Anyone who has seen the film Avatar will recognise these worms! They were the models behind some of the wierd and wonderful plants in the forest.
All the amazing coloured species here filter their food from the water with a complex network of cilia on their finely branched "Christmas Trees" or "Feather Dusters". Their fossils are the earliest known from the Jurassic.
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The tremendously colourful "christmas trees" of these worms are both complex feeding and breathing organs. The body of the worm itself is protected by the massive skeleton of the coral that it grew up in.
What sets this genus apart from the other fan worms is that they have a "chalk" cap to their tube. When danger treatens, the worms instantly retract and seal the tube with the cap, which is has a forked sharp spine.
Taking their name from the two "horns" of plankton-filtering hairs, they build their tubes in the same way as Feather-Duster Worms. They are even more sensitive to changes in light intensity, usually disappearing long before you can get close to them.
The bodies of these worms are hidden inside tubes built from mud or sandgrains held together with mucous, or as in Pomatostegus, a chalk-like tube in a massive coral colony (though without a protective cap as in the Christmas-tree Worms). They are all colourful, but within a species, not as variable as the Christmas-tree Worms. Very sensitive to changes in light intensity, they retract instantly to light changes.
They are all filter-feeders, capturing plankton from the current-swept habitats they prefer. Their "feather-dusters" also serve as gills.
These Feather-Duster Worms are individually rather small, but cement their calcareous tubes together to form branching colonies that can reach a substancial size. They are filter feeders like the other Feather-Duster Worms.
Relatives of these genera live on volcanic deep-sea vents.
Horseshoe Worms look vers similar to Feather-Duster Worms, but are in fact only remotely related to worms. Their closest relatives are brachiopods! The body has a "bulb" at the base of the tube. This is where the stomach is and is the centre of the circulatory system. Their blood contains haemoglobin, as in mammals, though there are no red blood cells. Haemoglobins' ability to stongly bind oxygen enables Horseshoe Worms to inhabit areas with poor water circulation and low dissolved oxygen levels.
The "fan" is a single structure called a lophophore. Each side is curled into a whorl giving the appearance of two "feather dusters". Blood is pumped through these to pick up oxygen. Minute beating hairs propel any captured food into the stomach. To excrete, the hairs beat in the other direction to bring waste products to the edge of the lophophore.