Shipworms aren't worms, at all. They're actually odd-looking clams with wormlike bodies and a pair of miniature shells capping the front end. They make the intricate tunnels you may see in pieces of driftwood washed up on the beach.
The clams use submersed wood for both shelter and food, scraping out twisting chambers with their ridged shells and feeding on tiny wood particles with the help of cellulose-digesting bacteria. At their rear end are a pair of tubes (siphons) that project from the open end of the tunnel.
Shipworms can be alternately male, then female. Sperms released into the water by males are sucked in through the siphons of females, and the tiny, free-swimming larvae -- a million or so per brood -- are ejected from brood chambers in the gills after a short period of development.
The larvae eventually attach themselves to wood, morph into burrowing adults, and the cycle continues.
Aptly called "termites of the sea," shipworms such as Teredo navalis can cause enormous damage by progressively degrading the integrity and strength of submerged wood. Their destructive activities have been known by mariners since ancient times, when countless shipwrecks were attributed to shipworm rot.
Columbus abandoned two of his ships because of shipworms. In 1731, shipworms caused the collapse of wooden supports used for dikes in the Netherlands, resulting in extensive flooding and the replacement of wood by stone. A major shipworm infestation in San Francisco Bay during 1919-1921 caused millions of dollars of damage to piers and other maritime structures.
Over the centuries, various methods were employed to protect wooden vessels from shipworms. These included a thorough drying-out of ships between voyages, the application of pitch, wax, or tar to the wood, and the use of lead or copper sheathing on the hulls.
Such tactics met with limited success, as noted by Thoreau in his poem "Though All the Fates":
"The vessel, though her masts be firm,
Beneath her copper bears a worm . . ."
Modern building materials and wood preservation practices have reduced the incidence of shipworm rot, but these tiny mollusks continue to cause extensive destruction around the world.
Vicky McMillan, a retired biologist formerly at Colgate University, lives on Hilton Head Island.