Luckily they were dead (so they think).
Zebra mussels are a huge threat to the water resources in the western United States. Each female mussel can produce tens of thousands of offspring each year. These attached to and inside things. First found in the Great Lakes, they rapidly wreaked havoc on the environment there. Impacts on humans were first noted, they clogged the cooling systems of nuclear power plants, sunk the navigation buoys in the shipping lanes. closed drinking water intakes for cities like Cleveland and you can no longer walk on the beach there barefoot without cutting your feet to ribbons. As they are filter feeders, they alter the environment also. Within four years of introduction to Lake Erie, scientists figure they were filtering the entire volume of the lake every four days. This crashed whole food chains.
An alert Oregon state employee saw a houseboat being trailered on I-5 recently and reported it. They found it about 300 miles later in Washington State. This one happened to be seen, how many have not? For more info:
http://blog.oregonlive.com/breakingnews/2007/05/ (scroll down)