Christmas Tree Worm

Spirobranchus giganteus, commonly known as the Christmas tree worm, is a tube-building polychaete worm belonging to the family Serpulidae.

The anatomy and morphology of Spirobranchus giganteus are characterized by its two chromatically hued spiral structures, which are the most common feature observed by divers. These multicolored spirals are highly evolved structures used for feeding and respiration.

Similar to other tube-building polychaetes, Spirobranchus giganteus has a segmented, tubular body that is approximately 3.8 cm (1.5 in) in length. The body is covered with chaetae, small appendages that aid in the worm's mobility. Due to its sedentary nature within its tube, the worm lacks specialized appendages for movement or swimming.

The most distinctive feature of these worms is their two "crowns" shaped like Christmas trees. These crowns are specialized mouth appendages known as prostomial palps. Each crown is composed of feather-like tentacles called radioles, which are ciliated and trap prey for transportation to the worm's mouth. These radioles also serve as respiratory structures, often referred to as "gills".

One notable difference between Christmas tree worms and other Sabellida fan worms is that the latter lack specialized structures to plug their tube holes when they retract into them. Spirobranchus giganteus possesses a modified radiole, known as the operculum, which is used to secure its hole when withdrawn into its tube.

As an annelid, Spirobranchus giganteus has a complete digestive system and a well-developed closed circulatory system. They also possess a well-developed nervous system, including a central brain and various supporting ganglia. Their mitochondrial sequence evolution is faster than other annelids due to their nucleotide composition and divergence of protein sequences. Like other polychaetes, S. giganteus excretes waste through fully developed nephridia. Reproduction involves the shedding of gametes directly into the water, where they become part of the zooplankton and are carried by currents.

Christmas tree worms have a wide distribution across tropical oceans worldwide, occurring from the Caribbean to the Indo-Pacific.

In terms of ecological significance, Spirobranchus giganteus is commonly found embedded within heads of massive corals, such as stony corals Porites and brain corals. They secrete a calcareous tube around their bodies, which serves as their home and protection. These worms settle onto existing living coral heads before secreting their tubes, enhancing their level of protection as the coral tissue overgrows the calcareous tube. The brightly colored radioles are used for filter feeding, where microorganisms are filtered from the water and deposited into the worm's digestive tract.

Few organisms prey on tube-borne polychaetes, and Spirobranchus giganteus is no exception. The symbiotic relationship between S. giganteus and its host corals is still not well understood. However, the movement of the operculum can sometimes damage the coral tissue, and mortality of coral tissue is increased when the worm's operculum hosts filamentous algae.

While the Christmas tree worm itself holds no commercial fishery importance, it is highly regarded by marine aquarists and divers. The eye-catching crowns of these worms make them popular subjects for underwater photography among sport divers. Some aquarists intentionally include coral heads inhabited by S. giganteus specimens in their miniature reef aquaria.