Despite their comical name and pretty appearance these worms are actually some of the most interesting animals you can find on coral reefs.
It’s that time of year again. You’ve picked out your perfect tree brought it home and plastered it in tinsel, baubles and lights. It’s standing in the corner of your living room with star atop and your carpet is littered with needles that you just can’t seem to hoover up. This time next month it’ll be long gone and you’ll be trying to keep to your New Year’s resolutions. But on coral reefs all around the world lives a much more permanent and naturally beautiful variation, the Christmas tree worm. Also known as Spirobranchus giganteus, the Christmas tree worm is a polychaete (segmented) worm that plays an important role in reef ecology. Its incredible colours, patterns and shape inspired marine biologists to name them after our beloved festive decorations. But there is so much more to them than first meets the eye and if they were given a different name you’d probably take them a lot more seriously.
More than just a pretty face
The beautiful plumes that have made these worms famous can come in a wide range of stunning colours and intricate patterns. But it’s not just for show. The feathery appendages that spiral out from the central spine are actually hair-like structures called radioles and they serve three vital functions for the worm’s survival. Firstly they are what the worms use to breathe with. The radioles help the worms extract oxygen from the water which the worms use for respiration. For this reason they are often mistakenly labelled as gills, but they are not. Although they serve a very similar function to gills in fish they are not actually the same thing. Secondly the radioles double up as eyes, well sort of. The radioles contain some exposed nerves that are light sensitive, but do not allow any form of traditional vision. It is only used to help tell when a predator may be swimming overhead which triggers a defence mechanism.
The third function of the radioles is to catch the worm’s food. Christmas tree worms are filter feeders, they use their radioles like a sieve to catch phytoplankton and other microscopic organic matter which makes up the worms diet. This sounds like a pretty basic process but once captured the tiny pieces of food are passed down a groove in the radioles by even smaller hairs known as a ciliate tract. These smaller hairs create tiny water currents that move food and mucus along the radioles and are even used to sort food from other particles. However nothing is wasted, the grains of sand that are often caught along with their food are stored in specialised compartments for later use.
At an average height of 3.8cm above the corals the Christmas tree worms are hardly the most imposing animals on the reef. But the beautiful structures we can see are only the crowns of the worms. Over two thirds of the worm are actually hidden in the corals they live on. Christmas tree worms are burrowers they dig themselves into the corals and form a calcium carbonate tube around them that acts as their bunker. The sand they can collect whilst feeding is also used to build these structures. Sometimes they don’t even have to do the work themselves at all and just lie on top of a coral and let them grow around them over time. The worms radioles are extremely sensitive to light and touch, when they are triggered by the proximity of a predator the worms are capable of retracting completely into their bunkers and remaining out if sight. They typically re-emerge about a minute later very slowly testing to make sure the coast is clear.
All good things come in pairs
As you may have already noticed each colourful crown is located directly next to one of the exact same colour. That is because they are actually the same worm connected underground by their calcium carbonate tubes. This buddy system not only creates interesting patterns for divers but could help the radioles perform their multiple functions. It is currently not proven but widely suspected that one crown will perform respiratory duties whilst the other feeds. It is also likely that these roles are interchangeable and not fixed to a single crown.
As well as the crowns coming in pairs the worms themselves also pair up for reproduction. Christmas tree worms are sexually dimorphic meaning they contain both males and females that are distinct from one another. You might think that would go without saying but most polychaete worms are actually hermaphroditic meaning they have both male and female reproductive organs and can swap gender roles. Therefore the fact they have fixed genders is actually quite rare. The Christmas tree worms reproduce in a very similar way to most fish species and spawn their gametes, also known as sex cells. The gametes then mix in the water column to form a new worm which is carried by currents to a new reef where it will make its new home.
Not just for Christmas
Unlike the Christmas trees that we put up in our houses for the holidays these worms remain present on reefs all year round. They also have remarkably long lifespans being able to survive up to 30 years, although 10-20 is more common, which is quite impressive for a worm. Filter feeding animals like these are also very important in marine ecosystems as they recycle nutrients back into the food web. There are around 13,000 species of polychaete worms but the Christmas tree worm is by far one of the most interesting. Although they are not an endangered species their life is tied to the reefs they live on which are under serious pressures due to global warming. These worms may be named after Christmas trees but they have been around long before we ever started using them and are far from just a festive novelty.