The Lowly Clam That Almost Sank Two of Columbus’ Boats

If your boat looks like this, you're up shit creek without a paddle, so...hopefully that thing has an engine?

It was the year 1731, and the Dutch economy was at the mercy of a clam. But not just any clam, mind you—give the Dutch some credit. This was a true oddity, or at the very least an eccentric. While your typical clam lives a life of security in its shell, the so-called shipworm has a tiny calciferous casing, from which erupts a skinny tube of flesh up to two feet long. Swarms of the things burrow into wood, be it ship hulls or piers or, unfortunately for the Dutch, wooden dikes.

The strange invertebrate had destroyed over 30 miles of Dutch dikes, and threatened another dozen miles. The Netherlands were in serious danger of flooding, and the economy was spooked. Peasants fled their farms and entered the cities, where they could find no jobs. Crews tried switching the wood out for tropical hardwood, to no effect. The only solution was to import stones and build dikes that way, at no small taxpayer expense.

It wasn’t the first time the shipworm had menaced humans, and it wouldn’t be the last. The things had already terrorized the ancient Greek and Roman navies, and ate away so badly at two of Columbus’ ships on his fourth voyage to the Caribbean that he had to abandon the vessels. Later on in the 1910s, a shipworm infestation hit the docks of San Francisco Bay, causing what would today be billions of dollars in damages.

But have shipworms ever actually sunk a boat out at sea? “More or less,” laughs molecular ecologist Luísa Borges of Germany’s Helmholtz Center for Material and Coastal Research. “If you believe in the historical reports, apparently they sunk quite a lot of ships, when they undertook these long voyages.” The loss of several vessels in the Spanish Armada, for instance, have been blamed on shipworms, which the Brits no doubt appreciated.

Barking Up the Wrong Tree

The first question to ask here isn’t how the shipworm manages to bore into wood—and in such numbers that it can threaten national economies, for that matter—but how the hell a marine creature could evolve to rely on a terrestrial material in the first place. Shipworms subsisting on trees that happen to wash out to sea is a bit like you trying to survive solely on dead things that wash ashore.

Shipworms are tireless advocates of recycling—as long as the material is wood, and they’re eating it. They’re somewhat meh about plastic.

But there are marine habitats where plenty of wood grows right in the saltwater: mangrove forests. Mangrove trees are incredible survivors. Saltwater would kill any other kind of tree, but the mangrove’s roots can filter out the salt, and some species have roots that grow vertically up out of the water to take in more oxygen. It was among these trees that the first shipworms likely evolved. But now, thanks to the global economy, the clams have hitched rides to coastal cities all over the world.

The shipworm’s life cycle begins as a larva that settles onto wood, be it a mangrove root or pier or ship. While the adult clam’s shell covers only a tiny fraction of its body, the larva looks like a regular bivalve. It begins drilling a tiny hole into the wood, either by releasing enzymes to break down the tissue or by rasping away with its shell, or a combination of the two methods. “So it’s mechanical, most of it,” says Borges. “But some people think there might be some enzymatic action there too. It’s not entirely known.”

“The animal has a very strong foot,” she adds, “as most bivalves and gastropods and so on have. And it uses it like a suck, and moves the shell sideways and uses it as a rasp and file device.”

Now, if you’re anything like me, you’ve gone your whole life without ever licking a two-by-four. But it’s safe to say that mucousy flesh does not get along well with the abrasiveness of wood, so this leaves the shipworm with a problem: As it grows and elongates and continues to bore deeper into the wood, the shipworm risks stripping its own delicate flesh away. To avoid this, it excretes a calcareous lining—the same calcium carbonate material that makes up its shell—along its worm-like body. Pull a shipworm out of some wood and it’ll leave the calcareous tube behind.

The business end of a shipworm. And business is booming.

Gut Instincts

Now, if you’re anything like me, you’ve also gone your whole life without eating mangrove trees. It’s safe to say that they’re indigestible for humans, but not for the shipworm. It can digest wood just fine with the help of some very special symbiotic bacteria. But here’s the strange thing: That bacteria doesn’t live in the shipworm’s gut. Instead, it lives in the gills and produces enzymes that make their way to the gut, where they break down the wood’s cellulose into sugars. Plenty of creatures use bacteria like this to digest weird foods, but the shipworm is alone in storing the microbes outside of its digestive system.

A diet of wood, though, leaves the shipworm wanting for protein. So out of that tiny hole it first bored when it was a larva, the shipworm extends fan-like structures that it uses to catch plankton floating by. “So imagine that you have a lot of shipworms in a piece of wood, and they don’t have much wood to eat,” says Borges. “They actually filter-feed more than they eat wood. Otherwise they destroy their own environment. So they can kind of adjust their diet to survive.”

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