Mollusks (Phylum Mollusca) are found in marine, brackish, and fresh waters. They include a diverse group of animals such as clams, mussels, oysters, scallops, abalone, conchs, shipworms, snails, nudibranchs, chitons, squids, and octopuses. Common methods of introduction include ballast water introductions, aquarium releases, and accidental release from aquaculture facilities. Displacement by competition is the most frequently observed impact on native species. The most notable nonindigenous mussel introduction is the zebra mussel (Dreissena polymorpha), a native of eastern Europe.