EVERYBODY RAVES ABOUT THE BEAUTY OF CHRISTMAS TREE WORMS (Spirobranchus giganteus), with their fantastic arrays of bright colors and shapes like perfect fir trees.
The wormy bodies behind the gorgeous finery, maybe not so much.
The spiraling crowns we see are specialized tentacles, called radioles, that filter plankton from the surrounding waters for food, passing it down to the worm’s mouth in cilia-lined grooves. They also work like “gills” to let the animals absorb oxygen.
TRADITIONAL WORM BODIES
But behind the plumes, they have regular worm bodies, just like the earthworms you find in your garden. They’re members of Phylum Annelida, Class Polychaeta, Family Serpulidae, worms with bodies of ring-like segments (This means you’ll often see them referred to as annelids and polychaetes). Each segment features parapodia – appendages with setae, or bristles, extending from them (Bristleworms, or fireworms, are free-ranging polychaetes).
DIFFERENT BASINS, SIBLING SPECIES
In the Atlantic/Caribbean, Spirobranchus giganteus constitutes the basic Christmas tree species. In the Indo/Pacific, it was believed that several different species existed, but Australian scientists who sequenced the DNA of a broad range of specimens unexpectedly determined that they they’re a single morphologically various species, Spirobranchus cornicultus, located throughout the Indo-Pacific basin.
A significant difference from your familiar garden polychaete (and from most other marine polychaetes) is that these guys are sessile; they embed themselves into the coral substrate and never leave, using special glands to convert sand into calcium carbonate and create strong tubes in which to reside within the coral (This means they’re also often referred to as calcareous tube worms. It’s hard to keep up).
CHRISTMAS TREE WORMS CAN’T RUN – BUT THEY CAN HIDE
The worms actually have fairly complex eyes – on their tubes, which don’t do them much good. But their radioles also sport cells that let them distinguish changes in light – like the shadow of a predator looking to munch on them.
Pretty much the only advantage to being worms living in tubes is the ability to retreat down into those tubes when danger threatens. Divers see this a lot as they approach.
CHRISTMAS TREE-SHAPED PLUMES COME IN PAIRS
All Christmas tree worms have two separate fir-shaped crowns, separated by a trumpet-shaped structure called an operculum. When they have to retreat back into their tube, they draw the operculum in to plug the entrance.
No sources suggest any purpose served by the range or intensity of crown colors.
THE CORAL COMES TO THEM
A lot of sources talk about their “boring” into the coral once they settle. It appears that this is incorrect. Rather, once a larva settles on a coral host, it remains in place and lets the coral grow around it over time.
If this sounds like a lengthy process, biologists have counted the coral layers around them, like the rings of a tree, to estimate that Christmas tree worms can live as long as 30 years (sources vary from 20 to 40 years; a more typical lifespan is likely to be 10 to 20 years). And they’re only 1.5 inches long – a few years’ growth for many corals.
Many sources note that the calcareous tube is usually significantly longer than the worm – as long as 10 inches – giving it room to scoot down in. This suggests that the worm is older and the coral has developed around it over the years. The worms line it with mucus to facilitate sliding up and down.
SETTLING ON DEAD CORAL WOULDN’T BE A PLAN FOR SUCCESS
This would explain why Christmas trees only settle on living coral. Christmas trees are apparently picky about what kinds of corals they settle on. Biologists know that but they’re still trying to figure out what the criteria are. One suggestion is that they try to avoid corals frequented by algae-eating parrotfishes, which tend to scrape away the corals and anything else in their way while ferreting out algae.
PRINCIPAL SOURCES: Marine Biology, Fourth Edition, Peter Castro, Michael Huber; Reef Creature Identification, Florida, Caribbean, Bahamas/ Reef Creature Identification, Tropical Pacific, Paul Humann, Ned DeLoach; NOAA, What are Christmas tree worms?; marinebio.org, Christmas Tree Worms; Encyclopedia of Life, Spirobranchus giganteus; Australia Museum, Exploring The Diversity Of Christmas Tree Worms In Indo-Pacific Coral Reefs; Science News, Christmas tree worms have eyes that breathe, gills that see.