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A byssus is a group of strong filaments that are secreted by some families of clams (bivalve mollusks), in order to attach themselves to hard surfaces. Well known for possessing a byssus are several families of clams, including the pen shells, the true mussels and the false mussels: the Pinnidae, the Mytilidae and the Dreissenidae.
The phrase "byssus cloth" is also used to mean a rare fabric also known as sea silk, which is made using the byssus of pen shells as a fiber source.
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The English term byssus or bissus (plural byssuses or byssi) originated in John Trevisa's translation (1398) of Bartholomeus Anglicus's Latin On the Properties of Things (1240), referring to finest white flax from Egypt. From originally meaning 'fine linen', the semantics of byssus gradually expanded.
The etymology of byssus, according to the OED2, began with Biblical Hebrew būts or butz בוץ meaning 'a fibre or fabric distinguished for its whiteness', cognate with Aramaic ܒܘܫ bus and Arabic باض bāḍa 'to be white'. This Hebrew term was translated as Latin byssus and Greek βύσσος "a fine yellowish flax, and the linen made from it, but in later writers taken for cotton, also silk, which was supposed to be a kind of cotton".
The OED2 lists five meanings of byssus in historical order (with dates of earliest references). Note that "†" denotes obsolete meanings.
Excluding the botanical meaning (4) "rhizomorphs or grouped hyphae of certain fungi that roughly resemble a bivalve shell" and the obsolete (2) "fungoid filament" and (5) "asbestos", byssus refers to (1) "a rare fabric produced in ancient Mediterranean cultures" which is manufactured from (3) "a thread-like mollusc filament".
Byssus cloth or sea silk is an exceptionally fine and valuable fabric from ancient times, usually made from the byssus of molluscs.
The Greek text of the (196 BCE) Rosetta Stone records that Ptolemy V reduced taxes on priests, including one paid in byssus cloth, usually translated as "fine linen cloth". In ancient Egyptian burial customs, byssus cloth was used to wrap mummies.
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.
When a mussel's foot encounters a crevice, it creates a vacuum chamber by forcing out the air and arching up, similar to a plumber's plunger unclogging a drain. The byssus, which is made of keratin, quinone-tanned proteins (polyphenolic proteins), and other proteins, is spewed into this chamber in liquid form, and bubbles into a sticky foam. By curling its foot into a tube and pumping the foam, the mussel produces sticky threads about the size of a human hair. The mussel then varnishes the threads with another protein, resulting in an adhesive.
Byssus is a remarkable adhesive, one that is neither degraded nor deformed by water, as are synthetic adhesives. This property has spurred genetic engineers to insert mussel DNA into yeast cells for translating the genes into the appropriate proteins.