Q&A - Dendronephthya
by Rob Toonen
Rob, I wanted to pick your brain about something. I have been researching information on the Pink Carnation Coral (Dendronephthya species) and from all I can find, they are considered very hard to keep. I would really like to add one to my aquarium, but I don’t want to do so if there is no chance that I can keep it alive. Based on what little information I can find on these corals, it seems that we have little understanding of what their exact requirements are. Am I correct in that assumption? Can you tell me what are the perfect water conditions they need, and what they need to be fed? I understand that Carnations do not have zooxanthellae and therefore need to be fed because they cannot make their own food from the aquarium lighting – is this accurate? Any other suggestions or recommendations that you can make about my chances with these amazing corals would be greatly appreciated. Thanks. Evan
Well, as usual I’ll start with explaining exactly what we’re talking about so that everyone is on the same page before I launch into a more specific answer to your question. Dendronephthya is a common genus of soft coral (Alcyonarian) which completely lacks photosynthetic symbionts (e.g., zooxanthellae). They are commonly found under ledges, hanging from rock walls and in caves in regions of high flow where high concentrations of suspended plankton abound. Because these animals lack photosynthetic symbionts, they are absolutely dependant on capturing plankton to support themselves, and therein lies the problem with keeping these corals. I would be lying to you if I said that your chances of getting and keeping one of these corals was even fair. In general, they are considered among the most difficult of the corals to keep in captivity, and should be avoided by any but the most experienced of aquarists. Their terrible survival record almost certainly results from our general inability to provide plankton in suitable concentrations to provide for the needs of obligate suspension feeders in an aquarium. Most texts rate this coral as extremely difficult, and it is not recommended for even the most advanced aquarists. The best suggestion I can provide you is simply to avoid buying one.
Unfortunately, with the exception of a couple of recent research papers, our understanding of their requirements is pretty limited. The largest hurdle to be overcome is a proper source and amount of planktonic food. Some species, in particular Dendronephthya hemprichii, D. sinaiensis and Scleronepthya corymbosa, specialize almost entirely on phytoplankton, and even though these corals did eat some zooplankton, their feeble nematocysts (stinging cells) and tiny polyps are only capable of capturing tiny and weakly swimming zooplankton (planktonic animals) such as molluscan veliger larvae, tintinids and rotifers. Furthermore, the abundance of phytoplankton (planktonic algae, better known as greenwater) is usually 10 or more times as higher than zooplankton in the nutrient-poor waters surrounding tropical reefs. Research has recently shown that several species of non-photosynthetic soft corals, and even some species with zooxanthellae (the photosynthetic symbionts responsible for generating much of the nutrition of corals) capture a large amount of phytoplankton food. Gut analyses revealed that Dendronephthya did not capture any prey as large as baby brine shrimp, and in fact even the tiny, feeble zooplankton that they could catch (listed above) accounted for only <5% of their daily energy requirement. The remainder of the diet was entirely composed of phytoplankton in the 3-20 micrometer (1 Φm = 1/1000 of a millimeter). The detailed studies necessary to identify the species prey preferences for these soft corals have not yet been done, but I suspect certain species of Dendronephthya prey preferentially on certain species of phytoplankton, because most suspension-feeding invertebrates show strong prey selectivity when tested in laboratory feeding trials. Dissolved organic matter (DOM), such as amino acids, may also play an important role in the nutrition of these corals, although it has not been investigated sufficiently to make any conclusions in this regard.
There are occasional reports of people keeping these corals alive in aquaria, but in my experience so far, when questioned about their technique, people often explain that they feed the coral heavily with newly hatched brine shrimp (Artemia). As I explained above, however, baby brine are simply far too large for these corals to capture and consume, and I must conclude that such reports of success with keeping these corals on baby brine are either inaccurate, or innocent misidentifications of the coral in question. Misidentification could very well be a potential explanation because there are a bunch of other soft corals such as Nephthea, Stereonephthya & Litophyton (among others) that could potentially be confused for Dendronephthya and are occasionally sold in the local pet shops as “Carnation, Christmas Tree or Cauliflower corals" alongside Dendronephthya. The average person wouldn't have any reason to suspect that the coral was misidentified, and unless they have a better than average knowledge of coral taxonomy, they would never know that their coral was not Dendronephthya. I suspect that many of the reported cases of success with Dendronephthya may actually be one of these other species that prey primarily on zooplankton, and for which newly hatched brine may prove a suitable food. Still, even for these species, rotifers will almost certainly be a better choice for the size preference of most of these corals. Research on some species of Alcyonium and Scleronephthya suggests that they should be capable of catching zooplankton of the size of Artemia nauplii (and if they were properly enriched, they should be a fine food), and should (theoretically) have a far greater success rate in the aquarium than does Dendronephthya, so if you’re really set on having something along those lines, perhaps you should consider one of these easier alternatives...
Part of the confusion is likely because there are quite a few Dendronephthya species. Dendronephthya was originally thought to be at least three different genera, but these species have been rejoined into a single genus. Different species often differ in their habitat preferences and behavior, so there is good reason to believe that they may differ in their feeding habits, too -- but we just don't know enough about many of them to make sweeping generalizations like this.
Although the corals can be found upright and in full sunlight in the wild, they are most common in deep water, or hanging from the upper sides of caves and ledges, typically with lots of turbulent water flow and a high suspended particulate load. In theory, these corals should be extremely hardy animals. Personally, I think that the fragile nature of this coral is an artifact of them being kept in far from ideal conditions in our aquaria. Some Dendronephthya species are actually found intertidally (living in the shallow zone exposed to air each day when the tide is low), which means that they are actually out of water for tens of minutes every day, and it is hard to find a more stressful thing for a marine invertebrate than to be subjected to baking in the tropical sun while out of water for extended periods of time! In fact there are very few tropical species that are intertidal in the first place, and Dendronephthya is one of only a couple of coral species (along with the very hardy button polyps, Zoanthus sociatus, for example) that can survive in the intertidal in the wild. This suggests that the animals should be extremely hardy if fed properly, but for a starving animal, obviously hardiness is not a relevant issue.
So why do these animals seem to do so poorly in aquaria? Well, one factor that seems to be quite important is that the flow rate they experience seems to be critical to both their feeding and their expansion. These corals are capable of amazing amounts of expansion, and a melted blob of nondescript goo can expand into an amazingly beautiful spiky tree when placed in suitable conditions. The animals actually go through regular periods of expansion and contraction, and that exertion alone requires a substantial amount of energy to complete – if the coral is starved for an extended period, it may not be able to expand properly to feed, and that will guarantee its demise. Even if the animals are expanded and appear to be happy, they may be incapable of feeding efficiently – as I mentioned above, flow is an important component of their feeding success and in the wild there is a direct correlation between the rate of flow and the amount of coral polyp expansion. Researchers found that as they increased the flow rate in which the corals were kept from 1 to 32 cm/s, the expansion of the coral polyps increased in direct proportion to the increase in flow. Furthermore, unlike many, if not most, corals, Dendronephthya seem to grow best in areas of laminar flow (constant, unidirectional) rather than oscillating or turbulent flow. The ideal flow rate seemed to be roughly 15 cm/s; thus, a powerhead delivering ~15cm/s pointed towards this coral ought to provide a suitable flow regime for both the effective feeding and expansion of the corals. Unfortunately, my own observations do not support that expectation – my corals were showing poor expansion and no growth until I moved them to a slightly lower flow regime (~5-10 cm/s) and turbulent rather than laminar flow. Since I have moved the coral, it has shown much better expansion and has actually grown a second “stalk” off the original branch of the coral. I have no idea why this seems to be the case, but it could well be an artifact of keeping the animals in an aquarium. Regardless, for the corals I am keeping, the lower flow regime rather than the stronger unidirectional flow predicted by research on corals under natural conditions seems to be working better for both the pink and orange Dendronephthya in my aquarium...
As I said above, although flow rate is an important factor, feeding is the real key to success with these corals. Because Dendronephthya capture almost exclusively tiny phytoplankton, you'll need to provide plenty of greenwater if you’re to have any hope of keeping one of these animals alive in your aquarium. Up until recently, the only way to do that was to culture your own phytoplankton, and although I have written articles explaining how to do that, relatively few people are willing or able to culture phytoplankton reliably and in sufficient quantity to provide food for an obligate phytoplankton feeder such as Dendronephthya. However, there are now a variety of live phytoplankton suppliers, cryopreserved (freezer paste) phytoplankton, as well as spray-dried products on the market that not only make the culture of phytoplankton at home unnecessary, they also provide a reliable and consistent source of greenwater to feed your aquarium. There are advantages and disadvantages to each product, primarily the cost of live phytoplankton is highest, it is easiest to overfeed the cryopreserved algae, and the dried phytoplankton requires mixing in a blender before particles of the correct size are produced, but a detailed explanation of these issues will have to wait for another time.
One other point of interest is that there are an entire suite of copepods that are specifically associated with Dendronephthya, and about which nothing is basically known. There are a couple of coral/crab symbioses in which neither species survives without the other (in nature, if you experimentally remove one, the other is out-competed or eaten), but I think this association is unlikely to have anything to do with the generally dismal record of these corals in reef aquaria. Food seems like the most likely problem, and I have actually kept one of these animals for long enough to drop some daughter colonies in my tank when provided with heavy continuous phytoplankton feeding. I lost my colonies when I discovered that my blue tuxedo urchins (Mespilia globulus) had quite a taste for these soft corals, and although this is the only thing they seem to have ever bothered in my aquarium, they completely eradicated all my Dendronephthya.
All-in-all, if you can provide an appropriate habitat and flow conditions for these animals, the biggest problem will undoubtedly be getting the feeding right. Despite their magnificent beauty and the frequency with which they are imported into pet shops, I would recommend that you resist any temptation to buy one, because your chances of keeping the animal alive in an aquarium are very low. As I said, feeding would seem to be the main issue with these corals, but even as phytoplankton products become easily available and commonly used, there are still not widespread reports of success with these corals. Until reports of success with these corals become routine, I would recommend that all but the most experienced aquarists avoid supporting any trade in these beautiful but virtually impossible to keep corals.