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)
