Its Beginning to look a lot like Christmas… Tree Worms
Whilst many of us frantically prepare for the onset on festive activities, one creature is ahead of the game – The Christmas Tree Worm. A sedentary polychaete of the family Serpulidae, known in the scientific community as Spirobranchus giganteus. Admired around the world for their beauty and festive appearance, these worms are an important bridge between polychaetes and people. Most of the worms we encounter in the marine environment can look hellish at the best of times, so if we wish to captivate and educate communities for the purposes of conservation, these genial little critters need to be at the top of our Christmas list.
At approximately 2.5cm long, Spirobranchus giganteus (Christmas Tree Worm) can be found globally across the equatorial tropics and they come in a wide variety of colours and sizes. From red, blue, white, orange and pink, these colourful polychaetes swathe the reefs they inhabit with an assortment of vibrant colour. But it is not their colour and rather their fanciful plumage that earns them their title as the Christmas Tree Worm.
Spirobranchus giganteus are tube dwelling polychaetes that use two spiralled gill structures for feeding and respiration. Their diets typically consists of the microscopic baubles of the marine world – plankton, as well as other suspended particulates. The ability to filter their food from the water column is owed to their radioles, feather-like appendages that act as miniature fishing nets, catching planktonic material as it drifts by. The radials pass caught food particles into a channel that leads toward the central spine. Food is moved down this channel by small hair-like protrusions called ciliary tracts. Arranged in rows the ciliary tracts beat in order to create a current, drawing food down the channel and towards the mouth. By extending these gill structures to feed above the coral the worms are able to filter large amounts of particulates from the water column and it is a highly effective feeding mechanism.
The rest of the worm is hidden in a burrow below the gills. Spirobranchus giganteus burrow into live calcareous coral to establish a firm footing on the reef, preventing them from being swept away by the very currents that bring them food. When a predator approaches, the extended gills help to detect the predator and allow the worm to retreat safely into its burrow. A passing shadow is enough to trigger a retreat by S. giganteus and re-emmergence is tentative. Slowly S. giganteus will emerge from it’s burrow, testing the water as it does so to ensure the threat has passed. Spirobranchus giganteus will remain in it’s burrow for the duration of it’s life, only leaving if forced by action or environmental pressures. The worms line their burrows with a tube, built using sand collected during feeding from the water column. Spirobranchus giganteus actively store sand caught during feeding in a specialised sac within their bodies especially for the purpose of tube building. When combined with mucus, the sand creates a versatile material that offers the worm and added layer of protection inside the burrow.
Spirobranchus giganteus are heterosexual organisms. Mating occurs in communities with females casting their eggs into the water column, followed by sperm from the males. The eggs are fertilised in the water column and become drifting planktonic larvae until they eventually are recruited onto a coral, where they will settle and burrow in order to build their tube.