Welcome to the Magical World of Christmas Tree Worms

glowing brilliantly colored purple and orange Christmas tree worms (Spirobranchus giganteus)

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There is such a thing as a Christmas tree worm, but it isn't the evergreen-eating pest its name might imply. It lives in tropical oceans, not tannenbaums, embedding itself in coral reefs and growing bizarre, colorful structures that sort of resemble a Whoville Christmas tree — hence the name.

Those Seussian plumes are how the worm breathes and eats. A Christmas tree worm makes its home inside a coral reef, burrowing down to create a tube where it might live for up to 40 years. Its "Christmas trees" extend out from the reef, often providing the only obvious sign of the worm's presence. Each worm has two trees, composed of feathery tentacles known as radioles that are part of its highly adapted respiratory system. In addition to serving as external gills, they're also covered with hairlike cilia that help the filter-feeding worm trap plankton and pass the food down to its mouth.

Where Christmas Tree Worms Live

white and orange Christmas tree worms grow on white coral reef

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Despite their variety of colors — including red, orange, yellow and blue — Christmas tree worms all belong to one species, Spirobranchus giganteus. They're widely distributed in Earth's tropical and subtropical seas, especially the Caribbean and the Indo-Pacific, where they live most of their lives anchored into coral reefs, hiding inside tubes they build with calcium carbonate taken from surrounding seawater. They prefer shallow water, typically living at depths between 10 and 100 feet.

How They Evade Danger

Christmas tree worms only grow to about 1.5 inches, but they're a common sight for divers thanks to their vivid colors and shallow habitats. They're also known for being skittish, quickly retracting into their tubes when they sense movement in the water. They can seal themselves in using an operculum, a specialized body structure that opens and closes like a door. The worms slowly re-emerge about a minute later, making sure the coast is clear before fully extending their plumes, also known as crowns. To see what that looks like, check out this video of Christmas tree worms in the Philippines:

Where They Stand in the Animal Kingdom

Christmas tree worms are polychaetes, a class of mostly aquatic worms that have colonized virtually every corner of the ocean, including the frigid abyssal plain and the steamy waters around hydrothermal vents. Some are mobile, but most burrow or build tubes — ranging from dainty, 1-inch wisps like Christmas tree worms to the nightmarish bobbit worm, a seabed-dwelling goliath that can grow nearly 10 feet long. Polychaetes are among the planet's most common marine animals, and they also account for more than 8,000 of the roughly 9,000 annelid worm species known to science.

How They Reproduce

Closeup of a yellow Christmas Tree Worm burrowed into a Brain Coral

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There are male and female Christmas tree worms, which reproduce sexually by releasing their sperm and eggs into the water. The eggs are then fertilized as they drift with the currents, eventually developing into larvae that settle on coral heads, burrow inside and construct tubes of their own. Many tube-building polychaete worms are also capable of reproducing asexually through a process known as paratomy.

Conservation Status of Christmas Tree Worms

As a species, S. giganteus seems to be doing pretty well. Its populations are stable with no major threats, other than local pollution or being taken from the wild by coral collectors. But as with many sea creatures, ocean acidification and warming could soon change that. Both problems are now deepening due to climate change, and both can threaten the coral reefs on which Christmas tree worms grow. And beyond that, acidification may endanger the worms more directly by reducing calcium carbonate minerals in seawater. Those minerals are a key ingredient in not just the calcareous tubes of Christmas tree worms, but also the shells of oysters, clams, sea urchins and countless other marine animals.

For now, however, there's no sign Christmas tree worms are struggling. And while they would be dwarfed by an actual Christmas tree, their natural beauty offers a priceless gift for anyone who meets them in their element. That can set a good example for our overall relationship with the sea, illustrating how to enjoy the experience of its riches without needing to own them. As Dr. Seuss famously wrote, "maybe Christmas doesn't come from a store. Maybe Christmas, perhaps, means a little bit more." Thanks to their relatively sedentary lifestyle, they're always home for the holidays.

Merry Christmas!