Spirobranchus giganteus, usually known as the Christmas tree worm, is a tube-building polychaete worm belonging to the species Serpulidae.
Both its popular and Latin names point to the two chromatically hued spiral structures, the most characteristic feature seen by divers. The multicoloured spirals are deeply derived structures for feeding and respiration.
The Christmas Tree Worm (Spirobranchus giganteus) is similar to most tube-building polychaetes. It has a tubular, segmented shape of an estimated length of 3.8 cm (1.5 in) covered with chaetae.
These small appendages aid the worm’s movement. As it does not travel outside its tube, the worm appears to not have any specific appendages for movement or swimming.
The worms’ most unmistakable features are two “crowns” shaped like Christmas trees. These are significantly modified prostomial palps, which are functional mouth appendages.
Each spiral is formed of feather-like tentacles called radioles, which are densely ciliated and cause any prey caught inside of them to be transported to the worm’s mouth.
While they are primarily feeding formations, S. giganteus also uses its radioles for respiration; therefore, the structures usually are called “gills”.
One significant difference between Christmas tree worms and the similarly related Sabellida fan worms is that the latter does not have any functional body structures to plug their tube holes when they retreat into them.
S. giganteus, like other parts of its family, owns a modified radiole, usually termed the operculum, that it utilises to secure its hole when retracted into its tube.
As an annelid, S. giganteus possesses a full digestive system and has a well-developed sealed circulatory system. Like other annelids, these worms include well-developed nervous systems with a primary brain and many aiding ganglia, including pedal ganglia, unique to the Polychaeta.
Like different polychaetes, S. giganteus discharges with fully developed nephridia. When they breed, they simply shed their gametes through the water where the eggs and spermatozoa become part of the zooplankton to be moved by the currents.
- 1 Christmas Tree Worm Habitat and Distribution
- 2 Christmas Tree Worm Ecology
- 3 Why is the Christmas Tree Worm Important to Humans
- 4 Christmas Tree Worm Conservation Status
- 5 These Colourful Sea Creatures Protect Coral
- 6 More In-Depth Discovery of the Christmas Tree Worm
- 7 Christmas Tree Worms and the Great Barrier Reef
- 8 Further reading
Christmas Tree Worm Habitat and Distribution
Christmas tree worms are widely scattered throughout the world’s tropical oceans. They have been recognised to occur from the Caribbean to the Indo-Pacific.
Christmas Tree Worm Ecology
S. giganteus is generally embedded completely in heads of extensive corals, such as stony corals Porites and brain corals. Like members of its species, it can secrete a calcareous tube encompassing its body.
This tube works as the worm’s home and shield. S. giganteus usually settles onto an existing body of living coral before discharging its tube, whereby enhancing its level of protection as coral tissue overgrows the calcareous tube.
When the worm withdraws into its tube, the opening is shut using an operculum (lid), which is further protected by sharp, antler-shaped thorns.
As sedentary or stationary occupants of coral reefs, Christmas tree worms feed principally by filter feeding. They use their brightly coloured radioles to filter microorganisms from the water, which are then dropped right into the worm’s digestive tract.
Few organisms are recognised to feed on tube-borne polychaetes, and S. giganteus is no exemption. The symbiotic connection between S. giganteus and its host corals is still inadequately understood.
Nevertheless, occasionally the movement of the operculum can scrape or wear away the coral tissue through abrasion. That mortality of the coral tissue is heightened when the worm’s operculum hosts filamentous algae.
Why is the Christmas Tree Worm Important to Humans
While the worm itself has no economic fishery importance, it is of interest to marine aquarists and divers worldwide.
The unpredictably coloured worm crowns make remarkably popular underwater photographic materials for sport divers. Many aquarists who have minute reef aquaria purposely add heads of coral that S. giganteus specimens inhabit.
Christmas Tree Worm Conservation Status
As the species is extensive and relatively common, no protection efforts focus on this species (or polychaetes in general).
This variety was thought to be solely found in coral heads. However, they have also recently been named an organism that lives on top of the giant clam variety Tridacna squamosa in the Gulf of Thailand.
The conservation status of the host varieties which it inhabits changes.
Opposite their name, colourful Christmas tree worms (Spirobranchus) decorate coral reefs areas all year round. Their benefit to the environment goes far beyond their good looks and colourful appearance.
According to Elena Kupriyanova, a senior research specialist at the Australian Museum who’s studied aquatic worms for more than 20 years, these colourful creatures are first and leading, protectors.
Much like the intricate tree-root systems of a vast forest, colonies of Christmas tree worms produce a calcium carbonate tube that digs into the corals they dwell on where they can live for up to 40 years, depending on their mass, the coral, and reef health.
These Colourful Sea Creatures Protect Coral
“They’ve been seen guarding their coral hosts against the crown of thorns starfish by pushing away the predator’s tubing feet, leaving living corallites intact around the worm’s tube orifice,” Elena told Australian Geographic.
“There were observations of fast restoration of living coral tissue near Christmas tree worm burrows following coral bleaching, predator feeding, and growth by turf algae. In these circumstances, recovery was aided by the proximity of the worm.”
Not only do these worms take their position as protector earnestly, but they’re also relatively modest about their looks.
“One needs a lot of patience and good buoyancy skills to photograph them underwater. They are very shy models and withdraw very quickly and come out again very reluctantly,” Elena says, adding that it makes it difficult for her to obtain specimens for her research.
“”Obtaining Christmas tree worms from the coral is not an easy task as it involves using a chisel and hammer.””
More In-Depth Discovery of the Christmas Tree Worm
According to Elena, the Christmas worm’s beguiling presence has created quite the academic absurdity.
“The major misconception is that they are well-known and well-studied, but nothing is farther away from the reality, there is this paradox – they are not studied well because of the assumption that such large and obvious animals should surely be already thoroughly studied,” Elena says, which has led to a crisis in taxonomy.
“For decades the iconic Christmas tree worms had been indiscriminately treated as a single tropical species Spirobranchus giganteus, the name one still sees in colourful guides on marine organisms.
Now we know this is not the case and this particular species is found only in the Caribbean. Many other species inhabit tropical coral reefs.”
Christmas Tree Worms and the Great Barrier Reef
In the year 2019, a group of divers reported a rare sighting of Christmas tree worms reproducing at Agincourt Reef, off Port Douglas on the Great Barrier Reef.
Shane Down, who succeeded in capturing footage of the event said it was the first time he’d ever witnessed Christmas tree worms spawning, despite many years located on the reef.
“I have seen coral spawning numerous times but to see this in the heart of the day on a dive was truly magical, very rare and special,” says Shane.
“It’s an amazing world under the water, and the corals never cease to astonish a long-time underwater advocate.”