Mussels are several families of bivalve molluscs. Mussels live in lakes, rivers, and creeks. They sometimes also live in intertidal areas along coastlines worldwide. The freshwater mussels (several allied families, the largest being the Unionidae) and saltwater mussels (family Mytilidae) are not closely related, despite the fact that they may look similar. They are grouped in different subclasses. The freshwater Zebra mussels and their relatives (family Dreissenidae) live attached to rocks in a manner similar to marine mussels, but are classified with the Heterodonta, the taxonomic group including most bivalves referred to as "clams".