where the shrimp grows algae and cyanobacteria for its food. The shrimp gains protection from predators in these trenches, while the coral obtains nitrogen from the shrimps’ waste products.
A number of fishes are known predators of Porites corals, including parrotfishes and some butterflyfishes, as well as the crown-of-thorns starfish (Acanthaster planci) and certain sea urchins (Diadema spp.). These are easy enough to exclude from an aquarium. The aquarist should be more concerned about cryptic enemies of Porites, such as some snail and nudibranch species.
For instance, the small snail Coralliophila violacea is a known predator of Porites. Even worse are nudibranchs of the genus Phestilla. In absence of natural predators, these small mollusks grow and reproduce quickly. They can strip living tissues from Porites corals in a matter of only a few days. Although their populations can be controlled (or even eliminated) by certain wrasses (saddle wrasse, Thalassoma duperrey), certain butterflyfishes (such as the threadfin butterfly, Chaetodon auriga), some xanthid crab species (Pilodius spp.) and undesirable mantis shrimp (Gonodactylus spp.), a better strategy is to quarantine any newly acquired Porites coral. Treatment with commercially available “coral dips” is also a good idea.
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Porites are sometimes infested with internal trematode worms, specifically the flatworm Podocotyloides stenometra. These infestations cause small areas of the corals’ tissues to swell and turn pink, giving rise to the common name of this malady — “coral zits.” These flatworms sap corals’ energy reserves and cause them to grow slowly but are not life-threatening. The life cycle of this flatworm involves transfer from the coral to a coral-eating butterflyfish to a snail and then back to the coral. It is unclear how this life cycle would progress in an aquarium without one or more of the parasite’s intermediate hosts. Also avoid most parrotfishes, many triggerfishes and butterflyfishes.
Porites corals are readily available in the pet trade and make excellent candidates for captive reefs. With an understanding of their requirements and provision of proper environmental conditions, the hobbyist can be rewarded with a thriving coral reef within the home.
- Gulko, D. 1998. Hawaiian Coral Reef Ecology. Mutual Publishing, Honolulu, Hawaii, 245 pp.
- Oren, U., I. Brickner and Y. Loya. 1998. “Prudent sessile feeding by the corallivore snail Coralliophila violacea on coral energy sinks.” Proc. R. Soc. Lond. B. 265:2043-2050.
- Veron, J.E.N. and M. Stafford-Smith. 2000. Corals of the World. Australian Institute of Marine Biology and CCR Qly Pty Ltd., Townesville, Australia.