The Mighty Shipworm

by Peggy Williams

    Shipworms are small mollusks (less than 1/2 inch in size) related to angel wings. Also called Teredos, there are several species, and all of them eat wood.

    As babies in the veliger stage, they float about in the ocean using velar lobes to catch the currents. When they grow large enough to be too heavy to float, they sink to the bottom where, if they land on wood, they settle and thrive. If they don’t land on a suitable environment, they die.

    As soon as they settle they begin eating. They develop two shells, making them bivalves, but, like the angel wings, the valves are not hinged. Instead, there are extra plates attached to the animal that hold the two valves together. This makes it possible for the valves to move independently and rotate, helping dig a burrow in the wood with the sharp shell  edge.

    Within seventy days they can excavate a five-inch tunnel in the wood. They live less than a year, but in that year they can weaken the wood drastically. Wooden-hulled ships, dock pilings, and even dams can be destroyed by shipworms.

    Shipworms cannot live in fresh water, so mooring in rivers is one way of avoiding them; they also cannot attach in swiftly moving water or on swiftly moving ships. Once ensconced, however, they are difficult to kill. Poisons such as arsenic have little effect; paint impregnated with copper works for awhile, but as soon as a scratch appears, the tiny shipworms can go to work.

    Much of the land in the Netherlands has been built up from the sea by means of dikes, and the dikes used to be made of wood. Two hundred years ago it was discovered that shipworms had eaten far into the timber defenses and there was fear that the sea would reclaim the nation.

    In 1588 King Philip II of Spain determined to attack and conquer the English by sea. He send his armada to Portugal (occupied then by Spain) for outfitting and final preparations for the attack. Lisbon is a superb harbor, but it is saline and has ideal conditions for shipworms to thrive. Had the armada been anchored up the Tagus River, it would have been immune, but during the four or five months spent in the harbor it must have been vigorously attacked not only by shipworms but also barnacles, which add weight to a ship and slow it by impeding water flow over the hull.

    Many other factors have been blamed for the beating the English gave the Spanish Armada and its defeat. The fleet did not set out early enough in the year and encountered spring storms that first kept the Armada in port for three weeks, then were so perverse that the fleet stayed in the latitude of Lisbon for twelve more days. Forward progress was less than 20 miles a day.

    The English, under Drake and Hawkins, were meanwhile acquiring fleeter, more streamlined galleons (moored in the fresh-water Thames) and long-range cannon. Unseasoned barrels holding food supplies caused food and water to spoil and become poisonous. Philip insisted the fleet stay together, and the ungainly freighters slowed the pace of the Armada.

    The fleet had to rest again at Corunna before heading once more towards England. There it was discovered that teredos and barnacles had badly damaged many ships. Spongy planking was dangerous but couldn’t be scraped because shipworm tunnels would be opened to let in the sea. But Spanish pride wouldn’t allow them to turn back, and they neared Dover on July 24, still making less than two miles per hour.

    Both sides broke off battle after exhausting their ammunition, but the English had only a short distance to go to port; the Spanish had to go to greedy northern ports or hostile Scots or Irish ones, or limp home. Of the 130 ships sent out from Spain, only 68 came back; half of them were irrepairable.

    Though the role shipworms played in this drama is unrecorded, it seems logical to assume that it was great. Who would image that such a tiny animal could change history?

     ref article in Sea Frontiers : “ Marine Science and the Armada” by Joseph J Betz