Aquarium Corals: The Bubble Corals: Species of the Genera Plerogyra and Physogyra
Of all the corals hobbyists can choose from, the bubble corals are definitely some of the most unique in appearance. Topped with oddly-shaped, swollen vesicles, these are eye-catchers in any aquarium. There's a lot more to them than the pretty bubbles though, as they can be very aggressive and need their "personal space", and are eager eaters, too. Nevertheless, they can be great choices for reef aquariums due to their hardiness and ability to live under a relatively broad range of conditions. So, I'll go over their basic biology and what you need to know about caring for them.
To start, there are three types of bubble corals, each of which can have some variation in skeletal form. Plerogyra sinuosa forms a skeleton that is typically flabello-meandroid in form, meaning the skeleton is shaped something like a laterally flattened cone at first, which grows into a more complex curved wall. However, some specimens maintain a cone-shaped form until they're quite large in size, while others may stay relatively flat, growing more outwards instead of upwards. Plerogyra simplex produces a skeleton that also starts as a small cone but becomes phaceloid in form, splitting up into large branches as it grows. And, Physogyra lichtensteini, produces a unique meandering skeletal form the can be rather flat to dome-shaped.
Also note that regardless of the species/overall skeletal form, their skeletons are always topped by a series of particularly large septa (especially those of Plerogyra spp.), which are the petal or blade-like structures that line the upper part of their skeletons and can sometimes be seen bulging through their soft tissues. They're especially easy to see when the bubbles are not fully distended.
As far as the bubbles themselves go, they really do look like bubbles most of the time. In the case of P. sinuosa and P. simplex these modified water-filled tentacles are usually only a half inch to an inch or so in diameter, but in some cases they can be inflated to as much as two inches and can have a variety of shapes, too. A large, smooth, symmetrical ovoid form is the most common, but they can also be smaller, asymmetrical, and/or have numerous little knobs all over them, as well. On the other hand, Physogyra lichtensteini has relatively small bubbles, but they can also be rounded in form or oddly-shaped with weird protuberances all over them. Note that they're often called a "pearl coral", or "pearl bubble" due to this smaller bubble size, too.
Whatever the form may be, the bubbles are typically translucent cream, white, or green in color, and many have little details on their surfaces that are quite interesting. A few have a shimmering band down the middle and are sometimes called "cat's eye bubbles", while many other bubbles have something of a fingerprint pattern covering them. Really, you could fill a large aquarium with various bubble coral specimens that all had slightly (or not so slightly) different colors, skeletal forms, bubble forms, and bubble details.
All three of these corals are also zooxanthellate, meaning they carry symbiotic algae in their bodies and rely on them for some of their nutritional needs. These entrapped algae create food for their coral host when exposed to light, and the bubbles are thought to be specialized tentacles that can regulate the amount of light that the algae receive (Borneman 2001). The structures can expand to increase their surface area, thus increasing the area exposed to light, or they can contract to reduce their exposure. Thus, they're typically filled with water and expanded during the day, then deflated and retracted into the skeleton at night when they're not needed.
When the bubbles are contracted, and sometimes when they aren't, you can easily see that bubble corals also have rather normal-looking feeding tentacles that can usually be extended up to a couple of inches in order to collect a wide range of food items. These look like typical coral tentacles, usually being clear or white and thin, and they're armed with stinging cells, like those of other types of corals, as well.
On top of the bubbles and feeding tentacles, they also possess some even longer and specialized tentacles produced specifically for stinging other corals that may be growing too close. These are called "sweeper" tentacles, and they can be as much as 3 or 4 times as long as the rest of the tentacles. Again, these generally stay out of sight between the bubbles during the day, but not always.
Most corals are spawners that spew out clouds of sperm and eggs in to the water from time to time as a means of sexual reproduction, and some hobbyists have been lucky enough to have this occur in their aquariums. However, I haven't heard of any cases of a bubble coral doing so while in captivity. Still, these corals have been known to reproduce asexually in two ways, so it is possible to find more in your tank than you add.
The first form of asexual reproduction is "budding", which can be seen when a small growth arises from the lower edge of a specimen's flesh. This growth can increase in size over time, and can eventually develop a small skeleton of its own. As it gets larger and heavier it will sag down, and eventually drop away, coming to rest somewhere and attaching to the substrate (Borneman 2001 and Fossa & Nilsen 1998). In aquariums, hobbyists can often speed things up a bit by clipping off buds before they drop, and can then place them in an appropriate location.
While this sort of budding is also typical of many other types of coral, the second method these particular species employ is quite unusual. Unlike many other corals with similar skeletal structures, the flesh of the bubble corals can wrap up and over the top of the skeleton and sometimes all the way down the sides, too. When it does, the soft tissue can actually spread out and over adjacent areas of substrate, as well. Then, such extensions of tissue can, at times, give rise to a whole new coral (Borneman 2001).
Plerogyra sinuosa and Physogyra lichtensteini have a broad distribution being found throughout the Red Sea and much of the Indian and West Pacific Oceans, while Plerogyra simplex is found in generally the same areas of the Western Pacific, but not the Indian Ocean or Red Sea (Veron 2000). They're also found in a variety of environments throughout these ranges where water flow is acceptably low. Some may occasionally be found on sandy bottoms in calmer areas, where sunlight is very bright, but they're more commonly found in protected areas, such as caves and under overhangs where they're typically attached in a vertical position. In these situations, light intensity is obviously much, much lower, and I've found many in Indonesian waters that were living under conditions where I was surprised any zooxanthellate coral could survive. So, they can obviously thrive in diverse conditions, with varying temperature and illumination.
These corals can accordingly thrive under various conditions in an aquarium, but there's one thing in particular to keep in mind when it comes to lighting. While hobbyists can choose what lighting to go with, specimens that were collected from dimmer areas around reefs are indeed susceptible to light shock if they are over-illuminated when introduced to an aquarium. So, if you intend to place a bubble coral in a very brightly-lit area in your tank (as in directly under metal halide lighting), I recommend that you first place the specimen lower in the aquarium (or in a shaded area), then slowly move it to brighter and brighter locations over a period of at least a couple of weeks. This will give the coral some time to adapt to the new conditions.
Water flow should also be low to moderate, and turbulent flow rather than a constant stream from one direction is best. It should be at least strong enough to get the tentacles moving around a bit and not just sitting still, but it also should never be so strong that it prevents expansion. None of these corals will open up and extend their tentacles if the current is too strong, as doing so could lead to tissue damage caused by any of the sharp edges found on their own skeleton. Oddly enough, I've found that sweeper tentacles are most often produced in higher currents, too. Not when it's strong enough to hurt them, but when flow is tolerable but fast. So, if you have troubles with sweepers, reducing the flow a little often helps.
When it comes to exact placement, as long as lighting and current are acceptable, the other thing to consider is what's nearby. The sweeper tentacles that can reach out several inches, so any specimen will need plenty of room. There are exceptions, though. While bubble corals can burn other corals up with their sweepers, they don't seem to mind each other at all, at least when I've put specimens of the same species right next to each other. I haven't tried putting different species in contact with each other though, so I'd be wary of doing so without a little experimentation first.
Lastly, bubble corals will do best if given some food from time to time. Under good lighting (and assuming you have acceptable water quality) they may look fine and grow, but feeding them appropriate foods will help them really thrive. Essentially anything meaty will suffice, as they can be hand-fed anything from brine shrimp to chopped fish, clam, or shrimp, and will also capture much smaller zooplankton-type foods. Feeding them directly by using a large needle-less syringe or baster full of food works great, too. Still, you never want to overfeed anything, as any leftover foods typically become great fertilizer for unwanted algal growth in your aquarium. So, don't overdo it.
In general, if any of these corals survive the first few weeks, I've found them to be quite durable and have few troubles. In fact, I'd say almost all problems are encountered very early on, being the result of injuries that occur during collection and shipping. The fact that these corals have such oversized and sharp skeletal septa makes them prone to skeletal damage and/or cuts if bumped and banged around, even when in a bag, and the septa may damage a specimen's flesh, or even break off inside its body. Thus, it's not uncommon to see a poorly handled specimen come in with broken pieces of septa, sometimes barely hanging on by strands of cut/torn flesh. Still, aside from such injuries, other typical problems are overwhelmingly due to poor water quality, which would be your own fault, and thus would not be a sign that bubbles corals are inherently weak by nature.
Unfortunately, when the above mentioned injuries occur they're often followed by brown jelly infections, which are typically fatal. When this happens, the cut areas become covered with a brown-colored mass of microbes that attack the injured area and look something like globs of mucus forming over the wound. And, they can increase in numbers quickly and consume the flesh of a victim within just a day or two, or maybe even in as little as a few hours in extreme cases.
The other problems are tissue recession and the growth of algae on the skeleton when it becomes exposed due to recession or injury. As noted, under normal conditions a bubble coral's soft tissue can extend well beyond the top part of the skeleton where the septa are, and can cover much, or even all of it. Even in the case of branched specimens, the soft tissue reaches down from the top of each branch and wraps a portion of the skeleton. So, the problem is that sometimes this extra tissue that covers these areas can start to essentially peel back from them and recede towards the top of the skeleton where the bubbles are, leaving previously covered areas naked in the process.
This malady is usually a symptom of unacceptably low alkalinity and/or low calcium and/or high phosphate levels, but might be brought on by other unknown factors, too. These can interfere with the precipitation of new skeletal material, and can make it difficult for a specimen's flesh to stay attached where it should. Thus, it'll be up to you to see that this doesn't happen by using additives that keep the alkalinity and calcium concentrations at acceptable levels, and by using all means possible to keep phosphate levels at a minimum.
Keep in mind that if recession starts and is allowed to continue, in the worst cases the entire coral animal can end up peeling right out of its skeleton. They can just wither away until there's nothing left. In addition, any newly exposed areas of skeleton are especially vulnerable to the growth of unwanted algae, and if it starts growing on the skeleton it can shorten the time to death. When this occurs, even if the conditions that led to the recession in the first place are remedied, specimens will have great difficulty reclaiming these areas. So, it is imperative that you act quickly if you see such recession.
Lastly, when looking for good specimens, the key is to look closely for any visible damage to the skeleton or the soft tissues. As mentioned above, these corals are prone to injury during collection and shipping, so if you do see anything that looks suspicious, I recommend that you move on to another specimen.
Aside from that, it's also best to choose a specimen that has its bubbles well expanded after the lights have been on for a while. If the bubbles are closed up during the day, there may be trouble. If a particular specimen is a new arrival and has not had time to acclimate to its surroundings at a shop, it may be fine later. However, if it continues to stay closed after several days, I would again suggest that you keep looking elsewhere. Never forget, regardless of how difficult it may be, it's better to be patient and thoroughly check out a potential purchase, rather than acting hastily and having something die in your aquarium.
- Borneman, E. 2001. Aquarium Corals - Selection, Husbandry, and Natural History. T.F.H. Publications, Neptune City, NJ. 464pp.
- Fossa, S. and A. Nilsen. 1998. The Modern Coral Reef Aquarium, Volume 2. Birgit Schmettkamp Velag, Bornheim, Germany. 479pp.
- Veron, J.E.N. 2000. Corals of the World, Vol. 2. Australian Institute of Marine Science, Townsville, Australia, 429pp.