Christmas Tree Worms (Spirobranchus giganteus)

Christmas tree worms are an unusual species distributed widely throughout the worlds tropical oceans, although the largest natural distribution is across the equatorial belt. The Christmas tree worms are named for their Radioles which are the Christmas tree shaped fans you see above the coral.

Christmas tree worms feed by using their radioles. Radioles are hair-like appendages that circle outward from the central spine to catch phytoplankton and some small zooplankton once the worm gets older which float in the water column. Once the worm catches the food, it is then passed down the food groove by tiny hair-like extensions on the surface, that generate water currents to move food or mucus. The food particles are sorted, but larger particles are discarded. Any sand grains the worm collects are directed to storage sacs to be used later for tube building. The radioles also act as a form of gill to allow the worm to gather oxygen from the water.

The radioles you see protruding from the coral are in fact only a small part of the worm; the main body is safely encased in a tube surrounded by the coral. When the worm catches food, as stated above it also catches sand which it uses to produce the tube in which it lives. The total length of the worm can be anything from 2-4cm and never moves from this tube as they don’t have any specialised body parts for movement.

The radioles are extremely sensitive and passing divers or snorkelers may see them retract back into the tube for protection. Around the island of koh tao many of the dive sites have high concentrations of Christmas tree worms giving them colour and fantastic photography opportunities.

Andy Cartledge

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