Meet the bizarre underwater worm that looks like a Christmas decoration and sees through its gills.
A game I like to play while scuba diving or snorkelling on a tropical coral reef is to slowly glide up to a boulder of coral and peer as closely as I can at clusters of Christmas tree worms without scaring them.
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 9–12 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.
Once they’ve picked a home, a young Christmas tree worm burrows into the coral and constructs the tube it’ll spend its life in, lining it with calcium carbonate. They can live for up to 30 years.
The worms’ hypersensitivity, seen in their disappearing trick, comes down to an unusual feature. Nestled among their radioles are hundreds of bright orange eye spots. No other animals can see with their gills, but for these worms it makes sense to be able to scan the water while their body is burrowed within its coral host. The eye spots contain light-sensitive opsin pigments, which send signals to the worms brains to warn them of shadows overhead that could be a predatory fish or crab.
Rather counterintuitively, worms in crowded neighbourhoods are more skittish than those living alone or in small communities. The more worms on a coral colony, the longer they spend hidden in their tubes. There isn’t safety in numbers for Christmas tree worms, perhaps because they’re just too eye-catching when lots grow together to form underwater Christmas forests.
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