Meet the Christmas tree worm

You’ll find no festive spirit in these defensive, unsociable creatures

Found far from the cold of the North Pole, Spirobranchus giganteus is a species of tropical polychaete (bristle) worm that lives in warm waters from the Indo-Pacific to the Caribbean and makes its home on living coral. Young worms burrow into the coral before secreting a hard tube, where they remain for the rest of their lives. As they can’t flee from danger, the worms must rely on quick reactions and a plug-shaped structure called an operculum to keep them from becoming meals for passing fish. 

Spirobranchus giganteus gets its nickname, the Christmas tree worm, from the pair of unusual cone-shaped crowns protruding from its head. These are the radiolar crowns – sets of delicate tentacles that supply the worm with everything it needs to stay alive. When a Christmas tree worm feels safe, it pushes the crowns out of the tube so they can waft in the open water, where they perform two vital tasks: collecting food and acting like gills to aid respiration. Microscopic hairs cover each radiole, trapping plankton and other particles of food that drift past and transporting them down to the worm’s mouth.

Being sessile creatures that remain in one spot forever, there’s no chance of a holiday romance for Christmas tree worms. Reproduction is a much more practical affair for this species; from their tubes, worms send sex cells out into the water to meet and combine with gametes (mature cells) from other individuals. Larvae resulting from the mass spawning floats through the ocean until they reach coral suitable for burrowing.

On the lookout 

Although Christmas tree worms might not appear to be doing anything as they sit in their tubes and filter food, they’re constantly watching for signs of danger. The eyes on their head aren’t much use as they can’t see out of the tube and into the surrounding water, but the radiolar crowns more than compensate with their 360-degree surveillance system.

The hundreds of eyespots dotted all along the radioles have evolved to respond to silhouettes in the water above. When another animal looms into view, signals from these light receptors trigger the Christmas tree worm’s efficient nervous system, prompting a rapid withdrawal of the crowns, so the worm is sealed inside its tube before a predator can try to take a bite.

Safety first

These soft-bodied animals are vulnerable, so the Christmas tree worm has developed multiple defences

Image credit: Future PLC/© Art Agency/Nick Sellers

This article was originally published in How It Works issue 132, written by Victoria Williams 

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