Christmas Tree Worm, Spirobranchus giganteus

The Christmas Tree Worm (Spirobranchus giganteus) is a species of small, tube-building polychaete worm in the Serpulidae family. It is widely distributed throughout the world’s tropical oceans, occurring abundantly from the Caribbean to the Indo-Pacific.

The worm’s common and scientific nomenclature refers to the two chromatically hued spiral structures, most prominently seen by divers. These multicolored spiral structures are actually part of the worm’s highly derived respiratory system. Each spiral is composed of feather-like tentacles called radioles, which are heavily ciliated and allow prey that are trapped by them to be delivered to the worm’s mouth, meaning the respiratory structures also act as a feeding system.

The Christmas Tree worm has a tubular, segmented body lined with small appandages that aid in mobility. Because the worm does not move outside its tube, it does not have any specialized appendages for movement or swimming.

As an annelid, this worm possesses a complete digestive system and has a well-developed closed circulatory system. It also has a well-developed nervous system with a central brain and many supporting ganglia (exterior nerve tissues) unique to Polychaeta. Reproduction occurs asexually and the worm simply sheds its gametes into the water where the eggs and spermatozoa become part of the zooplankton to be carried by the currents.

This species is commonly found embedded in the heads of massive corals. It can secrete a calcareous tube around its body which serves as home and protection from predators. The worm generally bores into the head of living coral before secreting the tube, thereby increasing its level of protection.

This worm is a filter feeder, primarily feeding on microorganisms it filters from the water, which are then deposited straight into the worm’s digestive tract.

The worm has little to no commercial fishery importance, yet it is of interest to marine aquarists and divers everywhere. These multicolored specimens make extremely popular underwater photographic subjects. Many aquarists that have miniature reef aquariums purposely include heads of coral that S. giganteus specimens inhabit.

Image Caption: Spirobranchus giganteus (Red and white Christmas tree worm). Credit: Nick Hobgood/Wikipedia(CC BY-SA 3.0)