An Undersea Christmas Tree

By Annabel Thomas

For The Bali Times

SEMINYAK ~ This being the last edition of The Bali Times for the year, and the one that comes out at Christmas, I thought I’d stick with the theme.

Both the Latin name (Spirobranchus giganteus) and common name (Christmas Tree Worms) of these tube-building worms refer to the two spirals of plumes (radioles, feathery tentacles) that you can see in this photo. These cones can be a variety of colors – red, orange, blue/purple, white, yellow, pink, grey, brown – and although they are quite small (approx 1.5inches/4cm in width), they make a pretty and appealing addition to the reef. While these are the only parts of the worm seen by divers/snorkellers, the segmented body of the worm is actually out of sight inside its tube within the coral.

The radioles are highly developed and have two purposes:

– feeding: Christmas Tree Worms are filter-feeders – the minute hairs on the radioles are coated with sticky mucus that catches phytoplankton and suspended particles that float by in the water. These are funnelled downwards and sorted into food (into the mouth) and grains of sand (which are then stored to be used for tube building at a later date).

– respiration (like feathery gills) for the exchange of oxygen and carbon dioxide.

As an annelid, Christmas Tree Worms have a complete digestive system along with a well-developed closed circulatory system and nervous system with a central brain.

There are male and female Christmas Tree Worms which simultaneously cast eggs and sperm into the water, where the eggs are instantly fertilized. The larvae then develop and in turn settle down onto calcareous (high proportion of calcium carbonate) coral heads.

Here they either bore into the coral or build a tube on the surface. As hard corals only grow approx 5mm per year, the worm only has a little work to do to increase the tube length as the coral grows to become encased in the coral’s skeleton and therefore fully protected. This is a form of symbiosis (interrelationship between two organisms) known as commensalism: where one organism derives benefit – in this case, the Christmas Tree Worm gets protection – and the other (the coral host) is neither hurt nor harmed.

Christmas Tree Worms are found on coral reefs throughout the tropics – have a look for yourself – and although they are very sensitive and will disappear back inside their protective tubes whenever a shadow passes overhead or there’s an unexpected change in nearby water movement, they soon reappear; so just stay still and watch them reemerge.

So, finally, a Happy Festive Season to all readers of The Bali Times … have you thought about YOUR New Year’s Resolution yet? To participate in an environmental project? Improve and educate yourself, your family and staff about recycling opportunities here in Bali (contact for weekly pickups) or go snorkelling, take up scuba-diving and see Christmas Tree Worms for yourself?

The writer is director of AquaMarine Diving – Bali.

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