VIEW THIS EMAIL AS A WEBPAGE >>

26 June 2024 — Shipworms: Maybe Marine Borers Aren’t That Boring?



Just this month the Washington Post ran a news item I really hadn’t expected: scientists had made a new discovery about how shipworms digest wood. The latest study of Teredo navalis, one of the most common varieties of shipworm, had revealed that symbiotic bacteria located in a section of their intestines called a typhlosole—researcher Reuben Shipway said that this little sub-organ looked “like Salvador Dali’s mustache upside down”—were responsible for the production of enzymes capable of digesting lignin, the toughest part of the wood they so love to eat. The chemical process used to convert wood to energy is of interest to scientists exploring topics like carbon conversion and biomass fuels. Who would have expected so much promise in a creature that has mostly been the bane of the maritime world?

line drawing of worm-shaped organism

Teredo navalis. Image: PD, source: Freshwater and Marine Image Bank

Because yes, shipworms have been viewed mostly as an expensive problem in the seafaring community. Just google the world “shipworm,” and the first pictures you will see will be huge pieces of wood, some of them that ought to have been sturdy pilings or crucial parts of a ship’s hull, riddled with parallel tunnels and therefore terribly weakened. Thus structurally compromised, ships and wooden structures like docks and pilings can become very dangerous. How can these smallish translucent, “slurpy” creatures cause so much harm? 

fragment of wood bored through with hundreds of worm tunnels

This piece of wood, seen on a beach in North Ayrshire, Scotland, gives us a pretty good idea of just how much damage shipworms can do on wooden structures. Photo: Creative Commons Attribution-Share Alike 3.0 license

The shipworm is actually a bivalve, not a worm, but it's a bivalve that got a little “inside out.” Whereas the clam, for example, wears its shell on the outside for protection, the shipworm wears its tiny shell on one end of its body near the mouth, and uses it to scrape away at wood, digesting the little scrapings as it goes. Eventually waste products are expelled through one of two siphons at its opposite end. The shipworm inches forward as its scraping creates a tunnel, and the longer it eats, the longer the tunnel becomes. When multiple worms invade, it doesn’t take long for damage to get dangerous and expensive. 

drawing of tunnel workers and tunneling frame

In a letter, engineer Marc Brunel (father of Isambarde Kingdom Brunel) described how the lowly shipworm inspired him in his plan for creating the ambitious Thames Tunnel: “I happened to see before me a piece of condemned timber, a portion of the keel of a ship, wherein the sea-worm… had made many erosions, even near the water edge…. I then said to myself these little things have made little tunnels, so might we, by adopting some corresponding means of protection….” Brunel then designed a tunneling shield to support the reinforcement of tunnel walls as the tunnel was being dug out. Drawing of the tunneling shield in action from Sketches of the Works for the Tunnel Under the Thames, from Rotherhithe to Wapping, by Robert Cruikshank.

We know from ancient literature that saltwater-loving shipworms have been a problem since humans took to the seas in wooden vessels. Some early vessels were coated with tar or wax to hinder shipworms’ access to the wood; a later method was to char the hull and then seal it with tar. In the late 18th century a common practice was to mix tar with hair, paper, or felt, and apply it to a ship’s hull and then add a half-inch-thick wooden sheath—“sacrificial planking,” which would have to be replaced every few years. The evolution of ship construction was accompanied by experimentation with coatings and coverings to protect the wood from shipworm and its companion scourge, marine growth such as barnacles and mussels, that would impede a ship’s progress through the water. Lead, used in ancient times and resumed in the Age of Exploration, was heavy and accelerated the corrosion of iron fittings. Copper, whose oxidation created a surface toxic to marine life, was expensive but effective. The more impurities in the sheathing, however, the less effective it was. Metal roller George Frederick Muntz patented a sheathing alloy of copper, zinc, and trace iron in 1832; it was judged to be more durable, easier to work with, and cheaper than the pure metal. His alloy was called Muntz’s metal, or yellow metal. The historic ship Cutty Sark was originally sheathed in yellow metal; today a similar, more readily available alloy protects her timbers. Zinc was sometimes also used as an alternative to copper. Today, hulls made of fiberglass or metals such as iron or steel aren’t a target for shipworm; wooden hulls are typically painted with anti-fouling paint.  

San Salvador is a replica ship modeled after the ship of the same name that brought explorer Juan Rodriguez Cabrillo to the waters of San Diego in 1542, the first recorded European vessel to sail along Southern California. She was added to the museum fleet in 2015. The threat of shipworms requires diligent maintenance to preserve this important ship, and those like her. Photo: Mark Albertazzi.

I reached out to Ray Ashley, President and CEO of the Maritime Museum of San Diego to ask about the battle against shipworms today; the museum’s fleet of historic vessels includes four with wooden hulls—Californian, Surprise, the 1914 Pilot, and San Salvador—that must be protected from wood borers. He described the museum’s process:



Teredo is certainly a problem in our area and the wooden pilings right next to our ships are infested with teredo. Our only means of protecting the wooden ships is bottom paint, haul outs at two year intervals, and trying our best to keep on top of leaks. Why leaks? We’ve found that even small leaks can facilitate the introduction of teredo larvae into the internal areas of the ship where there are areas like moist seams and deep bilges which usually retain some water even when pumped out but which are not protected by bottom paint. The larvae then establish residence, mature, and go to work. Difficult external areas include the space between rudder posts and stern posts where it’s hard to get a good coat of bottom paint into the narrow gap or anywhere else that wood might be difficult to get paint on to but yet exposed to sea water.

soup bowl filled with shipworms and broth

Wood-boring worms are a popular food in some parts of the world. Image: Shankar S. via Wikimedia Commons

If You Can’t Beat ’Em—Eat ’Em!


Shipworms are a major headache for folks caring for wooden ships and docks, but they show a lot of promise as a sustainable food source. They are already considered a tasty treat in parts of Southeast Asia like the Philippines, where they are enjoyed both raw and cooked. Afficionados will remind us that these creatures aren’t really worms, they’re bivalves, and that snacking on them is hardly different from enjoying their bivalve cousins, clams, oysters, and mussels. Proponents of more sustainable foods want to see these creatures evolve from local delicacy to worldwide staple, as they are fast-growing and easy to process, and they are loaded with protein and vitamin B12. A team led by Dr. David Willer, a Fellow of Murray Edwards College, Cambridge, created the world’s first system of farming the terednids, housing them in salt-water tanks on land and feeding them with waste wood, such as scraps from the foresting industry. The fact that the tanks are an enclosed system allows growers to keep out contaminants and also to manipulate the food sources of wood and algae to tweak the end product’s taste and nutritional value. Perhaps the team’s boldest move was to re-brand the little critters: Goodbye, “shipworms,” say hello to “naked clams,” coming soon to a restaurant near you. 



Extra Credit


Naked Clams to open a new sector in sustainable nutritious food production


Animals in Sea History: Columbus’s Worms



 


Sea History Today is written by Shelley Reid, NMHS senior staff writer. Past issues can be read online by clicking here.

Facebook  Twitter  Instagram  YouTube