Christmas Tree Worms
One of a series of notes on reef creatures encountered on our trips in Natewa Bay. These are not formal scientific accounts, but designed to be of interest to the curious lay person. References to more rigorous sources are included.
This Porites boulder coral has a festive outcrop of plumes, each one the triangular shape of a traditional Christmas tree, hence the name. I am more reminded of an irregular Christmas tree decorated by brightly coloured ornaments.
The main body of the worm is unseen, protected in a tube deep within the coral.
It is fun to get a finger close to these plumes – they disappear before there is any chance of touching them, withdrawing into their tubes which can then be sealed off with a hard lid (termed an operculum).
There are lots of reef creatures that would gladly eat these worms given the chance.
This photo was taken in the protected reefs of Natewa Bay - delicate creatures like these are usually limited to deeper, less turbulent water, but we encounter them just below the surface, ideal for snorkelers.
Image: A Crowd of Christmas Tree worms
The feathery plumes are extensions that filter plankton for food and allow the worms to respire (oxygen in, carbon dioxide out). They alwayzs occur in pairs. These are close-up views (all photos taken with the small Olympus TG camera) – the plumes are only about 40mm high.
At the Sandbank the worms are usually found in large numbers on the same species of hard coral as seen in the first photo.
A close look in reefs exposed to more turbulent water may reveal the occasional plume at shallow depths. Though usually they are found in deeper waters on a variety of corals.
Image: The yellow lipped operculum, ready to seal off the tube
Christmas Tree worms are polychaetes. Almost all polychaete worms are marine, some 8000 species have been recorded almost everywhere from the deepest oceans, in very hot water and in very cold. An extremely successful life form. Almost all are mobile, using their bristles to crawl about on the bottom or to swim. But not so the Christmas Tree worms. https://ucmp.berkeley.edu/annelida/polyintro.html
Microscopic planktonic stages of the Christmas Tree worm settle on particular corals (usually), and build protective tubes. In the examples shown here, growing coral has enveloped the tubes. The worms are now deeply embedded, and very well protected. Their swimming and crawling days are over – unlike most polychaetes, each adult worm is now sessile (meaning ‘fixed’ in place for the rest of their lives). It seems worms do not actively burrow into the coral, as some internet sources state.
It is generally accepted that all of the variously coloured Christmas Tree worms belong to a single polychaete species, Spirobranchus giganteus. The wide range of plume colours in this single species is no doubt due to interplay between genes and environment – wide colour variation is well described in other single animal species, although rarely with such exuberance.
An aside: Single coral species can be differently coloured, but unlike Christmas Tree worms, the different colours are usually due to the type of algae the coral cells take up. These algae are expelled when coral animals are stressed, resulting it bleaching, which is not immediately fatal but will kill corals if prolonged. If the bleached coral survives it can subsequently ‘re-absorb’ a different algal species and subsequently become differently coloured).
Do Christmas Tree worms damage or benefit corals? The jury is out. They have been observed inhibiting the predation of the coral polyps by the infamous Crown of Thorns starfish, and corals with the worms seem to do better, and grow faster than those without. As to harm, my suspicion is that any net adverse effects of Christmas Tree worms are likely limited to corals already under stress.
Christmas Tree worms can live in aquariums, and many internet sites advertise them for sale. However, descriptions of how they are sourced was hard to find. Since it is not possible to extract adult worms and then build them homes, the answer is unsurprising: sourcing of Christmas Tree worms involves wholesale removal of the corals they inhabited. The worms are quite common, found in tropical Indio-Pacific and Caribbean reefs and do not seem directly threatened. The removal of living corals is another matter.
And that is all we have to say about the extravagantly coloured Christmas Tree worms