Tonnidae is a family of medium-sized to very large sea snails, known as the tun shells. These are marine gastropod molluscs in the clade Littorinimorpha. The name "tun" refers to the snails shell shape which resembles wine casks known as "tuns". While the shells are thin, they are also strong. There is no operculum. They are found in all tropical seas, where they inhabit sandy areas.