Christmas tree worms bring a touch of holiday cheer to the world’s tropical seas throughout the year. These tiny creatures resemble festively decorated fir trees, but they are actually a type of worm called Spirobranchus. However, their beauty only scratches the surface as two-thirds of their body is concealed inside a coral reef’s calcium carbonate tube. So while we only decorate our trees once a year, the underwater Christmas tree worms add a little sparkle to the ocean all year long.
The Christmas tree worm constructs its own miniature dwelling using its own secretions and can reside in it for as long as four decades. It adjusts the growth of its burrow to correspond with the growth of the surrounding coral, ensuring that the opening that permits its brightly-coloured tentacles to protrude from the reef is maintained, providing the only visible indication of its presence. The worm’s two tentacles are called radioles, which usually extend around 2.5 cm and come in various shades, including blue, orange, yellow, white, red, and pink. These radioles not only act as external gills but are also covered in cilia-like hairs that enable the filter-feeding worm to capture plankton and transport it down to its mouth for consumption.
Christmas tree worms are a fascinating species with a wide range of colors. However, they all belong to the same species, called Spirobranchus giganteus. These unique creatures can be found in tropical and subtropical oceans all over the world, including the Caribbean and the Indo-Pacific. They tend to thrive in shallow waters and can often be spotted by divers and snorkelers at depths ranging from 10 to 100 feet (3-30 meters). Because of their striking appearance and preferred habitats, Christmas tree worms are a popular attraction for underwater enthusiasts.
The Christmas tree worms are popular for their ability to sense movement in the water and withdraw quickly into their tubes. They possess a unique body structure called an operculum that serves as a seal which they can open and close like a door. After a minute or so, the worms gradually come out of their tubes, making sure it’s safe before fully extending their crowns.
Despite being distinctively male or female, Christmas tree worms, like most polychaetes, do not mate; they practice external fertilization instead. Both genders release their sperm and eggs into the open water without leaving their burrowed tubes, where fertilization occurs. The fertilized eggs develop into larvae within a few hours, and these little creatures settle on coral reefs to build their own tubes. Interestingly, the baby worms have a natural inclination towards specific coral species or types, generally less aggressive ones, as the ideal location for their future homes. This symbiotic relationship benefits both organisms, as researchers have discovered that Christmas tree worms can save corals from bleaching, algae suffocation, and animal predation such as crown-of-thorns starfish.
Overall, Spirobranchus giganteus appears to be thriving as a species. Their numbers remain consistent and there are no significant dangers aside from the occasional local contamination and coral collectors capturing them from their natural habitat.