Spirobranchus is the second largest genus of tube-building annelid fanworms in the family Serpulidae. Members of Spirobranchus are known as Christmas tree worms.
The worms form a calcareous tube which becomes incorporated into their host coral. The prominent features visible externally are a pair of conical spiral radiole bundles ("trees"), and an operculum that closes when they retract into their tube.
The crown of radioles serves for both food-gathering and respiration. "Christmas tree" refers to the tiered, tapered shape, whilst "Spirobranchus" translates to "spiral gill".
↑Iñiguez, Abril; Starger, Craig; Kupriyanova, Elena; Willette, Demian; Varman, Tristan; Drew, Joshua; Toha, Abdul; Maralit, Benedict; Carpenter, Kent; Barber, Paul (2013-01-01). "Phylogeography of the Christmas Tree worm Spirobranchus corniculatus (Annelida, Serpulidae) from the Coral Triangle, Australia, and Fiji". 2013 International Polychaete Conference: IPC2013: Program and Abstract Handbook: IPC International Polychaete Conference, Sydney 2013: 4-9 August 2013, Australian Museum, Sydney, New South Wales. Christmas Tree [Worms] is a common name given to a group of large and colourful serpulid polychaetes of the genus Spirobranchus that are obligate symbionts of hermatypic corals commonly found in tropical and subtropical regions world-wide.
↑Blainville, Henri Marie Ducrotay de. (1818). Mémoire sur la classe des Sétipodes, partie des Vers à sang rouge de M. Cuvier, et des Annélides de M. de Lamarck. Bulletin des Sciences, par la Société Philomatique de Paris. 1818: 78-85., available online at https://biodiversitylibrary.org/page/4146409 (p. 79).