for National Geographic News
Flea-sized crustaceans with seven sets of legs, four moving mouth parts, and a voracious appetite for wood-borne bacteria could cause the edge of downtown Seattle, Washington, to slip into the Puget Sound.
Known as gribbles, the crustaceans have devoured portions of a wooden platform and supporting timbers designed to stabilize a steel-and-concrete seawall built along the Seattle waterfront in 1934. The wall allows deep-hulled ships to dock at the city's edge.
Thirteen feet (four meters) of dirt rests on top of the 60-foot (18-meter)-wide platform that helps serve as the foundation for the waterfront's bustling mix of restaurants, museums, warehouses, train tracks, streets, and a raised, double-deck freeway.
John Buswell, a senior engineer for the City of Seattle Department of Transportation, said that when the wall was built, engineers figured the steel, concrete, and dirt would isolate the platform and timbers in an oxygen-poor and water-deprived environment inaccessible to gribbles and other wood-boring creatures.
"But the theory on keeping them isolated from marine borers didn't prove to be the case," Buswell said. Over the years, saltwater corroded holes in the steel portions of the wall, allowing the soils behind it to get sucked out to sea as saltwater rushed in.
"This creates a little tunnel for marine borers to get to the timber and as long as there is a fresh exchange of seawater, they tend to survive," Buswell said.
The extent of the gribble damage manifested itself when the 6.8 magnitude Nisqually earthquake shook Seattle for about 45 seconds on February 28, 2001. The raised freeway, known as the Alaskan Way Viaduct, settled several inches.
When engineers dug down to investigate the platform, they discovered that sections of it were 100 percent gone. According to Buswell, engineering studies suggest another Nisqually-scale earthquake could cause the viaduct and seawall to completely fail.
To prevent such a catastrophe, the City of Seattle plans to re-build the seawall at an estimated cost of U.S. $700 million. Several billion more dollars will be spent to rebuild Alaskan Way.
It is well known that gribbles cause millions of dollars a year in damage to wooden piers, seawalls, and boats. But wood is an incidental part of the crustaceans' diet, according to Paul Boyle, director of the Wildlife Conservation Society's New York Aquarium.
Boyle, a gribble expert, said wood is a food that is rich in carbon but poor in nitrogen. Organisms need nitrogen to drive their metabolism, so a diet on wood alone is insufficient. The question he and other academics faced was how do the gribbles survive on a diet of wood.
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