Shipworm Not a Worm

Not only do shipwrights need to be concerned with gribbles but they also worry about the shipworm. The shipworm, which is not a worm at all, is actually related to the clam and mussel, therefore it is a mollusk.

These "termites of the sea" are able to digest the wood due to a symbiotic relationship with a bacteria that is stored in a special organ in their body. This relationship is much like the termite, which also needs a bacteria to allow it to digest the cellulose in the wood and turn it into glucose for energy.
The larva of the shipworm uses its small sharp shell to drill into wood. As they grow, the tunnels get larger and larger. Often, wood may have a tiny hole on the outside but the inside will be a maze of larger tunnels. The shipworm (teredo navalis) or sometimes known as the teredo worm, uses the shells near its head or siphon to burrow. Their ridged and rough surfaces rub the wood away as the mollusk turns its head one way and then another. It leaves behind a circular tube that is a bit larger than the shell. The shipworm eats the wood and the soft body's mantle leaves behind a white calcareous substance like chalk to line the burrow for protection and strength.

Shipworms have long been a global scourge of maritime activities. In 1502, during Columbus' 4th voyage to the Caribbean Sea, his ships survived a water spout, a hurricane, high seas, lightning and rocky reefs. However, luck ran out when two of his four ships had to be abandoned in the Hispanolas and Jamaica because of shipworm rot. He later used copper on the hull to protect it from shipworms.

Pier owners in San Francisco Bay saw an estimated $1.3 billion (today’s dollars) in shipworm damage in the early 1920s. In the last decade, the city of New York has spent well over $100 million to protect and replace worm-damaged structures.

One good way to decrease shipworms attaching and living on marine wood is to inhibit the symbiotic bacteria it needs to live. Some wood that inhibits the growth of the symbiotic bacteria is Douglas-fir, ebony, jarrah, koa, lignum vitae, narra, Osage orange, paduccah, purple heart, red alder, red oak, teak, verawood, and western red cedar. That is why Boothbay Harbor Shipyard spends much time and care choosing the right wood for each section of a ship for a new build or rebuild.