Scallops are a cosmopolitan family of bivalves, found in all of the world's oceans, though never in freshwater. They are one of very few groups of bivalves to be primarily "free-living"; many species are capable of rapidly swimming short distances and even of migrating some distance across the ocean floor. A small minority of scallop species live cemented to rocky substrates as adults, while others are more simply attached by means of a filament they secrete called a byssal thread.