It’s Christmas (worms) in July!

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Christmas Tree Worm. Photo Credit: Armando F. Jenik

Christmas in July has become a cultural staple and the recent pandemic has brought it nothing, if not more, publicity and believers. For those who celebrate Christmas, whether fad or tradition, tree decorating only comes once (or now twice) a year. In the world’s tropical seas however, living Christmas trees are erected year around. Christmas tree worms (Spirobranchus giganteus) are brightly colored marine polychaetes, or segmented worms. As you’ve probably guessed, their common name comes from the branched cones (or trees) that adorn their bodies.


Christmas Tree Worm. Photo Credit: Maite Pintenat

Each worm has two crowns (blue above) that protrude and tower above its tubular body (orange in this picture). The crowns are made up of radioles, hair-like appendages that extend outwards from the worm’s central spine. This system allows water to easily flow between the radioles, while facilitating exchange of gas and nutrients.

To eat and breathe, the worm pumps water up and over the crowns, allowing the radioles to catch phytoplankton and oxygen suspended in the water. Since the cones take in air, they’re often mistaken for gills, but the majority of transport mechanics actually involve food. After nutrients are taken in, they are passed downwards through a food groove by ciliary tracts, lines of tiny hair-like structures that line the surface of cells. Water and mucous push the food, ensuring it doesn’t get stuck.

Christmas Tree Worm with Visible Compound Eyes. Photo Credit: Michael Bok

In addition to their unconventional intake system, Christmas tree worms have eyes in an unusual location: at the base of their back radioles (neon orange above). This tucked-in spot limits the efficacy of the visual system, allowing Christmas tree worms to only see directly in front of and behind their bodies. Each banana-shaped eye is composed of 600-1,000 facets (or units), that together create a compound eye. Christmas tree worms have more facets than any other fan worm! Even more unusual, their eyes grow and develop from their tentacular radioles, a description that makes these worms seem more similar to aliens than marine organisms.

Surprisingly, the fan worm family (Sabellidae) relies exclusively on photoreceptors in their radiolar tentacles, so Christmas tree worms are just one of many species that use this method. Researchers hypothesize that radiolar eyes’ diversity in form and sophistication may represent a relatively recent evolutionary development that allows sessile (immobile) worms to cope with their hazardous tube-dwelling lifestyle. Like a burglar alarm, their eyes can detect approaching predators, startling the worm, and leading to a rapid retreat into its fortified, calcium carbonate tube.

Christmas Tree Worm. Photo Credit: Nick Hobgood

As sedentary organisms, Christmas tree worms need more than sophisticated eyes to avoid predation. A strong shell protects and anchors the bulk of their more-traditional worm body (yellow above). A chalky- white calcium carbonate secretion hardens the outside of the casing, while colored pigments and algae allow the Christmas tree worm’s body to blend into the surrounding substrate (usually coral). To make their homes, Spirobranchus bore into live coral and dig burrows. When frightened, the worms rapidly retract into their homes to hide from awaiting predators. Averaging about 1.5 inches, they have little trouble fitting snuggly amongst the coral.

Christmas tree worm startle reflex. Video Credit: Super Sea Monkey

Some groups take hiding one step further by colonizing stony corals. Instead of arduously drilling into the coral’s rock-like skeleton, they nestle their bodies against coral polyps’ living tissues, forcing the polyps to build around them. Over time, these coral bunkers can reach up to ten inches in length!

Christmas Tree Worm. Photo Credit: Cobra Toxin

Christmas tree worms are picky about which corals they settle on, but scientists are unsure of what influences landing decisions. Factors like material, coral type, local predators, and reproduction may be at play. Once settled, Christmas tree worms can live more than 30 years, so this is a big decision to make (even for those in the average 10—20-year groups)!

Broadcast Spawning in Christmas Tree Worms. Video Credit: Undersea Productions

Once settled and mature, it’s time to reproduce. Unlike most invertebrates, who produce asexually, Christmas tree worms are either male or female. To release their genetic material, males and females use broadcast spawning, shooting their gametes (genetic contributions) into the water. With luck and plenty of genetic material, sperm and eggs will find suitable matches (remember this post). After fertilization, larvae develop in just 24 hours. From there, it’s float and repeat.

The next time you or your acquaintances are craving Christmas, remind yourself that fantastic tree displays come every day— you just have to look to the ocean instead of the snow.

Have you seen a Christmas tree worm? What are you Christmas in July plans? Post your stories in the comments below. Share to spread the cheer.

 

© 2020 Sabrina L. Groves. Creative Commons Attribution-Noncommercial 4.0 International License.

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