THE WORLD'S WEIRDEST CREATURES

Under the sea and upon the land, some animals look - to us - pretty strange...

- DR HELEN SCALES, DR HELEN PILCHER

Christmas tree worm

Scuba divers or snorkellers on a tropical coral reef can slowly glide up to a boulder of coral and peer closely at clusters of Christmas tree worms. These tiny creatures look remarkably like tiny fir trees, although the artificial kind made of brightly coloured plastic and tinsel. They can grow in rainbow mixtures of reds, yellows, oranges and blues, and all of them are the same single species, Spirobranchus giganteus.

Get too close and, in the blink of an eye, the reclusive worms disappear into their tubes, which are burrowed into the coral, and slam shut little lids, operculums, behind them. Then it's a wait of a minute or longer before the worms decide it's safe to come back out.

Christmas tree worms can reach lengths of around 3.5cm and most of their bodies remain hidden in their tubes. The festive parts sticking out are pairs of feathery spiral-shaped tentacles known as radioles, which they use for breathing and feeding. As well as acting as gills absorbing oxygen from seawater, the radioles filter suspended food particles and plankton, passing them towards the worms mouths with microscopic hairs, like a conveyor belt. Close relatives of Christmas tree worms, members of the same Sabellidae family, include the feather duster and peacock worms, which stick up in tubes from the seabed with a mop of radioles poking out. All of these sedentary worms begin life as minute mobile larvae.

Female and male Christmas tree worms cast their eggs and sperm directly into the seawater, where they fuse and form larvae that drift for nine to 12 days before settling onto a suitable piece of coral. They seem to be quite choosy, only growing on particular species, such as brain corals, although it's not known exactly how they make their choice.