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The Zebra Mussel (Dreissena polymorpha)

Since 2011 is already shaping up rather grimly as the year of mass animal die-offs, it is time to revile the black-hearted invasive species which caused some of the the worst mass bird die-off in recent history.  I’m talking about the detested zebra mussel  an inch-long filter feeding bivalve mollusk with a pattern of brown zigzags on its shell.  The freshwater zebra mussel isn’t really that closely related to the marine mussels but shares many features with the Venus clams.

I am not too surprised if you feel ripped off that this dangerous invasive animal is a tiny shellfish.  But don’t dismiss the zebra mussel because of its diminutive size.  Zebra mussels are believed to be the source of deadly avian botulism poisoning that has killed immense numbers of birds in the Great Lakes since the late 1990s.  Additionally US powerplants and boat owners spend half a billion dollars a year scraping the creatures off water intakes for power plants and other underwater equipment.  The mollusks are also rather sharp and can injure wader’s feet–which necessitates wearing shoes in affected waterways.

Originally natives of the Black Sea and Caspian Sea, Zebra mussels spread via canal to the rest of continental Europe and then stowed away in fresh-water ballast and on anchor chains of ocean going boats to travel across the ocean to Great Britain, Ireland, Canada and the United States. The Great Lakes are being hit especially hard by them.  Whenever they spread to a new location the native freshwater mollusks are gravely impacted.

Zebra mussels on a Lake Erie beach (photo from Great Lakes Environmental Research Lab).

Of course the mussels’ impact is not entirely negative.  To quote the National Atlas of the United States (which, by-the-way is an interesting geography resource):

Zebra mussels do have a positive impact on some native species. Many native fish, birds, and other animals eat young and adult zebra mussels. Migratory ducks have changed their flight patterns in response to zebra mussel colonies. Lake sturgeon feed heavily on zebra mussels, as do yellow perch, freshwater drum, catfish, and sunfish. The increase in aquatic plants due to increased water clarity provides excellent nursery areas for young fish and other animals, leading to increases in smallmouth bass populations in Lake St. Clair and the Huron River. However, these native species do not feed heavily enough on zebra mussels to keep the populations under control.

We might as well enjoy the smallmouth bass and the clear water.  Nothing people have done has halted or even impacted the spread of the zebra mussels so it looks like we’ll have to learn to live with them.

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