"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.
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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