Monday, October 4, 2010


I’m going to try something a little different today on the Network. I have limited my topics to either local species, generally accompanied by a photo that I took myself or musings on natural history education in general. Today I’m going to start a series that introduces organisms that are oddities, phylogenetically speaking.

Perhaps my most nerdy passion is learning about phylogeny. Phylogeny is essentially the study of relationships in the tree of life. The organisms I want to take a look at are those that have diverged in interesting ways from their close relatives.

So today’s featured creature: the shipworm. I’ve been thinking a lot about the word worm. It’s one of those words that, speaking technically, carries almost no actual meaning. It’s a descriptor of body type (long and thin) and before biologists were able to start the project of classifying animals based on their actual relationships instead of what they looked like it got used as a label for all sorts of things. So we end up still using the word “worm” today to describe at least four major phyla (annelids, polychaetes, nemerteans and nematodes) and probably more that I’m missing as well as a host of other creatures that aren’t part of any of those groups.

Image courtesy of the US Geological Survey

Like this guy here. The “shipworm.” Folks tell me he’s not a worm at all. He fits the description more or less, though. He’s long and thin. But this animal is actually a bivalve mollusk, meaning his closest relatives are clams, mussels, oysters, scallops etc. and most closely the geoducks and soft shell clams. But this creature has diverged away from the “normal” bivalve physiology by growing into an organism with an elongated body. It retains the two shells of other bivalves but these have been altered into a cutting or boring device at one end of the animal.

Shipworms are so named because they are well known by sailors to bore into ships’ hulls, destroying them in the process. Wood can become so concentrated with the burrows of shipworms that it becomes nothing but an empty husk, brittle to the touch. The family of shipworms has also been dubbed “termites of the sea.”

This behavior of drilling into wood serves two purposes. First is protection. The animal will remain inside its burrow and secrete a calcareous compound to line it. The second is nutritional. These mollusks actually consume the wood and live symbiotically with a bacteria that takes nitrogen from sea water and synthesizes proteins. They also extend their siphons into the water through the open end of the burrow both to breath and to supplement their diet with plankton.

Even though a single piece of wood can become almost completely saturated with shipworm burrows they never make them long enough to interfere with a fellow shipworm. They simply stop growing. So apparently they are not only fascinating but also really great at sharing!

So the next time you pass some drift wood at the beach inspect it more carefully and you may find the little holes left by these bizarre bivalves.


Chesapeake Bay Field Guide:

Grave, B.H. Natural History of Shipworm Teredo Navalis, at Woods Hole, Massachusetts. Accessed at:



  1. cool! I just learned something new

  2. I learned many somethings new. But I still have a question: how exactly should I go about explaining this "worm" classification in a classroom of very young children?

    This couldn't have come at a better time because this particular week at my daycare one of the very new and very young teachers is doing a unit on worms, mostly consisting of ridiculous(ly fun) art projects like painting with spaghetti and stuffing tube socks; obviously a science lesson is not the primary goal. But I have spent the week cringing so far as she read a book about an inchworm the first day, and then the next claimed that caterpillars and worms were "basically the same thing."

    Now, I understand there are many colloquialisms that I cannot fight and recently I have been called out for being too picky to the point of rude--but I just can't go around letting children think that worms and caterpillars are the same thing. But when I went to correct the statement, I realized my own knowledge of the word "worm" was seriously lacking. I'd prefer to get my story straight before even more misinformation gets presented.

    So, again, how does one explain the classification of "worms" to children? And what of all the non-worm things that are called worms--is the misnomer really all about shape alone? Or, is this one of those times I have to learn to let go?

  3. Wow, let's see. First of all, is this something you should let go? Probably. It's kind of like the word "bug." I don't think, no matter the amount of science education, that people in general will stop using the word bug to refer to any kind of creepy crawly little critter.

    Same goes for worm. I was about to write that mollusks are so very far from worms. But imagine that you're living hundreds of years ago before DNA was even a twinkle in Watson and Cricks' ancestors' eyes and you see a soft bodied weird looking thing. That's a worm. Duh.

    I guess my point is that no matter how you slice it the vast differences between animals in different phyla will probably remain a mystery to most folks. Here's a great example (although it involves animals in the same phyla). I like to talk about horseshoe crabs. Some of you may know this. When explaining the difference between the HSC and a real crab to folks I have to go with strict morphological differences. Crabs have ten legs, so does the HSC. Darn. So I point out the long tail, the lack of claws, etc.

    When you're talking about inchworms you can point out to very young kids that inchworms have legs and earthworms do not. I'd also be very interested to see what their ability to understand what colloquial means. I assume that preschoolers wouldn't quite grasp it but I'm sometimes surprised by their wisdom.

    Not sure if that answers all the questions, so let's continue the conversation if you'd like more clarification.

  4. And something else I just thought of. Science and environmental education at the level of preschool should first and foremost be about appreciation. If you can get kids excited about worms I think you've been a good science educator.