Honolulu Star-Advertiser © Susan Scott
November 18, 2005
Last weekend, I received an e-mail from a reader in New Jersey. “I have a wood bulkhead that is being eaten by shipworms,” wrote Tom.
His bulkhead is a breakwater on waterfront property, and he wonders how the shipworms get in and if there is a way to stop them from wrecking a new wooden wall.
Shipworms can be rascals but they can also be artists. One of my best beach-walking finds last year was a piece of wood so riddled with shipworm tunnels it barely held together. The half-inch-diameter tunnels wound through the wood so ornately, I brought the small log home and called it worm art.
Shipworms, however, are not worms. They are bivalves with two shells, such as clams, oysters and mussels. But there the resemblance ends.
A shipworm has a cigar-shaped body with a pair of razor-sharp shells at the head. Between these two shells lies a muscular foot that clamps down, holding the animal in place while the shells open, close and rock their way through wood.
Shipworms eat the fruits of their labor, digesting about 80 percent of the sawdust they produce while burrowing. To digest these wood particles, shipworms, like termites, have bacteria buddies in their stomachs. The bacteria turn sawdust into usable carbohydrates.
This carbo-loading leaves shipworms short on protein. Some species (there are more than 60) poke siphons out of their wood holes and feed on tiny passing animals. Others rely solely on their bacteria to convert nitrogen into protein. In return for their services, the bacteria eat some of the carbohydrates they make from the wood.
Fighting these characters is tough. Females cover and protect their eggs until the surrounding current brings them sperm cast out by males. Fertilized eggs grow under the continued protection of the female, eventually hatching into tiny larvae that swim off in search of wood.
When they find a suitable spot, such as a crack or indentation in the grain that allows them to hang on in the current, these little Pac-men begin boring.
Give them an inch and shipworms might take 6 feet, the longest tunnels shipworms build. Some species live for several years.
Shipworms have plagued people for as long as we have put wooden structures in the water. In the past, boat builders nailed large sheets of copper onto hulls to keep the shipworms out. Today, we apply copper-impregnated paint to submerged surfaces, which keeps shipworms and other members of the fouling community (like barnacles) from settling down.
When the paint wears off, boat owners haul out their boats and reapply the paint. Repainting bulkheads and pilings, however, requires removing or replacing them.
Shipworms can be a royal pain, but besides their artistic affinities, they’re crackerjack recyclers. These animals greatly reduce the number of logs, planks and other wooden junk drifting around the ocean.
For that, we offshore sailors, whose nightmare is hitting a log, love these little wood-eaters. Especially those of us with fiberglass boats.