The Christmas Tree Worm (Spirobranchus giganteus) belongs to the family Serpulidae, a large group of sedentary polychaete annelids that inhabit calcareous tubes they construct from crystalline calcium carbonate and a mucopolysaccharide matrix using calcium glands in their bodies. The genus Spirobranchus includes at least 20 species, most of them found in the subtropics and tropics and many of them living in close association with corals. Spirobranchus giganteus belongs to a complex of species formerly lumped together as subspecies of S. giganteus. Spirobranchus giganteus (sensu stricto) is widely distributed in the tropical Western Atlantic (but not in the Pacific, where specimens belong to the S. corniculatus complex). (Fiege and Ten Hove 1999; Ten Hove and Kupriyanova 2009 and references therein)
Spirobranchus giganteus appears in the intertidal to subtidal zones as a typical species of coral reef polychaete, living mostly buried in coral skeletons. It grows on coral surfaces covered by living tissues and its tube is always covered by coral skeleton. Thus, the orifice of the serpulid tube is always present on the surface of living coral. According to one study of S. giganteus growing on Porites corals in Okinawa, individuals may live for a decade or more (some for more than 40 years), based on counting the annual growth bands in the coral skeleton overlaying polychaete tubes. (Nishi and Nishihira 1996)
The worm is aptly named, both its common and Latin names refer to the two chromatically hued spiral structures, the most common feature seen by divers. The multicolored spirals are highly derived structures for feeding and respiration.
Spirobranchus giganteus is similar to most tube-building polychaetes. It has a tubular, segmented body lined with chaeta, small appendages that aid the worm's mobility. Because it does not move outside its tube, this worm does not have any specialized appendages for movement or swimming.
The worms' most distinct features are two "crowns" shaped like Christmas trees. These are highly modified prostomial palps, which are specialized mouth appendages. Each spiral is composed of feather-like tentacles called radioles, which are heavily ciliated and cause any prey trapped in them to be transported to the worm's mouth. While they are primarily feeding structures, S. giganteus also uses its radioles for respiration; hence, the structures commonly are called "gills."
One major difference between Christmas tree worms and the closely related sabellida fan worms is that the latter do not have any specialized body structures to plug their tube holes when they withdraw into them. S. giganteus, like other members of its family, possess a modified radiole, usually called the operculum, that it uses to secure its hole when withdrawn into its tube.
As an annelid, S. giganteus possesses a complete digestive system and has a well-developed closed circulatory system. Like other annelids, these worms possess well-developed nervous systems with a central brain and many supporting ganglia, including pedal ganglia, unique to the Polychaeta. Like other polychaetes, S. giganteus excrete with fully developed nephridia. When they reproduce, they simply shed their gametes straight into the water where the eggs (and spermatozoa) become part of the zooplankton to be carried by the currents.
Christmas tree worms are widely distributed throughout the world's tropical oceans. They have been known to occur from the Caribbean to the Indo-Pacific.
Spirobranchus giganteus is commonly found embedded in entire heads of massive corals, such as stony corals like Porites and brain corals. Like members of its family, it can secrete a calcareous tube around its body. This tube serves as the worm's home and protection. S. giganteus usually bores a hole into an existing head of living coral before secreting its tube, thereby increasing its level of protection.
As sedentary inhabitants of coral reefs, Christmas tree worms feed primarily by filter feeding. They use their brightly colored radioles to filter microorganisms from the water, which are then deposited straight into the worm's digestive tract.
Few organisms are known to feed on tube-borne polychaetes and S. giganteus is no exception.
While the worm itself has no commercial fishery importance, it is of interest to marine aquarists and divers. The variously colored worm crowns make extremely popular underwater photographic subjects for sport divers. Many aquarists that have miniature reef aquariums purposely include heads of coral that S. giganteus specimens inhabit.
As the species is widespread and relatively common, there are no conservation efforts focusing on this species (or Polychaetes in general).