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    The Overlooked Joy of the Christmas Tree Worm

    By By Jennifer Frazer
    June 05, 2012, 9:32:48 AM EDT


    Put your gills in the air like you just don't care . . . Christmas tree worms, Spirobranchia giganteus. Creative Commons Nick Hobgood. Click image for link to image and license.

    While in the southern hemisphere, I’ve taken every opportunity I can to dive. It’s the hiking of the submarine world: good exercise, and lots of pretty stuff to see.

    One my final dives, on the reefs of the remote island of Rarotonga in the New-Zealand -dministered Cook Islands*(see here for locator map), I encountered an animal quite by surprise that I’d waited years to see. A Christmas tree worm. Actually, lots of Christmas tree worms.

    Once I realized what I was seeing, I couldn’t stop looking at them. You see pictures of these things and can hardly believe they’re real, or that they actually come in that stunning array of sour patch gummy-worm colors. But they do. I saw white, neon yellow, electric blue, neon pink, pink tips shading into a grey base, and grey tips shading into a pink base. I don’t have any actual pictures due to the lack of a Scientific American (TM) Fund for Ridiculously Expensive Underwater Cameras for Bloggers, but this picture should give you the idea.


    Creative Commons Nick Hobgood. Click image for link and license.

    And they’re just as awesome in real life as in the pictures. Once I realized what I was looking at, I swam right up to one. It was lounging on a giant, roundish, hard coral. And then just like that, it evaporated into its burrow and slammed its operculum/trap-door closed.

    Well. Merry Christmas to you too, buddy.

    This video gives you the idea:

    Not all the worms behaved that way. For some of them, no matter how close I got, they held their ground. I guess even Christmas tree worms have personalities.

    The Christmas trees always come in pairs because they are, in fact, the breathing/feeding structures of a polychaete worm, which I wrote a bit about here. In bio-nerd speak, they’re called “radioles”.


    You'll want to click on this one to link to a larger version where you can see the radioles in detail. Creative Commons Nick Hobgood. Click image for license and link.

    That feathery shape has evolved, as it so often has, to maximize surface area for the aforementioned eating and breathing. They burrow into their host corals, apparently causing little damage unless their numbers escalate, and secrete a calcareous shell to further reinforce their position. Within that tube, they have small legs and bristles called parapodia and chaetae, respectively, (as all polychaete worms do) that Christmas tree worms use pretty much exclusively for extruding themselves to eat, breathe, and procreate, and then yanking themselves back inside when a dumb American diver blunders near. They have few natural predators, and unlike a lot of other marine life, humans have left them pretty much alone. Life’s probably pretty good.

    Surprisingly, for a worm with such varied graphic design options, they all belong to one species — Spirobranchia giganteus, which is something like “gigantic spiral gills”. I can’t really argue with that name.

    A few years ago I was in the Sant Ocean Hall at the Smithsonian and spent a lot of time admiring their biodiversity exhibit. Christmas tree worms were on display, but only as a photo. Their colors apparently don’t preserve well in formaldehyde, so instead of displaying a jar filled with a dubious cream blob, they went for the next best thing. But people usually don’t look very closely at the photos in such exhibits. They may glance, but they don’t see. For Christmas tree worms, there’s just no good museum solution.

    I only wish there was a way of putting people in the water next to the real thing, slamming door and all. Like so many other things, there’s a magic to being in the presence of a creature that a photograph can never quite convey.

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