When biologists divide out the different types of corals and their kindred, one of the groups that is formed is called the Subclass Octocorallia. It's name is a giveaway when it comes to what sets them apart, as all of the members are corals that have a ring of eight tentacles circling their bodies (polyps). Within this group of eight-tentacled corals there is a sub-group called the Suborder Stolonifera, and these are the subject of this month's article. The members are most commonly known to aquarists as the clove ployps, the star polyps, and the pipe organ corals.
The vast majority of these are difficult to identify and classify, even for scientists working with them, but they do have some other common characteristics that distinguish them. Aside from having eight tentacles, the vast majority also have little side branches that arise from these tentacles. These tiny structures are called "pinnules" and they can give a tentacle a serrated look, or can even make it look more like a feather.
The stoloniferans also share a feature called a stolon (no surprise). This is a layer of tissue that connects all of the polyps in a stoloniferan colony, which typically forms durable mat-like encrustations or strands that attach to hard surfaces (although we'll go over a big exception in a minute).
So, with these bare basics covered, let's take a closer look at the three common types of stoloniferans that you'll be sure to come across in the hobby.
The Clove Polyps (also called daisy polyps):
Clove polyps, most of which belong to the Genus Clavularia, typically have the fanciest-looking forms of the bunch, with beautiful pinnate tentacles that look like fluffy feathers. However, while these plume-like tentacles are present on the most frequently seen specimens in stores, there are a few that have rather plain, thin tentacles, as well. All sorts regularly come in shades of brown, green, cream, or yellowish, and the polyps sometimes have lighter-colored centers, too.
Regardless of the polyp form, many of these grow as colonies that are rather loosely connected (relative to their relatives) by stolons that are thin and web-like. The stolons run like little creeping vines from the bases of each polyp and are often spread out a bit and only interconnected here and there. But, this isn't always the case though, as they can be quite tightly arranged when a number of polyps are situated close together. Likewise, there are those that form small clump-like bases/mats, too.
Another feature is their ability to retract into themselves when bothered. Just brush one side of a colony with your finger and you'll see the whole thing "shut down", with the polyps quickly deflating like little balloons and disappearing from sight by folding into the lower portion of themselves. Then the stolons become clearly visible, with a little nub/stalk sticking up where each feathery polyp was. They often retract like this at night, as well, so you can take a look in the morning when the lights first come on and you'll see the same thing. Something that looks quite dead, but isn't. If they're healthy, just give it a minute and they'll start to slowly emerge.
As far as environmental conditions go, in general clove polyps can tolerate a broad range of lighting. There are numerous varieties that hail from different environments, but they'll all do fine in aquariums lit by anything considered coral-worthy, from V.H.O. fluorescents to metal-halide systems. So, you don't have to worry too much about where they'll go with respect to lighting requirements. But, if you want them to really thrive they'll need the most they can get, with moderate to bright illumination being the way to go unless you see signs that they may be over-illuminated, such as a failure to open up.
Currents should be moderate. If they are too low, detritus/sediment can accumulate on a colony's base/stolons, so the current needs to be at least strong enough to keep them cleared off fairly well. Conversely, if the current is too strong the polyps will tend to stay retracted to avoid be overly blasted. So, a good turbulent current that stirs the water around a colony is best.
Unlike many other corals, clove polyps aren't going to take any foods you may try to give them. Even fine, plankton-type foods and powders are unlikely to provide any sort of benefit. But, they'll be fine despite this, as long as the lighting is acceptable, so feeding them is one thing you don't have to worry about.
You don't have to fret too much when it comes to where to put a colony with regards to fighting, either. They're highly resistant to the stings of most corals so they'll likely be just fine in crowded conditions. They won't sting anything they touch either, with the possible exception of zoanthid polyps (reported in Sprung & Delbeek, 1997). However, they can simply overgrow other low-lying neighbors, so you'll probably need to pay more attention to other nearby specimens rather than the clove polyps themselves.
Yet another plus is that they can be reproduced/propagated very easily using either of two basic methods. The easiest thing to do is position a small piece of shell or rock right up against part of a colony and let it do the rest. Over time the colony will spread onto this piece of substrate, which can then be cut away from the rest of the colony and moved elsewhere. Just use a razor blade to do the severing and put the newly produced little colony in a suitable spot, or even in another aquarium.
Likewise, if you don't feel like waiting for part of a colony to overgrow something, you can use a razor to cut away a part of the colony and then attach it to a piece of substrate yourself. You can dab the "cutting" on a piece of paper towel to get most of the water off, then use some gel-type "superglue" to attach it to a suitable piece of rock or shell, etc. This is much faster, but takes a little more skill and technique, so I suggest you do some homework specifically on the propagation of soft corals prior to trying it. Once you get the hang of it, it is very easy to do over and over, though.
The Star Polyps:
Years ago anything you read about star polyps very likely called them Clavularia viridis. But, calling them this was apparently a mistake made by a biologist long ago, as the correct name is Pachyclavularia violacea. When it comes to such things, sometimes they are simply mistakes that spread though literature one article or book at a time, but other times names are actually changed from one to another by taxonomists/biologists, as well. For example, Allen & Steene, 1994 call green star polyps Briareum, then Sprung and Delbeek, 1997 say that Briareum is synonymous with Pachyclavularia (typically called an encrusting gorgonian), then Fossa & Nilsen, 1998 and Borneman, 2001 call them Pachyclavularia violacea...
Anyway, star polyps are small colonial polyps that form rubber-like encrusting mats, rather than colonies arising from loosely connected stolons, as the clove polyps typically do. Star polyps can also retract, but even more so than the clove polyps, being able to pull completely into the mat when irritated, leaving only a small bump behind. But again, if they're healthy, they'll slowly start to come back out after a few minutes.
The polyps are most often a bright fluorescent green, but they may also be cream colored to almost brown, rarely yellow, and have finely-pinnate tentacles. They're pretty, but you won't see any that look like the awesome feathery tentacles of a clove polyp. They often do have a lighter colored center like them, though, making the whole package quite beautiful. Underneath them, the mats themselves are typically purple to reddish in color and are very tough. They can also cover any sort of surface in an aquarium, and often grow right onto the glass in sheets.
When it comes to lighting, like their cousins, they can live under a wide range of lighting systems, although in general the brighter the lights the better. Thus, if you want to see the stuff really take off, try something strong. Once they get going, you may regret it though, as they can really spread! Also like their cousins, star polyps can take a wide range of currents, and prefer a moderate, turbulent flow around them. Again, the reason being that it should keep them cleared of junk, but not blow the polyps out of their base. They won't be needing any feeding either.
Star polyps are also resistant to the stings of many corals, but unlike clove polyps, these can "burn" many other specimens that they grow into contact with. So, they can actually become a problem at times, as they can spread out over surfaces and can start to overgrow, irritate, and even kill their neighbors. As I mentioned, once a colony gets going, it can be surprisingly difficult to keep it under control at times, too. However, with all that said, they can be stung badly by some of the more aggressive corals found in aquariums, so don't make the mistake of thinking you can shove a colony in anywhere you wish.
Fortunately, they can be cut and cut and cut without any problems if they need to be trimmed back. Of course, this means they can also be propagated easily if you want. The methods are the same, as you can be patient and wait for polyps to grow onto a rock or shell, which can be moved, or you can speed things up by getting out the razor blade and glue.
Pipe Organ Corals (also called organ pipe corals):
Now for the weird one. The pipe organ coral, Tubipora musica, is an oddity because specimens build a hard skeleton even though they aren't "regular" stony corals. In fact, they have the eight-tentacled polyps like other stoloniferens, but the rest of the stony corals have only six tentacles (or a multiple of six) per polyp, so they aren't even in the same subclass with them. Pipe organ corals are definitely octocorals, but about 99% of the rest of the stonies are hexacorals. And, if that isn't enough, even though a pipe organ coral's skeleton is made of carbonate, like the regular stony corals', it is red in color instead of white. Odd, indeed.
Colonies typically consist of rounded, massive skeletons that are always attached to hard substrates, and upon close inspection it can be seen that they are comprised lots and lots of tiny hollow tubes. If you look at a cross-section, you'll also find that these tubes are joined laterally by numerous, thin horizontal skeletal layers that fuse them all together, which are formed by the stolons that connect the polyps. The polyps stick out of the tops of the these tubes and are typically light green, gray, or almost white in color. And, as far as form goes, they are commonly identical to star polyps, having thin tentacles with tiny pinnae down the sides. I say commonly though, because there are some specimens that look more like clove polyps, or that have fatter tentacles and look more like those of some a flowerpot corals (Alveopora).
There are still more differences, too. Unlike the clove and star polyps, pipe organ corals will need moderate to high intensity lighting and will not fare well under dimmer lighting. They also won't overgrow other specimens, are they are more susceptible to being stung by other corals, so be careful about that. And, while I've never seen it that I can remember, Lewis (1982) reports that pipe organ corals capture prey with their tentacles. So, assuming they do capture prey, they should benefit from the addition of specialty invertebrate foods and/or being placed in an aquarium that uses a deep sand bed to provide a natural food source for its inhabitants.
As far as propagating a pipe organ coral goes, you're going to have a long, long wait if you try sticking them next to a rock as your preferred method. Cutting up a colony with a razor isn't going to work either for obvious reasons. But, don't fret. If you want to make more colonies, want to make a specimen smaller, or just change it's shape get out a hacksaw blade a saw away. Be careful, of course, but don't worry too much. You'll likely lose a few polyps in the process, but under good conditions, a healthy colony will recover with no troubles. You can also use this procedure to "save" a colony that is severely injured/partially dead. Cut away the bad, and keep the good.
Lastly, they do seem to prefer a low to moderate current, and will not extend their polyps if the flow is too strong. But again, flow should never be low enough to allow junk to settle onto a colony and collect.
To finish up:
If you read my article titled "Toadstool Troubles", you may remember that many hobbyists have reported that the use of phosphate-removing products that contain aluminum can have serious negative effects on a range of soft corals, which includes toadstool leather corals. The same products may also have a strong effect on stoloniferans, so be careful.
Other than that, clove polyps and star polyps can be considered "bulletproof" when kept under proper conditions, and are more likely to become a problem due to too much growth, rather than not enough. However, when it comes to pipe organ corals, hobbyists and other authors seem to have mixed opinions. Some specimens seem to be among the hardiest corals in a tank at times, but others have slowly died off for no apparent reason. People will argue due to their own experiences, but if I had to guess, I'd say they are more dependent on food than typically thought, and don't receive enough in many aquariums. And/or it could also certainly have something to do with chemical warfare between corals, since different aquariums will have different combinations of corals and methods of maintaining water quality. While the entire bunch is still lumped into one recognized species, there are obvious differences between many of them, too. So, these different-looking varieties could also have very different care requirements, which have not been documented (yet).
All in all, a very good bunch of corals to choose from...
Allen, G. R. and Steene, R. 1994. Indo-Pacific Coral Reef Guide. Tropical Reef Research, Singapore.
Borneman, E. 2001. Aquarium Corals - Selection, Husbandry, and Natural History. T.F.H. Publications, Neptune City, NJ.
Fossa, S. and Nilsen, A. 1998. The Modern Coral Reef Aquarium, Volume 2. Birgit Schmettkamp Velag, Bornheim, Germany.
Lewis, J. B. 1982. Feeding behavior and feeding ecology of the Octocorallia. Journal of Zoology, London. 196: 371-384.
Sprung, J. and Delbeek, J. C. 1997. The Reef Aquarium: Volume 2. Ricordea Publishing, Coconut Grove, FL.