Background

Christened by mariners, "termites of the sea," shipworms are parasitic mollusks that thrive in and upon submerged wooden structures, including pilings, bulkheads and the untreated hulls of boats. The insignificant shipworm, capable of devastation, in fact is no worm at all, but a clam. According to several sources, the shipworm has been recognized for thousands of years as a nuisance species. Mariners as early as 412 BC attempted to protect their ships' hulls from the dreaded worm with combinations of chemicals, including arsenic, sulphur and oil.

Today methods of protection and materials have changed - pilings are made of pressure-treated wood that resists infestation, hulls are treated with anti-fouling paint or made of fiberglass - but the shipworm remains a formidable pest, capable of serious damage.

Hundreds of species of shipworm exist, but only one is common to the Chesapeake Bay. Gould's shipworm, or Bankia gouldi, was first identified from specimens found in Norfolk Harbor. The species can survive and even thrive in salinities far lower than those tolerated by the better-known tropical species, Teredo navalis, and withstands harsh winter conditions. (The term "teredo" probably was once the generic term for all shipworms. And while Bankia is the only species of shipworm that is truly a native of Chesapeake Bay waters, it's very likely that Teredo navalis also entered the Bay on the many ships that arrived from other Atlantic ports and the Caribbean.)
Bankia gouldi larvae attach themselves to any submerged unprotected wood surfaces, such as a tree trunk or branch, ship's planking or pilings. In just a few years Bankia can honeycomb the stoutest oak, beech or locust tree, even if only part of it lies underwater. Similarly, once the shipworm has infiltrated a ship's structural member or an untreated dock piling, it's a matter of time before the entire structure will become infested, weaken and collapse.

Despite its tolerance of low salinities, Bankia often die off in years when there is a strong freshet, or when the "host" ship moves upstream into fresher waters. Shipworms also perish when their wooden habitat freezes, becomes too hot, or if the hull is hauled from the water. By then, however, fatal damage to the hull often has been done.

The exterior of a vessel whose interior is riddled with Bankia ironically may reveal only a couple of tiny holes or ports of entry–and otherwise appear perfectly sound. On the other hand, the hulls of some heavily infested ships may exhibit clusters of these tiny holes. When submerged, the shipworms' snakelike siphons emerge, periodically emitting small pellets–the remains of the wood particles that the shipworm has carved with its shell, ingested, then expelled through its siphon.

Habitat and Life Cycle

• The shipworm is related to boring clams that prefer hard substrates or mud bottoms. The bivalve has two small shells that grow only to a length of about one-third of an inch, and from which extends a long, wormlike body, which the surrounding tunnel of wood supports and feeds.
• Shipworms do not depend solely on wood for food, but also feed on plankton that they suck in through their siphons.
• Each shell has toothed ridges, which shave away bits of wood as the worm grows and moves. The shipworms' calcified tunnels never intersect and are roughly the diameter of a pencil.
• Like other clams, shipworms have incurrent and excurrent siphons at the end of the body that has no shell. These siphons or spiracles remain in contact with the small, almost pinhole sized entry point, but the clam eventually grows too large ever to emerge again. The Bankia "worm" circulates water, dissolved oxygen and food and discharges its wastes through the siphons.
• Shipworms release planktonic veliger larvae (larvae that have developed swimming membranes or velum). The larvae exist in the water column for two or three weeks before settling, if they're fortunate, on untreated wood.
• The larvae have a small foot that enables them to travel over the surface of the wood. They eventually attach by means of a secreted thread. Larval shells develop into minute versions of the shells that they use to bore through the wood, where they develop further into adult shipworms.

The shipworm occupies an unusual place in Maryland history. It is believed that the famous three-masted pinnace, Dove, which, along with the Ark, transported colonists and supplies to the New World from England for the fledgling Chesapeake Colony in 1634, was fatally damaged by shipworm during its stay here. The Dove was forced to remain in the Rappahannock River for several months while its owners sorted out legal difficulties. According to some reports, the Dove's original crew, angered by the fact that their tour of duty extended past the usual year, and concerned that the ship was leaking badly, brought suit against Lord Baltimore for unpaid wages. Anecdotes from the Dove's former crew indicate that the ship at this time was no longer seaworthy. When the ship finally set sail for its return trip to England with a new crew, it apparently sank in the North Atlantic with the loss of all hands.

Ironically, almost 350 years after the original Dove sailed to Maryland, her reconstructed namesake also was nearly destroyed by shipworm after having remained too long in the Bay with an unprotected hull; she was rescued from complete wormshot and had her bottom coppered at a Solomons, Maryland shipyard.

In 1699, colonial botanist Dr. David Krieg traveled on an English vessel to the Chesapeake Bay. During his trips ashore he collected various submerged aquatic vegetation (SAV) specimens, such as widgeon grass (Ruppia maritima), to study and bring to his London benefactor. One of his letters home indicates that the ship's crew moved the vessel into the fresher waters of what was probably the Tred Avon River, to protect the hull "from the worm."

Thanks to biologist Kent Mountford for the use of his research on shipworms. Other sources include Life in the Chesapeake Bay, Second Edition, by Alice Jane Lippson and Robert L. Lippson.

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Last modified: 10/26/01