Porites and "Christmas Tree Worms"
Rob Toonen
Posted to Reefkeepers emailing list, Sunday the 12th of December 1999.
I'm not sure where the idea that Spirobranchus worms feed on the mucus from Porites first originated, but I see it quite often. As far as I know there is absolutely no research that supports that claim (although if someone knows of some references to the contrary, I would appreciate knowing about them!). The worms are feeders on 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 that they do *worst* in (I'll explain this more below). It's sort of a complicated system, and no one has really figured out exactly what is going on yet, other than the fact that Spirobranchus "giganteus" is an entire species complex rather than a single species, and several "morphs" of the species have turned out to be completely infertile when crossed.
The reason that the worms are found on specific corals seems to be a result of larvae showing a strong settlement preference for corals such as Porites asteroides and Millepora complanata and ignoring corals such as Siderastrea, Dendrogyra and Agaricia. However, when researchers 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, M. cavernosa, and Porites porites came in dead last in every measure of growth for the worms! Just to make things more complicated, different researchers find different results in different regions (such as larval preferences for specific corals), suggesting that there is a lot of variation among the worms throughout their range, or that there are a lot of similar but unidentified species that are mistakenly being tested as the same animal. Until these issues are properly addressed, it is difficult to make accurate generalizations.
As far as I can tell from the research done on these worms, the primary reason that they are found almost entirely in association with live corals has nothing to do with nutrition. In fact, researchers have examined the boring invertebrate communities in corals that are live, to those in which 50%, or 100% of the colony was dead, and they found big differences. In living corals, only 3 species were commonly found (a bivalve, a vermetid snail and the Xmas tree worms). The number of species boring through the colony and the amount of carbonate skeleton lost to that activity increases with the proportion of the colony that is dead. Completely dead colonies were rapidly colonized by 17-18 species of boring invertebrates (the most significant of which are the sipunculans, or peanut worms) which remove an average of 14.2 kg of carbonate skeleton per m^3. In addition, parrotfishes and grazing echinoderms (including urchins, brittle & serpent stars) removed an additional 5.25 kg per m^3 and together these boring activities are pretty likely to ensure that the worms don't survive for long once the coral dies. It also seems likely that the boring invertebrates would not differentiate between the calcareous tube of Spirobranchus and that of the dead coral, exposing the worm to attack from predators, pathogens and other additional stresses not experienced when the worms inhabit a healthy live coral. Hence, it is not surprising that the worms aren't found in many dead coral skeletons, but it doesn't mean that the worms are eating coral snot
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! 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 orfice diameter per year. In any case, these worms should easily outlive the average reef tank, and you cannot delude yourself into thinking that the animals died of old age if they fade in your aquarium.
As I mentioned above, there is no indication that these worms feed on anything different than other tube-dwelling serpulid polychaetes (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 (as I said above if anyone knows of any such studies that I have missed, please let me know!). 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 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...
