Are Christmas Tree Worms Bad for Corals?

If you have ever been diving or snorkeling in a coral reef you have probably seen and fell in love with the Christmas Tree Worms. Their bright colors and intricate anatomy make them a highly conspicuous feature on the reefs, which invites viewers in for a closer look. But what exactly are these animals, and what kind of role do they play regarding symbiosis on the coral reef?


The Genus Spirobranchus

Christmas Tree Worms are a member of the Spirobranchus genus of tube building annelid worms. They are found throughout the tropical oceans, including the Caribbean and the Indo-Pacific. The worms use their spiral, tree-like appendages (called radioles) for both respiration and to filter food from the water. They are sessile, meaning that after they settle as larvae they will secrete a tube in which to live, and never leave it. Over time, this tube is usually grown over by living coral tissue and becomes integrated into the coral’s skeleton, increasing the worm’s protection. So, the worm obviously benefits greatly by living in host coral heads, but what does the coral think of all this?

Christmas Tree Worms Effect on Corals

In 2015, a student from the Naturalis Biodiversity Center of the Netherlands, Roel van der Shoot, came to our program on Koh Tao to conduct his project, investigation the role that Spirobranchus plays on our reefs. His results were recently published in the scientific journal Ecology. He looked at the effect that the worms were having on the host corals they were incorporated into. As the worms feed, or when threatened, they retreat down into their protective tube, which is securely closed by an operculum (a sort of lid). This operculum is often equipped with antler like structures that further defer any potential predators. This is an effective behavior by the worm for survival, but Roel noticed that sometimes this movement can scrape the coral tissue, leading to scars or a pigmentation response similar to inflammation.

In Roel’s study, he observed 749 worms, and took information on whether or not they had caused damage to the coral, and also whether or not their operculum had filamentous algae growing on it (as is common). He found that in 21% of cases, the worm had indeed caused damage to the corals. This often leads to tissue loss which is then replaced by the growth of algae of sponges, inhibiting the regrowth of coral tissue. He found that Porites corals were most susceptible to damage, and that overwhelmingly this damage was caused by the worms that had filamentous algae growing on their opercula. This is interesting, as it implies some further reasons for the association between the worms and the algae.

So, Are They Bad?

We would say that the jury is still out on Christmas Tree Worms as a benefit or a detriment to their coral hosts. Although 21% had caused some observable damage to the coral, that still leaves a great majority of individuals which were not problematic. It is also not clear as to what degree other factors contributed, such as the orientation of the worm in relation to the coral, which could be an important factor. Further studies are needed to shed more light on the nuances of this relationship, such as the survival of coral containing worms and those absent of them during bleaching or disease events. What we do now know is that this relationship seems ever more complicated then we originally presumed, and is one that will continue to intrigue and excite our curious minds while diving for years to come.

Hoksema etal 2019

Figure from the paper, showing damage to Porites corals from Spirobrnachus Individuals (Hoeksema et al. 2019)

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