Bright and Bristly: Christmas Tree Worms and Their Festive Whorls

Bristly creatures slowly extended out of the coral, gracefully unfurling their brilliantly coloured, spiralling plumes. Each was like a Christmas tree lazily unfolding its colourful leaves, gently waving in the currents. A pair here, three pairs there, an entire bunch right across—and suddenly, colourful, feathery little trees were sticking out in pairs through small holes scattered all over the coral’s surface.

I breathed out “wow!” through my regulator, expelling air bubbles, and they all shot back into their little holes, sealing the openings with small, round flaps. These creatures seemed very sensitive to the slightest motion, something that their survival probably depended on because they seemed anchored to the coral. But, what were they?

What’s in a name?

These bright little Christmas trees—called so because they resemble the festive fir trees—are actually worms. Christmas tree worms are marine worms. They belong to a (mainly marine) subset of a larger group of segmented worms (called annelids), making them very distant cousins of the earthworm. Christmas tree worms and others of their class are referred to as polychaete worms (pronounced poly-keet), which means “many bristles”. Their class name highlights the defining characteristic that’s earned them the common name, bristle worms.

Classified under the genus Spirobranchus, these worms are found on some reef-building (hermatypic) corals in shallow tropical coral reefs worldwide, living in calcareous (made up of calcium carbonate) tubes. Less commonly, they’re also found on rubble, some artificial substrate, and clams.

The Christmas tree worm’s adventures start when an individual larva settles on the coral and starts secreting a calcareous tube around itself. It is a slow process, and over the years, the coral tissue around it grows too, often resulting in the tube being firmly embedded in the coral or just barely jutting out.

Bound entirely to a sessile lifestyle (fixed in one place) in the protection of this anchored tube, these worms can’t swim to escape predation, nor can they hunt, but like all fascinating marine creatures, they too have evolved mechanisms that just make sense. 

They are filter-feeders, using the tiny, fine hair-like bristles (called cilia) on each radiolar crown (or tentacle) to filter tiny food particles, which are then passed down to the mouth. They protect themselves by retreating deep into their tubes which they seal with a hard flap, the calcareous operculum, which is spiky in most Spirobranchus species, affording them another layer of protection against predators. Christmas tree worms are primarily preyed upon by sea urchins, fish, crabs, and other crustaceans. While they can swiftly shoot back into the safety of their burrows, they sometimes aren’t quick enough to avoid a random bite, and in such cases, one might see these little fir trees looking slightly ruffled, with a few branches missing. Interestingly, if only the crown is damaged, in most cases the worm can regrow it.

Spirobranchus spp. have eyes on their head, but they’re not of much use. To make up for that, the radioles on each crown have many tiny eyespots called ocelli or radiolar eyes, which are capable of detecting changes in light and some motion, and the worms sometimes retreat if even a shadow passes overhead. 

Spirobranchus spp. are not bristly and bright from tip to end as one might expect. The twin crowns that poke out of the burrow are the only parts that look festive—the rest of the body resembles just another worm. Each worm has two crowns, which are specialised mouthparts, with the feathery projections (radioles) playing the dual role of feeding and respiration. Most of the worms in this genus don’t grow beyond several centimetres, living in a tube usually much longer than the worm itself, and the crown accounts for just a fraction of the body’s length.

Though extremely popular among underwater photographers and recreational divers, these worms have not been studied extensively. The shy worms retreat into their tubes at the slightest disturbance, making observation difficult. Furthermore, getting any worm specimens to study would require breaking the coral around the tubes, damaging several years or even decades of coral growth. Several species have been classified based on the small differences in their appearance and the structure of the visible parts, namely the crowns and operculum. However, individuals across species look very similar, and the differences might not be very easy to spot, limiting the accuracy of visual identification. 

Christmas tree worm populations have not been thoroughly assessed yet, and they are not considered to be at risk. However, factors affecting their primary substrates—live coral— including ocean acidification, climate change, pollution, and some other natural and anthropogenic events could affect these beautiful worms.

A mutual relationship with corals

Previously misidentified as harmful borers inhabiting coral hosts, some studies show there might be a symbiotic relationship between Christmas tree worms and corals. In this exchange, corals provide a substrate for the worms to live on, while the worms provide some form of protection to at least parts of the coral. On some reefs, Christmas tree worms have been observed attempting to protect their coral hosts from the predatory crown of thorns sea star (Acanthaster planci) by pushing the spiky operculum and tips of their crowns against the animal’s sensitive tube feet. Other studies suggest that the feeding motion of the bristles might increase water circulation closer to the coral’s surface, bringing in more nutrients beneficial for the coral. However, there have also been some reports of the operculum scratching the coral tissue and exposing it to potential diseases, indicating that the association is not entirely without costs.

Reef health indicators

While the exact nature and extent of the association between Christmas tree worms and corals is unclear yet, some studies have looked at the potential role these worms could play in indicating coral reef health. Usually observed in relatively unpolluted and clearer, less turbid waters, and primarily on live coral, it is believed that Christmas tree worms could be tentative biological indicators of water conditions. Some Spirobranchus spp. individuals have been documented to live as long as 40 years in unpolluted waters, averaging approximately 20 years otherwise—this is another point that has led to speculation about them being indicators of water quality.

I must have spent several minutes just staring at these unique creatures, trying to fathom whether they were plants or animals, if the bristles would sting, or if their bright colours advertised some level of toxicity to predators. With time, I would get some answers through scientific literature, but also more questions. But for that moment, I was content clicking a few quick photographs of colourful Christmas trees on little coral hills in a world where the festive trees were not seasonal but displayed their vibrance all year round.