Christmas tree worms growing on coral

Spirobranchus giganteus, commonly known as Christmas tree worms, are small, tube-building polychaete worms belonging to the familySerpulidae.

The worm is aptly named; Both its common and Latin names refer to the two, chromatically hued spiral structures that are most commonly what is seen of the worm by divers. In actuality, these multicolored spirals are merely the worm's highly derived respiratory structures.

S. giganteus is similar to most tube-building polychaetes. It has a tubular, segmented body lined with chaeta, small appendages that aids the worm with its mobility. As it does not move outside its tube, this worm does not have any specialized appendages for movement or swimming.

The worms's most distinct features are the two "crowns" that are shaped like Christmas-trees. These "crowns" are actually highly modified prostomial palps which are specialized mouth appendages of the worm. Each spiral is composed of feather-like tentacles called radioles, which are heavily ciliated which allows any prey that are trapped in them to be transported straight towards the worm's mouth. While they are primarily feeding structures, S. giganteusalso uses its radioles for respiration. It is because of this that the structures are commonly called "gills".

One of the major differences between Christmas-tree worms and the closely related sabellida fan worms is that the latter do not have any specialized body structures to plug the holes of their tubes with when they withdraw into them. S. giganteus, like the 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members of its family possess a modified radiole, usually called the operculum, that it uses to secure its hole when withdrawn into its tube.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. It 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, which are 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) will become part of the zooplankton and can be carried by the currents.

Christmas Tree Worm (Spirobranchus giganteus) in Porites coral

Christmas tree worms (Spirobranchus giganteus) from East Timor.