Christmas Tree Worm

Its larva actively selects and settles onto a living coral colony, then bores a tube into the coral's skeleton, and as the coral keeps growing outward over the following years it gradually grows around and encloses the worm's tube, leaving only the entrance exposed at the coral surface. This gives the worm a permanent, well-protected home embedded within one of the most solid structures on the reef, sheltering its soft body from predators and physical damage, while positioning its feeding crowns exactly where they need to be, out in the water column above the coral where plankton and food particles drift past.

The two spiral, fir-tree-shaped structures are highly modified feeding appendages called radioles, arranged in a tight helix, and they serve two functions at once. Their fine, feathery side branches are covered in mucus and tiny beating hairs called cilia that trap microscopic plankton and organic particles from the water and transport them down grooves to the worm's mouth at the base, and the same large, thin, well-exposed surface also acts as the worm's primary site for taking up oxygen from the water, effectively working as both its mouth and its gills.

The worm is highly sensitive to sudden changes in light levels and to vibrations or pressure changes in the surrounding water, both of which can signal an approaching predator such as a fish. When it detects a passing shadow or a disturbance nearby, specialised light-sensitive and touch-sensitive cells trigger an almost instantaneous muscular contraction that pulls both feeding crowns completely down into the protective tube in a fraction of a second, after which the worm seals the tube opening with a plug. It then waits, often for several minutes, before slowly and cautiously extending its crowns back out to resume feeding once the threat seems to have passed.

Yes, indirectly but significantly, because the worm depends entirely on living coral for its home. When a coral colony bleaches severely and dies, the Christmas tree worms embedded in it lose the living tissue and ongoing skeletal growth that maintains their tube and surrounding structure, and while a worm may survive for a while in dead coral, the long-term prospects for a population decline sharply as its host corals are lost. Widespread and repeated coral bleaching events driven by ocean warming therefore pose a real threat to Christmas tree worm populations across affected reefs, even though the worm itself is not directly harmed by the warmer water.