On the Great Lakes, people often see clusters of small mussels stuck to docks, rocks, and boats. These organisms, zebra mussels, don't belong there, and they do much harm. Readers of this book, an important addition to any science library, will learn how zebra mussels came to these freshwater lakes and how they act as an invasive species to ecosystems. Full-color photographs, fact boxes, and a quiz complement the accessible content, which addresses several standards of elementary STEM curricula.