I really want to get a Christmas Tree Worm rock, but I have been told that the worms will only survive if I can also keep the Porites coral in which they live alive. I have seen several nice pieces of Christmas Tree Worm rock in the local pet shops, but the coral is already dead or dying when I find them, and I don’t want to spend a lot of money on a rock that won’t last. Why is it that the worm needs the live coral to survive, and what conditions do these worms need to survive in my aquarium?
I'm not sure where the idea that the Christmas Tree Worm Spirobranchus feeds on the mucus from Porites first originated, but I see it quite often. Sometimes this idea is more general, as in the information that you have been offered, while other times it is highly specific and I have even been told that the coral mucus traps phytoplankton which the worms then eat, or that there are nutrients in the coral mucus without which the worms simply cannot survive. As far as I know this is all aquarium urban legend, and there is absolutely no scientific research I have ever been able to locate that supports any of these claims. The worms are suspension feeders on tiny plankton and although they are obligate associates of live corals, they are found in a variety of species, and Porites is actually one of the species in which they do worst in the wild (I'll explain this more below).
It's actually sort of a complicated system, and no one has really figured out exactly what is going on yet, so if you’re really interested in marine biology and want an interesting system to figure out for your graduate work, you might want to consider starting a research project on this topic. In terms of the worms, we don’t even really know which species we’re talking about, because it seems that the worm we’ve called Spirobranchus giganteus is not a valid species at all, but rather an entire species complex of cryptic species, and several of the "morphs" of this so-called Christmas Tree Worm species have turned out to be completely infertile (incapable of fertilizing one anothers eggs) when crossed in the laboratory.
Researchers have recently discovered that the worms are found on specific corals as a result of their larvae showing a strong settlement preference for corals such as Porites asteroides or Millepora complanata while completely ignoring corals such as Siderastrea, Dendrogyra and Agaricia. Perhaps I need to back up a step here. In case you don’t already know this, these worms (together with about 80% of all marine animals) produce tiny larvae that develop on their own until they metamorphose into the adult body form (like a caterpillar into a butterfly as a more familiar example). Obviously the choice of a suitable habitat is of critical importance to the survival of an animal that is permanently affixed to their choice once they metamorphose (such as a Christmas Tree Worm), and the chemical and physical cues associated with mature larvae selecting an appropriate habitat and metamorphosing into the adult body form are the focus of a great deal of research. Unfortunately for us all, this is a difficult problem to study, and the exact mechanisms and cues to which larvae respond remain largely mysterious to scientists. However, researchers are good at identifying the source of such cues and determining the conditions under which most mature larvae will settle into a given habitat and metamorphose from the microscopic larva into a tiny version of the adult.
Researchers have examined the settlement preferences of mature larvae of Christmas Tree Worms, and found that the larvae of some worms prefer corals such as Porites asteroides or Millepora complanata over others (as I just mentioned above). When researchers went a step further, and examined the growth and survival rates of worms on different coral species, worms that settled on Diploria strigosa did the best, followed by those living in Montastrea annularis, and M. cavernosa, with Porites porites being solidly in last place for every measure of growth or survival measured for the worms! It’s not really that simple, however, and just to make things more complicated, different researchers find different results in different regions (including everything from which corals are preferred by mature larvae, and in which corals juvenile worms do best). The differences found among researchers suggests that either there is a lot of variation among the worms in different portions of their range, or (as I mentioned above) that there are a lot of similar but unidentified species that are mistakenly being tested as the same animal. Until we know what causes the observed variation in these results, it is difficult to make accurate generalizations about the habits and care of any particular worm.
As far as I can tell from the research done on these worms to date, however, the reason that they are found almost entirely in association with live corals has nothing to do with nutrition. In fact, the live coral seems to provide protection to the worms rather than nutrition. Researchers have examined the boring invertebrate communities in corals that are live, and compared them to those communities in which 50%, or 100% of the colony was dead, and they found big differences. In living corals, only 3 species of tunneling invertebrates were commonly found (a bivalve mollusc, a vermetid tube snail and the Xmas Tree Worms). The number of species boring through the colony and the amount of the coral’s carbonate skeleton lost to that activity increases quickly with the proportion of the coral colony that is dead. Completely dead colonies were rapidly colonized by 17-18 species of boring invertebrates (the most significant and destructive of which are the sipunculans, or peanut worms) which remove an average of 14.2 kg of carbonate skeleton per cubic meter of reef. In addition, parrotfishes and grazing echinoderms (including urchins, brittle & serpent stars) removed an additional 5.25 kg per m3 and together these boring activities are pretty likely to ensure that the worms don't survive for long once the coral dies. These boring invertebrates are responsible for the breakdown of dead coral skeletons into coral sand and play an important role in natural reef communities. However, they make life very difficult for the Christmas Tree Worms, because it is unlikely that the boring invertebrates would differentiate between the calcareous tube of Spirobranchus and that of the dead coral. Removal of portions of the worm tube exposes the body of the worm to attack from predators, pathogens and other additional stresses not experienced when the worms inhabit a healthy live coral. It is therefore not surprising that the worms aren't found in many dead coral skeletons, but it doesn't mean that the worms are somehow eating coral mucus.
Unlike many worms that only live for relatively short times (a few years), Spirobranchus are a relatively slow growing worm that lives for a long time in the wild. Researchers used X-rays to determine the age of the worm by counting the annual growth rings of the corals in which they were embedded, and found that most animals were over 10 years old and some were more than 40 years of age! On average, a healthy worm grows about 0.2mm in the diameter of the tube opening each year, but under ideal conditions the maximum growth rates are up to 1.0 mm increase in orifice diameter per year. In any case, these worms should easily live 10 years or more if provided with proper care, and as comforting as it may be, we cannot delude ourselves into thinking that the animals died of old age if they fade in our aquariums.
As I mentioned above, there is no indication that these worms feed on anything different than the majority of other tube-dwelling serpulid polychaetes do (a mixture of primarily phytoplankton with some small zooplankters thrown in as the worm grows to maturity), but I am not familiar with any specific studies of prey preference or gut content analyses of Spirobranchus in the wild (so anything is possible, and a new study may warrant a new article). The fact that Spirobranchus are generally imported with pieces of Porites does not mean that they are obligate associates of this coral, nor does it mean that they are feeding off the mucus of the coral. In fact, depending on the source you check, the most likely associate for Spirobranchus is likely to be P. lutea, P. lobata, P.lichen or even Montipora informis. In any case, until I see a scientific study demonstrating that Spirobranchus is physically dependant on the living coral for anything other than protection (as outlined above), the assumption that this worm feeds in a substantially different manner than the other serpulids inhabiting live corals seems utterly unfounded to me, and I will continue to recommend they be fed and cared for as other feather-duster worms (which I have covered in previous columns).