Christmas Tree Worm

• GeneralIts two coloured spirals are gills, which it whips back in at the first shadow.
• The two spirals are gills and feeding filter at once, and they are covered in cilia that carry particles down to the mouth.
• It settles as a larva on living coral and forces it to grow around the tube, so that worm and coral end up forming a single structure for decades.
• Its two spiral crowns, the only part of the animal that shows, are not gills but modified mouthparts: each branch is a radiole covered in cilia that drive food particles down to the mouth.
• They also serve for breathing, which is why they are colloquially called gills.
• One of the radioles is modified into an operculum, a lid with which it seals the tube as it withdraws. It is extremely sensitive: it snaps back at a touch, a shadow or a movement.
• It lives its whole life in the same tube, which it secretes from calcium carbonate inside the coral skeleton. The larva settles on living coral and the coral tissue ends up growing around the tube, giving it extra protection.
• It can live more than thirty years without moving from there.
• The relationship with the coral is not entirely clear: movement of the operculum can wear away coral tissue, especially if filamentous algae grow on it.

Habitat & environment

Massive reef corals

Sources consulted: https://es.wikipedia.org/wiki/Spirobranchus_giganteus