Return of a Castaway

The gripping story of a boring clam

Among the great scourges to plague humanity, the shipworm must be one of the most underappreciated. Since antiquity, these wood-boring sea animals have sunk ships around the world and quietly altered history (see box, below). They disappeared into obscurity in the past century because metal and fiberglass replaced wood in boats and builders of piers and bulkheads started using chemically treated wood. But don’t dismiss these would-be skunks at the boating party yet. Each year, shipworms still cause an estimated $1 billion in damage to wooden vessels and maritime structures worldwide.

SWISS CHEESE? Shipworms bored through this oak piling from a harbor in Belfast, Maine, and caused it to collapse. The animals dig parallel tunnels along the wood’s grain and never cross into one another’s burrows. L. Healy and Eckelbarger/Darling Marine Center

BORING CLAM. Shipworms are clams, not worms. The adult has a tiny helmet-like shell that covers its head and files through wood. Eckelbarger/Darling Marine Center

MICROSCOPIC COCONSPIRATOR. Symbiotic bacterium gives a shipworm the power to digest wood and to survive on an all-wood diet when other food is scarce. Distel/International Journal of Systemic and Evolutionary Microbiology

They’re especially burdensome in developing nations, where fishermen can often afford only wooden boats and cheap paint for the hulls. Moreover, they’ve been forging a costly comeback in some U.S. harbors.

Scientists have always had a love-hate relationship with shipworms. While ship captains cursed these “termites of the sea,” 17th- and 18th-century naturalists marveled at the animal’s diversity, adaptability, and symmetry. All parties agreed that shipworms were a malicious pestilence from God.

Researchers who study shipworms say these mislabeled animals–they’re clams, not worms–are actually a scientific treasure. In fact, shipworms may be useful for projects as far flung as making stonewashed jeans and tracking global warming.

They’re also offering researchers the chance to study a unique bacterium-mollusk symbiotic relationship that enables the clams to digest wood as they tunnel through it.

Abetted by their symbionts, shipworms thrive wherever there’s wood and saltwater–in shallow harbors, floating logs, even shipwrecks at the bottom of the ocean. Only the coldest seas elude them. Although shipworms look like worms, they, like any good clam, have shells. Their shells are tiny, sharp vestiges that sit on their heads like helmets. The animals use this carapace as their wood-boring tool.

As tiny larvae, shipworms sneak into wood through pinprick-size holes. Once inside, they grow to between a fraction of an inch and several feet long, depending on the species and food availability. The clams dig tunnels along the wood’s grain, each one scraping out its pathway right up to its neighbor’s but with astonishing courtesy, never breaking in.

Shipworms wreck timber much as osteoporosis wrecks bone. Structural flaws develop gradually and invisibly, and the first sign of damage is often a devastating and unexpected break.

The only outward signs of shipworm intrusion into a piece of timber are the barely noticeable entrance holes. But stick a screwdriver into infested wood, and the wood crumbles like old paper.

“You can look at a piece of wood that looks solid and in fact is virtually a hollow honeycomb inside,” says marine biologist James T. Carlton of Williams College in Williamstown, Mass. “That’s why, on occasion, piers precipitously collapse when people didn’t even realize the shipworms were inside.”

Shipworms have also eaten the wood from shipwrecks, laments maritime historian and archeologist Warren C. Riess of the University of Maine in Orono. Shipworms have even devoured the hard teak decks of the sunken Titanic. The only exceptions are ships that descended deep into mud or went down in oxygen-deficient water like the Black Sea or fresh water like the Great Lakes (SN: 01/10/98, p. 24).

“We’re all jealous of the people who work in fresh water because they don’t have these problems,” Reiss says. “Their ships look like they just sank.”

Forgotten, not lost

Shipworms have been making a comeback along northern U.S. coasts and in other climes that have been either too cool or, ironically, too polluted for the animals.

“For years and years, pilings around the country were sitting in what amounted to raw sewage” that killed shipworms, Carlton says. Since the Clean Water Act of 1972, “wooden pilings that had been in the water for 100 years began to fall down,” he says.

In the past decade, New York has spent hundreds of millions of dollars fixing wooden pilings that support the city’s piers and even some of its bridges and roadways. In 1997, six people plunged into the East River when a 20-foot section of a Brooklyn pier collapsed unexpectedly from shipworm damage.

Shipworm populations have also been known to explode near the warm effluent of newly built nuclear power plants. Some scientists hold that shipworms are creeping northward as global warming slowly increases water temperatures and makes winters milder.

Consider the shipworms that started showing up in droves in the cold waters of Maine 3 years ago. These creatures hadn’t been a problem in Maine as long as most people could remember, making their outbreak seem quite mysterious, says Kevin J. Eckelbarger, a marine biologist at the University of Maine.

In the summer of 2000, Eckelbarger started getting calls from frantic harbormasters in the state saying that docks were collapsing. In Belfast, Maine, newly installed oak pilings 30 feet long and 10 inches thick were reduced to rubble in 7 months, Eckelbarger says.

The marine biologist and his colleagues discovered that the oak had been hollowed out by Teredo navalis, a type of shipworm common farther south in New England. The name is Latin for “to bore ships.”

Some scientists suggested that global warming might be to blame for the local shipworm eruption. While Eckelbarger agrees that climate change could be a factor, he says the evidence isn’t clear-cut. Octogenarian fishermen actually do remember shipworm problems in Maine in the 1950s, he says, suggesting that the shipworm infestations may have a natural cycle.

An excess of enticing, untreated wood may also be to blame, Eckelbarger adds. Creosote, the common but environmentally unfriendly wood preservative used through most of the 20th century, was banned in Maine 10 years ago. So, creosote-treated wood has simply been replaced by untreated wood in many places, Eckelbarger says.

“If you put raw, red oak in the ocean, you might as well put a chocolate sundae out for a 5-year-old,” he says.

Pick your poison

Wood preservatives are impregnated into wood under high pressures. Treated wood in turn slowly releases those toxins into the water, killing everything nearby and preventing shipworms and other animals that cause wood rot from getting a foothold. The same toxins that make wood preservatives effective also make them an environmental concern, however.

The two most widely used wood preservatives in the United States are creosote and chromated copper arsenate (CCA). Creosote, a derivative of coal tar that became an important wood treatment in the late 1800s and early 1900s, is now banned in some states because it contains many carcinogens and toxins. “This stuff is not good,” says marine biologist Michael J. Kennish of Rutgers University in New Brunswick, N.J.

CCA leaches out of wood at a slower rate than creosote does, but it discharges traces of arsenic, a particularly insidious carcinogen. Spurred by widespread public fear over CCA-treated wood in playgrounds, the Environmental Protection Agency recently announced a phase-out of CCA use in residential settings by 2004.

Although this ban doesn’t apply to marine pilings, CCA may eventually be prohibited altogether, says Dan L. Distel, a microbiologist at the University of Maine.

Producers of CCA in the United States have recently developed copper-based alternatives that don’t contain arsenic or chromium, CCA’s most toxic elements. Pier and boat builders are also seeking structural alternatives to chemically treated wood. These include cement, recycled plastics, and tropical hardwoods, such as greenheart, that are more resistant to shipworms than other woods are.

These more costly materials aren’t foolproof, however. Another type of clam bores into cement, piers made of recycled plastics can melt from heat and lightning, and the use of tropical woods contributes to the degradation of tropical rain forests.

Distel and other scientists are attempting to exploit the unique biology of shipworms to design less toxic, more shipworm-specific wood defenses. “Shipworms are not alone among the things that cause trouble,” Distel admits. “But they are one of the main deterrents to using wood.” They’re also one of the fastest-acting wood demolishers, he says.

Distel is particularly interested in a microscopic co-conspirator of the shipworm. Bacteria living in the shipworm’s gills secrete enzymes that permit the clam to survive as the only marine animal known to do so on a diet of wood alone.

Wood is made mostly of cellulose, a polymer of sugar molecules. As such, it’s an abundant source of energy, but most animals lack the enzyme cellulase, which breaks the bonds that lock the individual sugar molecules together. Shipworms have teamed up with bacteria that make cellulase, enabling them to get energy out of wood.

Sugars from wood, however, don’t provide an adequate diet even for a wormlike clam, Distel explains. “It’s like trying to live on cotton candy–lots of calories, but you don’t get the building blocks you need to make essential amino acids.”

The best-studied shipworm bacterium, Teredinibacter turnerae, makes not only cellulase but also a second enzyme useful to the clam. With this nitrogenase, the bacterium can turn nitrogen from the air into the basic ingredients of proteins, something else most animals can’t do. No other oxygen-dependent bacterium has this unique combination of cellulose-digesting, nitrogen-fixing skills.

T. turnerae was identified and isolated in 1983. John B. Waterbury of the Woods Hole (Mass.) Oceanographic Institution and the late Ruth D. Turner, the shipworm specialist for whom the bacterium was later named, led the discovery effort.

Bacterial symbionts could be the shipworm’s Achilles heel, some scientists propose. Distel says that if he or others could design a chemical to disrupt the association between shipworms and their bacteria or to disable T. turnerae‘s enzymes, shipworms could be controlled without wood-impregnating poisons.

Bug for all seasons

“Controlling shipworms is not our only motivation here,” Distel says. “We have other major motivations to understand these bacteria.” He, Waterbury, and biologist Scott M. Gallager of Woods Hole have been studying T. turnerae intermittently since its discovery almost 20 years ago. Their explorations have been limited by time and funding, but they all agree that the bacterium has enormous potential. “It’s a bug for all seasons. It does so many different things,” Gallager says.

For instance, the bacterium’s production of cellulase could be commercially important, Distel says. Most industrial cellulases currently come from fungi. Their greatest use is in producing stonewashed denim. By eating away the outer layer of cellulose-rich cotton fibers in jeans, cellulase releases dye. The enzyme is also becoming an increasingly common ingredient in household laundry detergents because it removes stains and frayed cotton fibers to make clothes look newer.

Novel properties of cellulases from the shipworm bacterium may lead to applications for which fungal enzymes aren’t well suited, Distel says. For instance, T. turnerae cellulase might be useful in producing biofuels. It might hasten the conversion of agricultural and paper-mill pulp waste, which are mostly cellulose, into sugars that can be fermented into fuels such as methanol or ethanol.

There may be special value to T. turnerae itself because it both cleaves cellulose and fixes nitrogen, Waterbury says. Cows’ diet of cellulose has to be supplemented with nitrogen, he explains. In the 1980s, the Department of Agriculture experimented with using T. turnerae to convert cellulose into nitrogen-rich cattle food, but efforts were stopped because of difficulties of working in saltwater. Waterbury thinks the application may still be viable, however.

Scientists are also studying the bacteria-shipworm liaison as a model for symbiotic and pathogenic animal-microbe interactions. For example, several scientists have been investigating how T. turnerae manages to infect shipworms. Do the shipworms pick up bacteria from the environment or pass them down from generation to generation in their eggs? Some data support each scenario. The answer may help scientists understand how other infections spread.

Bacterial symbionts enabled shipworms to evolve into animals that live in and eat wood–transforming them into economic and historic giants. “Otherwise,” Waterbury concludes, “they were just lousy clams.”

Shadow of History

Sailors have battled shipworms since ancient times

In the first known historical reference to shipworms, a Greek philosopher in 350 B.C. described them as a terrible plague for which there was little remedy. Historians say that both the Greeks and the Romans covered their wooden boats with lead, pitch, and tar to protect against shipworms. Even earlier, more than 3,000 years ago, the Phoenicians and Egyptians slathered their wooden boats in pitch and wax–probably to protect against shipworms, barnacles, and myriad other organisms that malign wood.

Shipworm devastation was rampant through the European age of exploration. During Columbus’ fourth voyage, which began in 1502, he was forced to stop in the Caribbean because his ships were too damaged by shipworms to continue. Shipworm appetites also helped Britain sink the Spanish Armada in 1588.

Shipworms hitchhiked all over the world on explorers’ ships. For 500 years, wooden ships opening the way to newfound seas were “Johnny Shipwormseeds” spreading wood-boring clams instead of apple trees, says James T. Carlton of Williams College in Williamstown, Mass.

Sixteenth- and 17th-century sailors tried just about everything to guard against shipworms. They coated their ships with a veritable stew of noxious and obstructing materials: heavy black tar, pitch, calfskin, cows’ hair, ashes, glue, moss, and charcoal. They also brought their boats to fresh or freezing water for a cure.

Salt-loving shipworms can seal themselves off from the deadly freshwater for only a few weeks. Freezing them brings a swifter death. Sailors would also burn the outer surface of their boats–a method that on occasion led to more damage than it prevented.

In the late 18th century, at great expense, the British Navy covered the bottoms of all its warships with copper plating, the most effective deterrent against shipworms at that time. Besides forming a barrier, copper leaks ions that are toxic to animals, including shipworm larvae. In the 19th century, copper alloys and paints replaced copper plating.

Given shipworms’ prominence throughout history, it’s amazing how little modern people know about them, Carlton remarks. He adds that if a ship captain from 1775 beamed himself into a present-day maritime museum, he’d be surprised at the lack of references to shipworms. The big issue for such an ancient mariner was shipworms. Carlton asserts, “They are the great shadow of maritime history.”