Byssus

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 the pen shells, the true mussels and the false mussels: the Pinnidae, the Mytilidae and the Dreissenidae.

Byssus cloth is a rare fabric, also known as sea silk, that is made using the byssus of pen shells as the fiber source.

The filaments

Byssus filaments are created by certain kinds of marine and freshwater bivalve molluscs, which use the byssus to attach themselves to rocks, substrates, or sea beds. In edible mussels, the inedible byssus is commonly known as the "beard", and is removed before cooking.

Byssus often refers to the long, fine, silky threads secreted by the large Mediterranean pen shell, Pinna nobilis. The byssus threads from this Pinna species can be up to 6 cm in length and have historically been made into cloth.

Many species of mussels secrete byssus threads to anchor themselves to surfaces, with Families including the Arcidae, Mytilidae, Anomiidae, Pinnidae, Pectinidae, Dreissenidae, and Unionidae.