shipworm or teredo (tĕrēˈdō) [key], marine bivalve mollusk of the family Teredinidae, specialized for boring in wood. A shipworm is not a worm, but a greatly elongated clam. Its two shells, enclosing only the front end of the body, function as a tool, rather than a protective covering; their ridged and roughened surfaces are used for boring. The burrow (lined with a calcareous coating produced by the clam's mantle) is begun when the animal is in its larval stage and is expanded as it grows. The common shipworm of the North Atlantic Ocean, Teredo navalis, may grow up to 2 ft (60 cm) long, although its shells remain only 1/2 in. (12 mm) long. Shipworms feed on wood particles and minute organisms. They do enormous damage to piers and ships, and although they are deterred by chemicals, control is still a problem. Shipworms are classified in the phylum Mollusca, class Pelecypoda or Bivalvia, order Eulamellibranchia, family Teredinidae.