Seattle Waterfront Falling to Gribble Invasion

John Roach
for National Geographic News
April 23, 2004
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.

Gribble Science

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.

"If gribbles bore in on wood for protection from predators like crabs, why do they continue to bore and eventually cause the wood to fall apart? It seems counter productive," Boyle said.

One theory was that the gribbles, like some termites, have nitrogen-fixing bacteria living in their guts, making a need for an external nitrogen source unimportant and thus wood a good source of nutrition. But when Boyle and his colleagues examined gribbles' guts they found no bacteria.

So the researches looked at the wood itself and there they found the answer: wood-borne microbes chock full of nutritious nitrogen. By burrowing through the wood, the gribbles "are creating a massive increase in surface area and that surface area is available to microorganisms," Boyle said.

When the gribbles come back to older burrows, they graze on these colonies of microorganisms. As the gribbles graze, they widen the burrow, further increasing the surface area. Eventually, this process breaks down all the wood.

Gribbles also pass their nitrogen waste as ammonium through plate-like appendages called pleopods, Boyle said. Bacteria hang out on the gribbles' pleopods and capture the ammonium the moment it is released, making for "a closed nitrogen loop," Boyle said.

While this incidental wood-devouring life is a nuisance to port cities, Boyle said that the gribbles have carved out a niche as one of the few organisms that break down the woody debris that floats down rivers and into the seas. Fungi, which break down wood on land, do not thrive well in saltwater.

Gribble Defense

With their niche so nicely carved out, gribbles have been a scourge to seafaring communities for centuries, boring through ships and docks with abandon, at times bringing commerce and exploration to a halt.

Christopher Columbus once had to delay a return to Europe because gribbles had rendered his ships un-seaworthy. When National Geographic Explorer-in-Residence Robert Ballard discovered the Titanic in 1985, gribbles had consumed all the exposed wood.

Today, most piers and boats are constructed with materials that are resistant to the borers, such as concrete in the case of piers. "The technology has moved in a direction to have wood be a sacrificial facing on more substantive structures," Boyle said.

Buswell, the City of Seattle Department of Transportation engineer, said that one possible alternative for rebuilding the seawall and Alaskan Way Viaduct in Seattle is to bore a tunnel through concrete reinforced seawall soils. "The gribbles will have to find somewhere else to eat," Buswell said.

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