The last time people thought very seriously about shipworms—people other than shipworm scientists, that is—was likely in the late nineteenth and early twentieth centuries, when wooden ships still reigned. A shipworm is in fact not a worm but a clam that looks like a worm and eats wood. The hinged shell of the typical bivalve has been reduced, in the shipworm, to a tiny pair of scrapers at the front end, which the animal uses for burrowing, like the tunnel-boring machines grinding away under Manhattan’s Second Avenue to extend the new subway line.
For tens of millions of years, shipworms were limited in their diet to floating logs and sunken stumps. Eventually dugouts, galleys, schooners, and frigates appeared as a side dish, and shipworms became notorious as the termites of the sea. Pliny and Ovid wrote about shipworms; the Romans coated the hulls of their vessels with tar and pitch as protection. In 1503, shipworm damage sank two of Christopher Columbus’s vessels and ended his fourth and final voyage to the Americas. “My ships were more riddled with holes than a honeycomb,” he wrote. Between 1919 and 1921, the naval shipworm destroyed nearly every wharf, pier, and ferry slip in San Francisco Bay. Shipworms are even thought to have helped the British, in 1588, defeat the Spanish Armada, which may have been severely compromised after spending months moored in the warm, shipworm-friendly harbors of Portugal, Spain, and southern France.
“My freshman microbiology professor liked to say, ‘If it wasn’t for shipworms, we’d be speaking Spanish today,’ ” Daniel Distel, a marine biologist and the director of the Ocean Genome Legacy Center, at Northeastern University, told me recently. Distel is a shipworm scientist, and earlier this week he and an international team of colleagues announced the discovery of live specimens of a five-foot-long shipworm called Kuphus polythalamia. The species is legendary; its existence has been known since the eighteenth century from fossil remains, but living ones have eluded scientists. Unlike other shipworms, Kuphus burrows into mud rather than wood. For protection, the animal builds a thick, chalky casing around itself, called a crypt. In the dissection video that was released with the study, it looks like a dark eel spilling from a long plaster cast.
A century ago, when shipworms were a common scourge, governments around the world paid scientists to study them. But the funding slowed with the advent of metal-hulled ships. “When I began, three decades ago, very few people were left working on shipworms,” Distel said. He spent the early part of his career researching the ghostly creatures that had recently been discovered at the bottom of the sea, around hydrothermal vents—giant clams, giant tube worms. They were able to thrive there, it turned out, because they harbored bacteria that leveraged the energy in hydrogen sulfide to generate nutrients, much as plants use the sun’s energy to turn carbon dioxide into carbohydrates. The vent creatures had internalized agriculture.
Distel wanted to learn more, but deep-sea research requires ships, submarines, and millions of dollars; plus, he said, “it was becoming a crowded field.” He found a more accessible animal in the shipworm. Its dietary staple—cellulose, an essential fibre in plants—is the most abundant biological material on Earth. Though it is loaded with energy, it isn’t easy to break down. Cows, goats, and termites manage the feat only because they carry bacteria in their digestive tracts that can do it for them. But with shipworms, Distel found, the bacteria live inside the animal’s own cells. “If you or I have bacteria in our cells, we’re sick, we need to go to the hospital right away,” he told me. “That’s true for most mammals.” But marine invertebrates, he said, have figured out a way to benefit from infection.
As Distel worked on his research, he kept an eye out for Kuphus, which had gained a reputation as the Loch Ness monster of malacology. “It’s always been obscure,” he said. “Obscure but known.” The hard casts of Kuphus, which can survive for millions of years, had turned up around the world, but the only description of the animal’s soft tissues came from a couple of deteriorating museum specimens from the nineteen-thirties. “I spent twenty years scouring the literature and not finding anything,” Distel said. He caught a lucky break in 2010, while on a project in the Philippines exploring the marine mollusk as a potential source of drug compounds. One day, a Filipino student showed him a news story, in Tagalog, about locals eating shipworms for medicinal reasons. An accompanying video clip showed an enormous organism in a tubular casement: Kuphus. “After all these years, we found it on YouTube,” Distel said. His colleagues collected several specimens from a shallow lagoon, though he would not say precisely where. “Those tubes—shell collectors do find them and sell them, and they get a pretty good price for them,” he said. “I don’t want to see those destroyed.”
Initially, the animal’s appearance was “a little shocking,” Distel said. “When it first came out of the tube, we had no idea what color it would be. Most shipworms are sort of beige, or whitish, maybe slightly pinkish. This thing comes out and it’s jet black, like gunmetal black, almost bluish black.” And the creature was far from delicate. “When we take other shipworms out of wood, eight times out of ten we break them in half,” Distel said. “This thing was a beast—solid, and quite slippery.” (When I asked him whether he had ever eaten shipworm, for health or pleasure, he said no. “I can’t get past all the bacteria that I know are in the environments where these animals live,” he told me. “Eating raw shellfish is hard for a microbiologist. Once, in a restaurant in Costa Rica, I ate a clam that I happened to be working on. I got the worst case of dysentery I’ve ever had in my life.”)
Kuphus’s innards were intriguing. The giant animal barely eats; its mouth, deep in the mud, is capped shut, and its digestive system is tiny, all but atrophied. At its hind end, two gilled siphons emerge into the lagoon like snorkels. The gills teem with sulfur-digesting bacteria, which are related to those found in deep-sea hydrothermal creatures. The bacteria essentially use the energy in swamp gas—the lagoon smells like rotten eggs—to make organic compounds for the shipworm, enough to enable the animal to grow to enormous size. Strangely, though, when Distel and his colleagues performed a genetic analysis of the symbionts, known for now as 2141T, they found that Kuphus acquired them relatively recently. Wood-boring shipworms first appeared on Earth about thirty-five million years ago, having evolved from wood-eating bivalves, which appeared at least a hundred million years ago. Many biologists assumed that Kuphus would be the primitive forerunner of both lines—the mud-dweller from which all others arose. In fact, though, it arose later; wood-eating bivalves came first, and somewhere along the way one acquired the sulfur-oxidizing bacteria.
Distel and his colleagues have a lot to learn about Kuphus, starting with the age of their own specimens. “Are they two years old, or two hundred?” he said. Also, how quickly do they grow? The mouth end of the shipworm’s tube, deep in the mud, is closed off. But Distel suspects that it must open from time to time so that the animal can extend itself and burrow deeper. “We think they grow in fits and starts, kind of like a lobster will shed its shell, form a new one, and grow into it,” Distel said. “But we don’t know for sure.” The team will also continue to look for smaller specimens, to shed light on the animal’s life cycle. Shipworms reproduce in all kinds of ways. Teredo navalis, the naval shipworm, is a protandrous hermaphrodite: it starts out male and turns female. Other species can fertilize themselves, and others still are live breeders, holding their young inside until a good food source is available. Some even have dwarf males, less than a millimetre in size, that stay attached to the females for their entire lives. “They’re sponges,” Distel said, not speaking literally. “They’re parasites that live off the females.”
Maybe more than any other creature, shipworms make plain that, in biology, everything is a vessel. Our bodies, the decaying trunks of trees, the gills of wormlike mollusks, even bacteria carry smaller forms of life somewhere, to do something. “All organisms are sort of like Russian dolls,” Distel said. “There are organisms within organisms; they’re in us and on us. Understanding how we became what we are isn’t just about studying our genes and their ancestry but also studying the bacteria that live along with us.” It’s shipworms all the way down, in other words, and it’s becoming ever harder to tell who is the ship and who is its commander.
