Zebra Mussel (Dreissena polymorpha)

The zebra mussel costs North America over $1 billion annually, a financial impact that dwarfs #3's ecological damage. These fingernail-sized molluscs from the Caspian Sea clog water intake pipes with colonies of up to 700,000 per square meter, choking infrastructure. They smother native mussels, reducing their populations by 95% in invaded lakes, and filter plankton so efficiently that they rob fish of food. Their economic toll is 50% higher than the average invasive species, with Great Lakes communities spending $500 million yearly on control. This outcompetes #3's python, which focuses on mammals, as zebra mussels reshape entire aquatic food webs.