The Archaeology of Shipworms, the Real Dread of the Ancient Mariners

Homer is the earliest mention of the extreme measures ancient Greeks used against the teredo, which could cause warships to break apart mid-sea.

Philippe Bohstrom
Philippe Bohstrom
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A replica of the HMS Bounty sinks during Hurricane Sandy off the coast of North Carolina, October 29, 2012.
A replica of the HMS Bounty sinks during Hurricane Sandy off the coast of North Carolina, October 29, 2012.Credit: U.S. Coast Guard/Wikimedia Commons
Philippe Bohstrom
Philippe Bohstrom

Forget rogue waves, hostile rams or hidden reefs. The real terror of the ancient shipwrights was the teredon worm.

Infestations of this remorseless mollusk could be kept at bay only by vigilant maintenance, including drying out the hull on shore and constant applications of pitch. The Greek poet Homer's poetical reference in "The Iliad" to “black ships” referred exactly to that.

This week a new species of shipworm was discovered alive for the first time. The giant jet-black creature had been known for centuries but remained unfound and despite being several feet in length, intriguingly, does not eat and has no digestive system. The more conventional shipworms had very healthy appetites.

Scientist removing the top of the shell revealing a giant shipworm living inside, at a laboratory in Manila, by Marvin Altamia, April 19, 2017Credit: MARVIN ALTAMIA/AFP

In summer, the Mediterranean and temperate seas the world wide seethe with the spawn of the teredo, actually a sort of extreme clam. Each teredo tiny larva swims about desperately evading predators and seeking timber: driftwood, dock pilings, or the hull of a passing ship. Fastening to the wooden surface, the larva bores a hole, rapping at it with the razorlike edge of its vestigial shell.

Once ensconced in its hiding place, the teredo never emerges. It keeps its mouth fixed to the opening of its hole, sucking in life-giving seawater. Meanwhile, it grows and as it does, the sharp shell at the other end of its body burrows ever deeper.

As the burrow extends into the timber, the animal grows to fill its ever-lengthening home. Within a month, the sluglike teredo can reach 30 centimeters and is now ready to eject swarms of larvae of its own into the sea, starting a new cycle.

Their wood planking and ribs riddled with their holes, ancient ships would suddenly break up and sink mid-voyage.

Only conscientious maintenance – new applications of pitch, drying out and inspecting of hulls, and prompt replacement of unsound planks – could keep the wooden ships seaworthy, for up to about 25 years.

Olympias, a reconstruction of an ancient Athenian triremeCredit: Templar52 / Wikimedia Commons

Not Man's best friend

Thus the worst enemy of wooden-hulled navies in the Mediterranean waters was less the neighbors than the misnamed worm. The maiden trires in Aristophanes resigned herself to a worm-eaten old age:

Soon shall age these timbers eat, and give the worms a lasting treat” – Aristophanes, "The Knights"

The earliest written allusions to protection against shipworm is in Homer, whose most common epithet for ships after “swift” is “black” or “dark”, a reference to the coating of pitch that made the ancients ships seaworthy and protected against the dread clam.

The length of a giant shipworm, a species never before studied, after removal from its shell, by scientist at a laboratory in Manila, by Marvin Altamia, April 19, 2017Credit: MARVIN ALTAMIA/AFP

Herodotus (6.119) informs that the Phoenicians, from the 5th century B.C.E., seemed to have plated their ship with bronze or bitumen to waterproof their vessels and deter the teredo.

Some ancient merchant ships solved the problem by being lead-sheathed, but the weight of lead made it unsuitable for oared warships, which needed to be lightweight and swift in the sea. The Greeks would haul their warships into huge shipsheds, where harbor workers would inspect the keel, replace infested planks and apply fresh pitch. Inventories of ships at the time frequently labeled wooden gear “worm eaten”.

The pitch was made out of resin and lime, tar and Sulphur, mixed with oil.  In later times, the keels were also waxed. The Romans used the same methods to protect their ships.

The Vikings, however, seemed to have put more effort into beautifying their long-ships with ornaments only above the water-line (although there is a description of a smaller boat that was coated with tar).

Teredo navalis infestation in a branch.Credit: Rosser1954 , Wikimedia Commons

The technology did not improve much over the millennia. The great seafarers Francis Drake (1540-1596), Christopher Columbus (1451-1506), and James Cook (1728 -1779) suffered badly from infested keels during their exploration voyages. Their main method of protecting the ships seemed to have been  to coat the keels every month with a layer of beck, oil and tar. Yet on Columbus' fourth voyage, they survived storms, hurricanes, coral reefs and diseases but had to abandon the ships due to the shipworms.

During the 16th and 17th centuries, a technique developed involving anhydration of the wood exterior using animal hair and tar. The ships of the East India Company were built in this manner, then coated with a bottom paint made out of arsenic, broken glass, ceruse and tallow.

After thousands of years, the industrial revolution brought back a forgotten solution: From the mid-18th century, ships were plated with metal, in this case copper, which prevented the larva from latching on to infest the keels.

The collapse of the San Francisco bay piers due to shipworm in 1914 led to the development of new prevention methods, to hinder the immense damage caused by the mussel.

So humans don't appreciate its charms and the monster shipworm species newly discovered burrowing into swamp mud in the Philippines is unlovely. But as much of nuisance the shipworm may be, it plays a crucial ecological function by breaking down and replacing organic material in nature. The shipworm has existed for over 60 million years, and is also happily eaten by peoples around the globe, including the Amazonas and the Philippines.

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