The Unwelcome Cousin
The thumbnail-sized zebra mussel, Dreissena polymorpha, is invading America. It is believed to have arrived from its native Eastern Europe in the 1980s by hitching a ride in the ballast of a foreign ship to the Great Lakes.
These inch-long mussels are D-shaped and, as their name suggests, have yellow and brown stripes. They accumulate in huge numbers by attaching themselves to almost any surface with threads (called byssal threads) just as our blue marine mussels attach to rocks. They can clog canals, irrigators, sprinkler systems, and have even clogged four-inch pipes. They will also attach themselves to other mussels in great numbers and compete for the same food drifting in the rivers. In the Pacific Northwest, there is concern that they could attach to fish ladders, screens, and hatcheries. Their shells are very sharp and could injure the passing fish, they could build up and disrupt water flow, and ultimately, they could threaten salmon recovery efforts.
Zebra mussels reproduce faster and more efficiently than our natives because their larvae are free swimming and do not need a host fish to grow to adult stage. They live in such large numbers that they also attach themselves to drifting aquatic plants—an ability that can contribute to their spread. When the plant dies and sinks to the bottom, the mussel detaches itself and moves to the bottom.
Zebra mussels have not been seen in Puget Sound yet. But in only 10 years the species infested the Great Lakes, the Mississippi River, and is now a problem in eastern states. They reproduce in large numbers, can live on almost any surface, and are therefore very successful at inhabiting US waters. There are no known natural enemies of the zebra mussel in the US at this time and once they are established, they are hard to eradicate or control.