What do Christmas Trees have to do with Coral Reefs?
No doubt for this festive season a lot of us would have spent time putting up Christmas trees. One of the long-standing traditions of the Christmas season is decorating our Christmas trees often times with the help of our families. During this period trees are ubiquitous from shopping malls, business places to homes.
The marine environment has its own Christmas trees that adorn the seafloor all year round, creating quite a display for those who dare to seek them out. Although these Christmas trees stand no more than 4 cm high, this in no way diminishes their brilliance and beauty. The organisms to which I am referring are the polychaete worms, known as “Christmas Tree worms” of the genus Spirobranchus. Interestingly, there is much more to these organisms than just their beauty.
Christmas tree worms are often found closely associated with coral reefs. Brightly coloured crowns protrude from the tube like body, each composed of hair like appendages called radioles, which radiate from the worm’s central spine. These appendages allow these organisms to respire and are the means by which microscopic plants or plankton are caught for food. Spirobranchus come in many colours including orange, yellow, blue, and white and, with an average 3.8 cm in span.
Unlike other polychaetes, Spirobranchus are sedentary. Once they have decided on a spot on a living coral colony, they will bore a hole and secrete a tube of calcium carbonate around themselves and call this home for up to 40 years. This makes them very picky about where they live. These worms show a particular affinity for the finger coral (Porites sp) and brain corals (Diploria sp).
Once a spot has been chosen, Spirobranchus engage in mutualistic symbiosis with the coral, that is, a biological relationship where two organisms benefit by living together. The corals provide a habitat and facilitate their feeding habits while they provide several forms of protection to the corals. These shy beauties are pretty fierce protectors of their coral hosts. One Australian researcher observed Spirobranchus pushing away the feet of the Crown of Thorns Starfish (Acanthaster planci) from its coral host, thus preventing predation. Colonies of living coral also showed faster rates of recovery when they are adjacent to Spirobranchus giganteus following events of coral bleaching, predation and algae overgrowth. Think of the coral as Gotham City and Spirobranchus as batman but pleasant to look at.
Although spectacularly coloured, Spirobranchus are shy about expressing their beauty. I dare say these are not the kind of organisms that would have a social media account. At the first sign of disturbance they retract pretty quickly into their calcium tubes and are very reluctant to come out. This makes photographing them very difficult. Spirobranchus are very much the type of organisms that do not like to be seen.
Despite their spectacular appearance these organisms are not well studied. This underscores the importance of coral reefs and their inhabitants, both the spectacular and not so spectacular. Coral bleaching, the effects of global warming and instances of pollution continue to threaten coral reefs worldwide. All reef inhabitants are invaluable members of coral reef society. They deserve to be understood and protected before unchecked anthropogenic effects cause us to lose them from this planet forever.
By: Attish Kanhai
Institute of Marine Affairs
Several brightly coloured christmas tree worms on a head of coral.