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HERE'S a quiz: 1. On his fourth voyage to the Americas in 1502, Christopher Columbus lost all his vessels to: a. fire, b. pirates, c. hurricane, d. shipworms.

2. Legend has it that to save his town, the Little Dutch Boy plugged a hole in a wooden dike with his finger (or more likely his entire arm). Who made that hole? a. vandals, b. woodpeckers, c. lumberjacks, d. shipworms.

3. Why was so little wood found on the sunken liner Titanic? a. it drifted away, b. there wasn't much to begin with, c. divers took it as souvenirs, d. shipworms ate it.

OK, there's clearly a shipworm theme here, and indeed, shipworms is the correct answer to all the questions. But in addition to the facts above, there's something else interesting about shipworms: They aren't worms. They're clams.

Shipworms get their name from their long, narrow, cylindrical bodies. But the worm resemblance ends there. A closer look at the creature reveals a shell at its front. This shell has two halves with a gap in between, like a clamshell. In the gap pokes a muscular foot that acts like a suction cup, holding the shell in place while its razor-sharp edges scrape the wood ahead of it.

WHY scrape wood? Shipworms eat sawdust. The shipworm's stomach has a pouch for storing sawdust and a special gland for digesting wood particles. These termites of the sea also have an organ full of bacteria that digest wood. The bacteria take nitrogen from the water and convert it to protein for the worm, since wood doesn't supply protein. The bacteria, in return, get nutrients from their host.

A shipworm begins life like most marine invertebrates: as a tiny piece of meat in the plankton soup of the sea. When it finds a piece of wood, the worm goes to work, using its shell to eat its way into the wood.

As the shipworm grows, so does the burrow. (The creature's breathing siphons remain at the surface of the wood.) Depending upon the species and the length of their wooden homes, shipworms can be as short as 6 inches or as long as 6 feet.

Once a shipworm claims a home, its stuck there for life. Even when researchers removed mature worms intact and uninjured, they were unable to dig new burrows.

The damage shipworms cause is legendary. Greek literature from 350 B.C. mentions them and early explorers dreaded them.

Shipworms are sometimes called the mollusk with the million-dollar appetite. These creatures are credited with single-handedly destroying the Hudson River piers in New York City. Researchers estimate that untreated timbers, such as pier pilings, exposed to Hawaii's ocean waters will last less than two years.

In recent history, East Coast researchers lowered wooden panels about 5,500 feet to the ocean floor. When they recovered the panels 104 days later, they were completely riddled with the wood-eating worms.

This voracious appetite has a purpose. Large amounts of wood get into the oceans by means of rivers, mangrove forests and humans. Shipworms play an important role in reducing the amount of driftwood in the world's oceans.

After writing this column, I wanted to see some of these remarkable little clams so I paddled my kayak around the Ala Wai Boat Harbor. I was disappointed. Nearly everything in the water is made of concrete or fiberglass. The few wooden boats moored here are coated thickly with protective paint.

But there's always the driftwood that washes up on the beach. I'll never pass another piece.