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You are here: Home Volume I March 2002 Invertebrate Non-Column: Gorgonians

Invertebrate Non-Column: Gorgonians

By Robert Toonen Posted Mar 14, 2002 07:00 PM Pomacanthus Publications, Inc.
Rob responds to a reader's question about gorgonians.

Rob,

I just got an orange and a red branch warty gorgonian (I think that they are from the Caribbean) for my tank. I have them in areas that get[s] low light and medium-strong current. It took them a few days to acclimate, but now their polyps appear to be fully open. As far as I can tell, the gorgonians seem to be doing fine in my tank so far. My question is besides DT's Live Phytoplankton, can I also feed them any small meaty foods like Prime Reef or should I get some sort of invert food or something else? If so, how many times a week? Anything else that I should know about them? Thanks! Mike

Mike,

Just to bring everyone to same starting point, let me start off by explaining exactly what a gorgonian is. Gorgonians are in the Class Anthozoa and together with the sea pens, the 'true' soft corals, and the stoloniferans (like the pipe-organ coral, Tubipora) form the subclass Octocorallia. The other subclass of anthozoans is the Hexacorallia, which includes all the stony corals, the anemones, the zoanthids and the corallimorphs (commonly called mushroom polyps). Although greatly oversimplified, the octocorals usually have feeding tentacles in multiples of eight, while the hexacorals usually have multiples of six feeding tentacles per polyp. There are a lot of other differences, of course, and Eric Borneman (2001) has a nice description of the classifications of cnidarians in his book Aquarium Corals, if you're looking for a more detailed and accurate explanation than I have given here. The Order Gorgonacea includes the sea fans, sea whips, sea rods, black corals, and other gorgonians that have a firm interior axial skeleton (the hard rod inside the animal that keeps it upright). This axial skeleton is usually composed of a tough proteinaceous compound called gorgonin (although some may be calcareous), and the colony forms a thin, rough layer of living tissue (usually loaded with spicules) that grows over that axial skeleton.

Photo

Non-photosynthetic gorgonian. Photo: Alf J Nilsen.

I'm glad to hear that your animals seem to be settling into your tank, but in truth, it is pretty hard to say for sure whether or not a gorgonian is doing well in an aquarium over the short term. Although a complete failure to extend the feeding polyps is likely to be a sign that the animal is not doing well, the converse is not necessarily true. Simply extending the feeding polyps is not an obvious sign either way; gorgonians can continue to expand and look 'normal' while slowly starving for months before showing obvious signs of decline. The only really clear indication that your animal is thriving in the aquarium is that it shows significant new growth through time.

Although gorgonians are amazingly resilient, and many of them can make a complete recovery from losing even a considerable amount of tissue (in fact, the animals are often propagated by simply cutting off a healthy branch), it is still generally a good idea to avoid purchasing an animal with exposed skeleton. If there are extensive portions of the skeleton showing, you know that the live tissue has died and rotted (or been cut or rubbed) away from that area. Unless you really know what you're doing, the problem with selecting a gorgonian that shows exposed skeleton is that: 1) you know that something has caused some of the colony to die recently, but you don't know whether that was mistreatment during collecting or a systemic problem, such as a disease; and 2) the exposed skeleton provides substrate upon which algae can grow, and that makes it more difficult for you to establish the animal in your aquarium. If the animal looks to be in great health otherwise, and you are determined to buy an animal with exposed skeleton, it is a good idea to simply cut off the dead portions of the colony before securing it in your tank.

I'll assume that you are correct that your gorgonian has come from the Caribbean, and will guess that the colorful warty sea rod you describe is most likely Diodogorgia nodulifera (there are plenty of suitable coral identification books out there, such as the Aquarium Corals book I mentioned above). These lovely gorgonians are very common throughout the Caribbean, and are both hardy and adaptable to a wide variety of habitats in deeper waters. In nature, they are found almost exclusively in deeper waters. They usually appear at about 40 or 50 feet and are common to depths as great as about 600 ft. They are often found in shaded areas, such as under ledge overhangs on walls and in caves, on shallower patch reefs. At greater depths, they are much less restricted, and are found out in the open on deep reefs, as well as growing from hard substrates on both sandy and rocky bottom areas.

Because these animals lack symbiotic algae from which to gain any nutrition, there is no specific lighting requirement for these animals. I think that your choice of a lower light area is a good one; however, I have seen them kept in tanks with both intense and subdued lighting, but if their natural distribution is any indication, they should probably be kept out of strong lighting. If nothing else, these gorgonians make a beautiful addition to a shaded area which would be suboptimal for most corals, and shading also reduces the risk of algal growth on the gorgonian. In my experience, people generally have more success with D. nodulifera which are kept in shaded regions of the aquarium.

I am not sure exactly what "medium-strong current" means in terms of a flow rate, but I am glad to see that you are thinking about the flow conditions. Flow is critical to the survival of most gorgonians, because their feeding ability is more-or-less directly related to the current flow in which they are kept. Unfortunately, recommended flow rates are also where my ability to give you a specific recommendation for your gorgonian becomes much more complicated, because to the best of my knowledge, there are no specific studies of feeding or current requirements for D. nodulifera. The conditions under which an animal will thrive varies with each species of gorgonian, and no single set of conditions or current velocities are best for all species. That makes it pretty hard to offer generalities about how best to keep a species, such as D. nodulifera, for which there has been no specific research.

Photo

A photosynthetic seafan from the carribean - these are much easier to maintain in a home aquarium.

One clue that we can use to guess the appropriate flow conditions for a gorgonian is the size of the polyps. Research by Dai and Lin (1993) found that each of the three species of Taiwanese gorgonians they studied occurred in specific habitats, apparently as a result of their ability to feed effectively. The larger the polyps, the greater they were deformed by high current speeds, and the less effectively the animals were able to feed in extreme currents (e.g., Sponaugle 1991, Sponaugle and LaBarbara 1991, Dai and Lin 1993). As a result, gorgonians with the largest polyps had the narrowest range of currents in which they were able to feed, and the smaller the polyps, the more capable the animals were of feeding in a wide range of current speeds. For example, Dai and Lin (1993) found that the gorgonian with the largest polyps in their study (Subergorgia suberosa) was restricted to lower reef slopes or sheltered outcroppings where currents averaged less than 10 cm/s. In contrast, the species of gorgonian with the smallest polyps in their study (Melithaea ochracea) was able to feed at the highest flow rates (as great as 40 cm/s in this study).

It is interesting to note that regardless of the polyp size, or the range of current speeds at which the animals were capable of feeding at all, all three species tested by Dai and Lin (1993) fed best at about 8 cm/s. In a similar study with two Jamaican gorgonians, Sponaugle and LaBarbara (1991) found that both species fed best at current velocities between 10 - 15 cm/s. D. nodulifera has pretty large polyps for a gorgonian, and in my experience, D. nodulifera is found primarily in the same approximate current regimen as S. suberosa, so in the absence of any specific data on the feeding behaviors and flow preferences of D. nodulifera, we'll just have to assume that intermediate flows are also going to be the best bet for successfully feeding your gorgonian. If we're willing to accept that assumption, I would guess that variable current speeds on the order of about 6-10 cm/s would likely be the best bet for the successful feeding of D. nodulifera.

OK, great, but what exactly does 10 cm/s mean when it comes to flow in your tank? Well, although it sounds silly, the best way to determine current speed in your tank is to simply measure it. Locate a bubble, a baby brine shrimp or some other tiny marker you can easily follow, and place one finger on the aquarium where that marker is first seen (this should ideally be upstream of where you want to locate your gorgonian). Watch that marker for 2 seconds and place another finger on the tank where ever the item you were following has ended up. Simply divide that distance by 2 and you have a very rough estimate of the current speed in your aquarium. Do this a few times to get an idea of the average flow velocity in that spot, and that will help you to decide whether there is sufficient flow in the area in which you hope to keep the gorgonian.

Before I go any further, I have to say that in general, asymbiotic (lacking symbiotic zooxanthellae, and therefore non-photosynthetic) gorgonians do not have a good record of survival in reef aquaria to date. You have taken the first step and are asking about the feeding requirements of your animal, so you are obviously prepared to start feeding it regularly. Of course, I would prefer that you had asked these questions before you purchased the animal rather than afterwards, but that's another issue. Feeding is essential for any asymbiotic gorgonian, because in even the best stocked tanks, it is unlikely to get enough food without some effort to directly feed it. Again, there is no specific studies of the feeding requirements of D. nodulifera, so we're left with looking at the feeding requirements and prey preferences of other gorgonians, and hoping that your gorgonian is fairly similar.

Sadly, there are not many studies on the natural diet or feeding behaviors of gorgonians on coral reefs at all. The studies that have been done suggest that most gorgonians likely feed on a variety of small plankton, detritus, marine snow, phytoplankton and even bacteria (e.g., Coma et al. 1994, Ribes et al. 1998, 1999). Although these studies suggest that gorgonians will take a variety of prey items, all prey items consumed by gorgonians in the wild are very small, and gorgonians eat a lot of them. In fact, based on the studies cited in this article, I would go so far as to say any gorgonians kept in aquaria require some food, regardless of whether they contain symbiotic zooxanthellae or not.

I say this because even gorgonians with photosynthetic symbionts (such as the commonly imported Plexaura, or Eunicea species) are capable of capturing and ingesting a considerable number of small to medium-sized zooplankton (in the approximate range of 5 - 700 micrometers). In addition to the nutrients provided to the gorgonian by symbiotic zooxanthellae, prey capture may be an important source of food energy for all gorgonians studied to date. One recent study (Ribes, et al. 1998) on feeding in Plexaura found that this 'photosynthetic gorgonian' eats roughly 7.2 ± 1.9 prey items per polyp per day (primarily ciliates, dinoflagellates, and diatoms in this study). That is a lot of food for an animal that isn't supposed to require any feeding! Although these prey contributed a negligible proportion of the annual carbon budget (~ 0.4%), feeding was necessary for the animal to meet it's energy requirements for growth and reproduction; almost 20% of the annual nitrogen requirement for growth and reproduction in Plexaura flexuosa came from zooplankton and microbial prey items (Ribes et al. 1998).

Among those gorgonians lacking symbiotic zooxanthellae, much of the diet appears to derive from a range of tiny prey including invertebrate eggs and larvae, ciliates, copepods and other microzooplankton, particulate organic detritus, diatoms, dinoflagellates and other phytoplankton. For example, in the small-polyped gorgonian Paramuricea clavata, Coma et al. (1994) found that copepod eggs, copepod nauplii, other invertebrate eggs and larvae, and other small zooplankton (primarily 100-200 micrometer size range) prey items accounted for > 75% of the diet. Adult calanoid copepods (roughly the size of a newly hatched brine shrimp) were also captured from time-to-time. Although the copepods were not captured as often as the food items listed above, they are so large and nutritious relative to the other plankton prey that they are likely an important component of the natural diet. There is no mention of phytoplankton capture by P. clavata by Coma et al. (1994), but another study found that this species also feeds extensively on diatoms, phytoplankton, ciliates and detrital particulates (Ribes et al. 1999). Neither of these studies found any evidence for significant uptake of dissolved organic compounds, bacteria or other nanoplankton less than about 3 micrometers in diameter by this small-polyped gorgonian.

Given the results from studies such as these on gorgonian feeding, I suspect that the large-polyped D. nodulifera you plan to keep would probably prey primarily on planktonic prey such as diatoms, ciliates, rotifers, invertebrate eggs and larvae, the occasional copepod (if they are lucky enough to catch some), and the like. It is hard to say for sure what is the smallest prey item that an animal can consume, but studies on a variety of gorgonians suggest that nanoplankton less than about 3 - 5 micrometers in diameter are a minor component of the natural diet (e.g., Coma et al. 1994, Ribes et al. 1998, 1999). As such, although DT's Live Phytoplankton is a great product, it is probably not the best choice for a primary food item for D. nodulifera. DT's is a live culture of one of the smaller phytoplankton species (Nannochloropsis), and the phytoplankton cells contained in this product are probably a little small (< 3 micrometers, on average) to simulate the natural diet of any of the gorgonian species studied to date. That is not to say that the animals cannot or will not eat DT's (they may very well eat it quite happily), just that based on the gorgonians studied to date, the vast majority of the natural diet of these animals is composed of larger particles.

So, what should you be feeding, then? Well, if you happen to have a tank with a well-established live sand bed or refugium of some sort, live foods (such as ciliates, copepods, invertebrate larvae and eggs) are likely being released on a regular basis and providing your tank with some plankton from time-to-time. Especially if you're feeding DT's regularly to your tank already, then the reproductive rate of some of these tiny invertebrates can be quite high, and may provide a considerable number of invertebrate larvae to the planktonic food chain in your aquarium (Shimek 1997). Furthermore, food supplied by the occasional algae scraping from the aquarium walls (usually consisting primarily of diatoms) and resuspension of particulate organic detritus from the aquarium substrate and live rock structure is also a bonus treat for these animals.

However, while such foods may be sufficient for a gorgonian with photosynthetic symbionts, they should not be solely relied upon to feed a gorgonian that lacks zooxanthellae. Personally, I consider any food captured in this way a treat for a non-photosynthetic gorgonian, and an effort to feed them directly seems important to the long-term survival of the animal. Most gorgonians will gladly capture rotifers, and if you happen to culture (e.g., see Moe 1997, or Wilkerson 2001) or buy live rotifers to feed your tank already, that will certainly help with keeping your gorgonian well-fed. Gorgonians with polyps as large as those of D. nodulifera are also likely to consume newly-hatched brine shrimp nauplii. It is important to keep in mind, however, that capture rates for relatively strong-swimming plankton (such as baby brine shrimp) are often much lower than for more easily captured prey (such as invertebrate eggs and larvae), and even a large-polyped gorgonian like D. nodulifera may have trouble capturing many baby brine shrimp unless you feed a lot of them. If you don't have access to these foods, there are a variety of artificial plankton foods, frozen and preserved Daphnia, frozen copepods, commercial invertebrate foods, and even finely ground fish foods that will probably help to keep the animal alive (see Toonen et al. 2002 for a comparison of the relative performance of some common phytoplankton products).

The problem with using choices such as frozen plankton, commercial invertebrate foods, or pulverized fish foods is that they don't tend to stay in suspension very long, and it is easy to overfeed your aquarium if you're not careful. That usually means that hobbyists provide food relatively infrequently, and very little at a time. By all reports from studies of feeding in the wild, most gorgonians appear to be capturing numerous prey items per polyp per day, and occasional sparse feedings may not be sufficient for keeping them healthy over the long-term in captivity. Although live foods would certainly be the best bet, they are generally more expensive and difficult to provide than an artificial alternative, and I know that compromises are often made for cheaper and easier to use products. Perhaps the easiest food to provide for these animals would be a dried plankton replacement such as ESV's Spray Dried Marine Phytoplankton or BSD'/ s Golden Pearls Clusters, both of which appear to be a highly nutritious and eagerly accepted by my D. nodulifera. Research into effective artificial feeds is ongoing, and there are new products such these appearing on the market all the time. This is fortunate for us as hobbyists, because many of them appear to provide a suitable alternative to the tiny zooplankton (on the order of 100 micrometers or less) that so many reef organisms prey upon. Hopefully, with a bit more experience, hobbyists on the whole will be much more successful with keeping non-photosynthetic animals than we have been in the past.

Even after giving you all this information, you may still have difficulty caring for these animals. In fact, these gorgonians are not recommended for beginners, and generally not recommended for even advanced hobbyists who do not plan to feed them at least a couple of times a week (and preferably every day). If you chose a healthy gorgonian, place it in a suitable environment, and make sure that it gets enough food, there is a good chance that you will be successful in keeping one of these beautiful animals. Of course, the same can be said of almost any animal added to an aquarium, and knowing what constitutes a "suitable environment" and "enough food" is one of the keys to success with any of the animals we try to maintain in an aquarium. The bottom line is that success rates with these animals among most reef enthusiasts remain pretty low. I hope that the information that I'/ ve given you here will help you to provide your animals with a good home, and prevent your gorgonian from the sad fate that so many of these nonphotosynthetic cnidarians face in the aquarium.

Good luck!

Rob

Literature Cited:

  1. Borneman, E. 2001. Aquarium Corals: Selection, Husbandry, and Natural History. Microcosm, T.F.H. Professional Series, Neptune City, NJ.
  2. Coma, R., J.-M. Gili, M. Zabala, and T. Riera. 1994. "Feeding and prey capture cycles in the aposymbiotic gorgonian Paramuricea clavata." Marine Ecology Progress Series 115:257-270.
  3. Dai, C.-F., and M.-C. Lin. 1993. "The effects of flow on feeding of three gorgonians from southern Taiwan." Journal of Experimental Marine Biology and Ecology 173:57-69.
  4. Moe, M. 1997. Breeding the Orchid Dottyback, Pseudochromis fridmani: An Aquarist's Journal. Green Turtle Publications, Plantation, FL.
  5. Ribes, M., R. Coma, and J.-M. Gili. 1998. "Heterotrophic feeding by gorgonian corals with symbiotic zooxanthella." Limnology and Oceanography 43:1170-1179.
  6. Ribes, M., R. Coma, and J.-M. Gili. 1999. "Heterogeneous feeding in benthic suspension feeders: the natural diet and grazing rate of the temperate gorgonian Paramuricea clavata (Cnidaria: Octocorallia) over a year cycle." Marine Ecology Progress Series 183:125-137.
  7. Shimek, R. 1997. It's (in) the water... Aquarium.Net 09/97:1-8. http://www.aquarium.net/0997/0997_4.sht
  8. Sponaugle, S. 1991. "Flow patterns and velocities around a suspension-feeding gorgonian polyp: Evidence from physical models." Journal of Experimental Marine Biology and Ecology 148:135-145.
  9. Sponaugle, S., and M. LaBarbara. 1991. "Drag-induced deformation: A functional feeding strategy in two species of gorgonians." Journal of Experimental Marine Biology and Ecology 148:121-134.
  10. Toonen, R., K. Batchelor, and T. Mai. 2002. "Planktonic Foods for Reef Aquaria: If you feed the tank, are these foods for you?" Marine Fish & Reef USA Annual 2002:18-31.
  11. Wilkerson, J. D. 2001. Clownfishes: A Guide to Their Captive Care, Breeding & Natural History. Microcosm, T.F.H. Publications Professional Series, Neptune City, NJ.
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