A byssus is a group of strong filaments that are secreted by some families of clams (bivalve molluscs), 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.
History of the word 
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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.
- 1. "An exceedingly fine and valuable textile fibre and fabric known to the ancients; apparently the word was used, or misused, of various substances, linen, cotton, and silk, but it denoted properly (as shown by recent microscopic examination of mummy-cloths, which according to Herodotus were made of βύσσος) a kind of flax, and hence is appropriately translated in the English Bible 'fine linen'." (1398)
- †2. "A name formerly given to filamentous fungoid growths of different kinds, which are now more accurately classified. Obs." (1753)
- 3. "Zool. The tuft of fine silky filaments by which molluscs of the genus Pinna and various mussels attach themselves to the surface of rocks; it is secreted by the byssus-gland in the foot. 'These filaments have been spun, and made into small articles of apparel.. Their colour ranges from golden yellow to rich brown; they also are very durable.. The fabric is so fine that a pair of stockings may be put in an ordinary-sized snuff-box'" [William Beck's 1886 The Draper's Dictionary]. (1836)
- 4. "Bot. 'The thread-like stipe of some fungi'." (1866)
- †5. "A name formerly given to asbestos." (1864)
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".
The fabric 
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.
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.
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.
See also 
- Oxford English Dictionary, 2nd ed., Oxford University Press, 1989.
- Quoting A Greek-English Lexicon. The OED2 also quotes Wilhelm Gesenius that būts applied to "the finest and most precious stuffs, as worn by kings, priests, and persons of high rank or honour".
- For example, Byssus herbarium is an old synonym for Cladosporium herbarum "a fungal plant pathogen".
- Translation of the Greek section of the Rosetta Stone.
- Maeder, Felicitas (private communication 14 Feb., 2011) says that the "byssus cloth" used on mummies was certainly fine linen and the mention of "byssus cloth" on the Rosetta stone quite possibly also refers to linen.
- Robert L. Strausberg, et. al. (December 31, 1989). Development of a Microbial System for Production of Mussel Adhesive Protein. doi:10.1021/bk-1989-0385.ch032.
- Ecsedy, Hilda 1975. "Böz – An Exotic Cloth in the Chinese Imperial Court." Hilda Ecsedy. Altorientalische Forschungen 3: pp. 145–153.
- Starr, Cecie and Taggart, Ralph. Biology: The Unity and Diversity of Life. Belmont, CA: Thomson Learning, Inc., 2004.
- Hill, John E. (2009) Through the Jade Gate to Rome: A Study of the Silk Routes during the Later Han Dynasty, 1st to 2nd Centuries CE. BookSurge, Charleston, South Carolina. ISBN 978-1-4392-2134-1. See Section 12 plus "Appendix B - Sea Silk."
- Hill, John E. 2004. The Peoples of the West. A draft annotated translation of the 3rd century Weilüe - see Section 12 of the text and Appendix D. 
- McKinley, Daniel L. 1988. "Pinna and Her Silken Beard: A Foray Into Historical Misappropriations". Ars Textrina: A Journal of Textiles and Costumes, Vol. Twenty-nine, June, 1998, Winnipeg, Canada. pp. 9–223.
- Maeder, Felicitas 2002. "The project Sea-silk – Rediscovering an Ancient Textile Material." Archaeological Textiles Newsletter, Number 35, Autumn 2002, pp. 8–11.
- Maeder, Felicitas, Hänggi, Ambros and Wunderlin, Dominik, Eds. 2004. Bisso marino : Fili d’oro dal fondo del mare – Muschelseide : Goldene Fäden vom Meeresgrund. Naturhistoriches Museum and Museum der Kulturen, Basel, Switzerland. (In Italian and German).
- Turner, Ruth D. and Rosewater, Joseph 1958. "The Family Pinnidae in the Western Atlantic" Johnsonia, Vol. 3 No. 38, June 28, 1958, pp. 285–326.