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A byssus () 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.
Many species of mussels secrete byssus threads to anchor themselves to surfaces, with families including the Mytilidae, Arcidae, Anomiidae, Pinnidae, Pectinidae, Dreissenidae, and Unionidae.
The purpose of the byssus is to keep the mussel attached to the desired surface, and to this end byssal threads must be able to withstand strong cyclic motion due to tidal action near the shorelines mussels inhabit. Mechanical testing of live mussels has shown that byssal threads can extend 39% before yield and 64% before breaking, at a nominal strain rate of 10 mm/min. Tensile testing shows that threads exhibit three distinct phases: initial stiffness from both the distal and proximal regions, softening due to yield in the distal region, and finally stiffening directly preceding tensile failure. The ability of the distal region to yield before breaking gives the mussels their characteristic hardiness even under strong tidal forces. Many variables that influence the performance of byssal threads have been studied, including species variations, seasonal variations, temperature effects, and aging effects. Temperature effects in particular have revealed a glass transition temperature of 6 °C.
The number of threads used by a mussel to attach is typically between 20 and 60; this can vary by the species, season, or age of the mussel. Under cyclic tidal conditions, the radial spread of fiber placement allows the mussel to dynamically align most of its fibers in the direction of applied force. This lowers the stress on any one thread, reducing the chances of failure and detachment. Mussels are also capable of ejecting the entire byssal complex, including the central stem, without damaging themselves. The complex can simply be regenerated, with fibers placement resuming within 24 hours.
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 a liquid form (similar to injection moulding in polymer processing) which then bubbles into a sticky foam. By curling its foot into a tube and pumping the foam, the mussel produces sticky threads each about the size of a human hair. The mussel then varnishes the threads with another protein, resulting in an adhesive. The attachment dynamics of the plaque are studied with the goals of artificially replicating the strong adhesive and of developing coatings to which the plaque cannot adhere. Foul release strategies such as fluoropolymer paints and lubricant-infused coatings are an active research area, the aim being to prevent the fouling of marine structures by invasive mussel species such as the zebra and quagga mussel.
Applications of biomimetic byssus adhesive include biomedical adhesives, therapeutic applications, and anti-fouling coatings.
Byssus was used by Carl Linnaeus as a genus of plants (some later known to be cyanobacteria). It has little relevance in current taxonomy, as most specimens are either lost or not identified. The ones that are identified have been either synonymized ( B. jolithus, B. aurea) or turned into nomen rejiciendum ( B. cryptarum).
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