Byssus

A mussel (genus Mytilus), attached to a rock by its byssus
Illustration of the byssus of Dreissena polymorpha, the freshwater zebra mussel

A byssus (/ˈbɪsəs/) is a bundle of filaments secreted by many species of bivalve mollusc that function to attach the mollusc to a solid surface. Species from several families of clams have a byssus, including pen shells (Pinnidae), true mussels (Mytilidae), and Dreissenidae.