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May 15, 2009

Hudson River safari: Teredos and gribbles

New York Magazine surveys the lower Hudson River:

2. Teredos and Gribbles

Two kinds of hungry pests gnaw away at the pilings that hold up structures like the FDR Drive, the U.N. school on East 25th Street, and the Con Ed plant at 14th. Teredos, which start life looking like tiny clams, grow up to be worms “as big around as your thumb, and nearly four feet long, with little triangular teeth,” says commercial diver Lenny Speregen. Like underwater termites, they devour wood. And Limnoria tripunctata, a.k.a. “gribbles,” are bugs about the size of a pencil dot that look like tiny armadillos, and eat not only wood but also concrete. Speregen says he’s seen fifteen-inch-diameter columns that have been gnawed down, hourglass style, to three inches. The city has tried jacketing pilings in heavy plastic to keep the critters out, but it hasn’t worked well: Floating ice tears up the jackets in winter. “I never said this wasn’t a war,” says Speregen.

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Teredos, which start life looking like tiny clams

Because Teredos, aka shipworms, are clams. The adult body is worm-elongate and the shell is reduced to a pair of corrugated, crenelated rasps at the business end of the beast. They tunnel through wood but feed in the ordinary clam way by filtering water drawn through a siphon. The wood is shelter rather than food. Since they don't actually consume the wood they bore through, preservatives like creosote mainly hinder only the colonizing larvae. Interestingly they somehow percieve the presence of other shipworm tunnels and the limits of the wood -including bolts, nails, etc.- and avoid them. Thus, even though an individual shipworm's tunnel may be convoluted and tortuous, it is nowhere breached and is open to the outside only at the site of initial penetration. There are a number of different species, with the ones of most economic significance having been transported worldwide long ago via the worm-riddled hulls of wooden ships.

The state fossil of North Dakota is, oddly enough, petrified Teredo-tunneled wood. During the Paleocene, some sixty-odd million years ago, in the last marine incursion of central North America, Teredo-drilled driftwood floated about on the so-called Cannonball Sea. Some of it sank, mineralized, and is now the object of rockhounds that dig it, along with shark's teeth, from the Cannonball formation shales of North Dakota. Sliced and polished it's a sort of combination petrified wood and mineralized swiss cheese. In the Hatfield Marine Science Center on the Oregon coast, I saw a big hunk of local fossilized Teredo drilled wood that had been drilled in its turn by rock-boring piddock clams. (Piddocks will also drill concrete.) Some of the piddock burrows had been colonized by clams specialized for that particular habitat after the piddocks had died. The rock was lately being used as a doorstop. - Reduce, reuse, recycle.

I've heard that there's a kind of Teredo in Palawan in the Philippines that the indigenes dice up and pickle like seviche. That alone might be a reason to go there.

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