Ship Worm

Teredo navalis inhabits warm seas and estuarine waters throughout the world. Like other molluscs, the worm passes through several stages before assuming its adult form. The female worm produces tens of thousands of eggs, a small percentage of which are fertilised by males. When the worm larva hatches it is covered with cilia enabling it to swim. However, within a day or so of hatching its looses much of its cilia as it develops a small bivalve shell that covers and protects most of its soft body. The part of the body remaining outside the shell remains fringed with cilia, allowing the organism to swim until it encounters wood.

Once the immature worm locates wood it secures itself by a secreted thread and immediately begins boring with its bivalve shell. It lives the remainder of its life in the tunnel it makes in the wood, growing to a width of about 10 millimetres and a length of between 20 and 45 centimetres. While the worm digests some of the wood its grind away as it tunnels, it feeds primarily on minute organisms they strain from seawater.

As the creature's Latin name Teredo navalis suggests, the worm posed a serious risk to large wooden vessels sailing warmer seas, such as the Mediterranean, Caribbean and southern Pacific. Because the hole by which the worm entered planking was very small, an infestation could be well advanced before it was discovered. In fact near half the weight of a piece of wood could be destroyed by worms before the outer surface showed signs of the worms' presence.

In all likelihood, the worm was a contributing factor to the sinking of numerous eighteenth century vessels in stormy conditions.

During the course of the eighteenth century, European navies tried various ways of protecting ships' hulls from worms. The Admiralty tried fitting ships with an outer protective hull, separated from the main planking by a protective coating of tar. However, this proved ineffective. From 1761, the Admiralty began exploring the use of copper sheathing. This proved effective in preventing infestation, although it was found that the copper sheets were badly damaged by corrosion due to the galvanic action of the copper on the iron bolts used to fix the sheets to the hull. This problem was solved in the early 1780s, and by a general order of 1783 all British vessels with sheathing were fitted with copper and zinc bolts.

Both the Dolphin and the Endeavour were fitted with copper sheathing.

Smaller boats carried by vessels were equally at risk of being destroyed by marine worms, and here too various protective measures were employed. The hull of the pinnace and other boats carried aboard the Endeavour were coated with a thick, toxic solution of powered cerussite, or white lead (lead carbonate). According to Cook, the coating proved very effective.

• Snow, Charles Henry, Wood and other Organic Structural Materials, McGraw-Hill, New York, 1917, http://cdl.library.cornell.edu/cgi-bin/chla/chla-cgi?notisid=ACP7606. [ Details ]