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By Erik Baard

 

Today we celebrate the 14-legged messenger of estuarine recovery, the gribble.

 

Gribbles might look like Pikachu  (the preview box doesn’t work for this link, but clicking it will lead to an image) but countless of these speck-sized crustaceans (photo by Seattle Department of Transportation, click to enlarge)are devouring piers throughout New York Harbor, forcing reconstruction at a cost of millions upon millions of dollars. And in forgotten little historic corners of our once-working harbor, derelict barge-to-rail lifts, docks, and railroad ties disintegrate into their four-part maws.

 

Gribbles (of which there are 56 species) and shipworms, another woodborer, were absent from large swaths of the harbor for generations because pollution levels were too high, and oxygen levels too low, for them to survive. Indeed, in a truly Orwellian twist of language, ours was known to sailors as a “clean harbor” because local waters were such a toxic broth that unwanted critters would die off of ships.

Now we appreciate that the gribbles’ return heralds the ecological recovery of our vital waterways.

 

For a long time the logic of the gribble diet was a mystery: gribbles lack the nitrogen-fixing bacteria in their bellies that symbiotically allow termites to survive on the carbon riches of wood. In short, they were endlessly gnawing through wood that provided little nutrition or energy to them. The secret, it turns out, is geometry. Old wood in seawater, like our harbor estuary, has a slimy surface. That, scientists have discovered, is the true diet of a gribble. And by tunneling through wood, and having succeeding generations of gribbles widen those tunnels, gribbles dramatically increase the surface area on which the nitrogen-rich microbial slimes can grow. A feast for generations of gribbles to come. What’s even weirder is that microbes cling to gribble’s excretory plates, called pleopods, to catch nitrogen bound in waste ammonium. It’s a partially self-perpetuating cycle.

 

So when you see those riddled hunks of rotten wood break away from a pier, think “gribble farm.”

 

Humanity may have played an important role in transporting gribbles aboard wooden ships, but the species has been thriving long before the invention of the simple raft. In terms of ecological niche, the crustaceans take the place occupied by fungi on land, converting the tough material of dead wood into a mush that is a rich base for new growth. Over time, natural driftwood and unanchored seaweed (gribbles munch down the holdfasts that anchor seaweeds) can travel thousands of miles. One might argue that our acceleration of this process and the increase of volume but not diversity of wood (only a few types make useful keel lumber) might have reduced the biodiversity of gribbles. And our marine construction techniques might also be introducing an artificial selection: one species, L. tripunctata, can take down piers treated with the toxic wood preservative creosote (a carcinogen for humans) through a symbiotic partnership with bacteria that degrade the coal-derived chemical.

 

One question I’d like to explore is whether gribbles limit the width of their tunnels, or at least entrances, to the diameter of their pleotelsons, a rear end plate that they can use as a defensive barrier. Or does their burrowing cause the home log to crumble before any particular tunnel is likely to reach such widths? Or has a communal effort to adjoin pleotelsons been observed?

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