The Zebra Mussel originally inhabited central and eastern Europe but by 100 years ago it had spread throughout the continent, abetted by the interlacing canal systems connecting river drainages.
The first find in North America was in Lake St. Clair just upstream from our own Lake Erie. Most likely they arrived in freshwater ballast; evidently a freighter loaded fresh water in Europe and steamed over to Canada and up the St. Lawrence Seaway before emptying its ballast tanks somewhere near Detroit. (Captains are supposed to see that ballast tanks are flushed with saltwater on the way over but apparently not everyone does this.) Within a few years the mussels had spread to at least 19 states and Ontario. Much of this dispersal is due to the mussels’ attaching themselves to hard surfaces including the undersides of ships and boats.
Zebra Mussel young (“larvae”) are free-floating before they settle down and start forming a shell. Rocks and shells of native mussels are other favorite attachment sites but shipwrecks, concrete breakwalls and water intakes will also suffice. (I saw my first “wild” Ohio Zebra Mussel on a marker buoy at Huron in 1989.)
Zebra Mussels may live up to 5 years and a single female may lay several million eggs during that time. Zebra Mussels are filter feeders, straining microorganisms (mostly algae) from over a gallon of water per mussel per week. Thus for the last two decades gazillions of mussels have been busily filtering Lake Erie and this has definitely changed the ecosystem (which partly defines a true “invasive”). The water is clearer, meaning sunlight can penetrate further and algae can grow at greater depths, replenishing oxygen critical to bottom feeding insects such as mayflies, which have returned to high numbers not seen since the 1950s.
The clearer water must make it easier for fish-eating birds to see their prey and this may contribute to the cormorant population explosion around Lake Erie. Most diving ducks love them (mussels, not cormorants).
There was much initial concern that Zebra Mussels could clog water intakes along the lakeshore but apparently technological developments have kept pace with that. The greatest conservation concern seems
to be the tendency of the Zebra Mussel to attach to the shells of live native mussels, many of which are already threatened or endangered (due to water pollution). Up to 10,000 Zebra Mussels have been found attached to a single shell of a (large) native mussel. Populations of native mussels inevitably decline once the Zebra Mussel is established.
What can you do? You can become informed (starting with http://www.fws.gov/midwest/endangered/clams/zebra.html). And if you have a boat, or know someone with a boat, see that the hull is completely cleaned off before moving it from one body of water to another, especially if you cross a divide between river drainage systems. (Live Zebra Mussels have been found attached to boats crossing from Nevada into California.)