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: http://www.chesapeakebay.net/shipworm.htm
Grave, B.H. Natural History of Shipworm Teredo Navalis, at Woods Hole,