![Mytilus with byssus](https://upload.wikimedia.org/wikipedia/commons/b/bc/Mytilus_with_byssus.jpg)
A byssus () is a bundle of filaments secreted by many species of
bivalve
Bivalvia (), in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class of marine and freshwater molluscs that have laterally compressed bodies enclosed by a shell consisting of two hinged parts. As a group, bival ...
mollusc that function to attach the mollusc to a solid surface. Species from several families of clams have a byssus, including pen shells (
Pinnidae
The Pinnidae are a taxonomic family of large saltwater clams sometimes known as pen shells. They are marine bivalve molluscs in the order Pteriida.
Shell description
The shells of bivalves in this family are fragile and have a long and triang ...
), true mussels (
Mytilidae
Mytilidae are a family of small to large marine and brackish-water bivalve molluscs in the order Mytilida. One of the genera, '' Limnoperna'', even inhabits freshwater environments. The order has only this one family which contains some 52 gener ...
), and
Dreissenidae
The Dreissenidae are a family of small freshwater mussels, aquatic bivalve molluscs. They attach themselves to stones or to any other hard surface using a byssus. The shells of these bivalves are shaped somewhat like those of true mussels, and ...
.
Filaments
Byssus filaments are created by certain kinds of marine and freshwater
bivalve
Bivalvia (), in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class of marine and freshwater molluscs that have laterally compressed bodies enclosed by a shell consisting of two hinged parts. As a group, bival ...
mollusk
Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda, the members of which are known as molluscs or mollusks (). Around 85,000 extant species of molluscs are recognized. The number of fossil species is e ...
s, which use the byssus to attach themselves to rocks,
substrates, or seabeds. In edible
mussel
Mussel () is the common name used for members of several families of bivalve molluscs, from saltwater and Freshwater bivalve, freshwater habitats. These groups have in common a shell whose outline is elongated and asymmetrical compared with other ...
s, the inedible byssus is commonly known as the "beard", and is removed before cooking.
Many species of mussels secrete byssus threads to anchor themselves to surfaces, with
families
Family (from la, familia) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its members and of society. Ideal ...
including the
Arcidae,
Mytilidae
Mytilidae are a family of small to large marine and brackish-water bivalve molluscs in the order Mytilida. One of the genera, '' Limnoperna'', even inhabits freshwater environments. The order has only this one family which contains some 52 gener ...
,
Anomiidae
Anomiidae is a family of saltwater clams, marine bivalve molluscs related to scallops and oysters, and known as anomiids. It contains seven genera.
The family is known by several common names, including jingle shells, mermaid's toenails, and sa ...
,
Pinnidae
The Pinnidae are a taxonomic family of large saltwater clams sometimes known as pen shells. They are marine bivalve molluscs in the order Pteriida.
Shell description
The shells of bivalves in this family are fragile and have a long and triang ...
,
Pectinidae
Scallop () is a common name that encompasses various species of marine bivalve mollusks in the taxonomic family Pectinidae, the scallops. However, the common name "scallop" is also sometimes applied to species in other closely related families ...
,
Dreissenidae
The Dreissenidae are a family of small freshwater mussels, aquatic bivalve molluscs. They attach themselves to stones or to any other hard surface using a byssus. The shells of these bivalves are shaped somewhat like those of true mussels, and ...
, and
Unionidae
The Unionidae are a family of freshwater mussels, the largest in the order Unionida, the bivalve molluscs sometimes known as river mussels, or simply as unionids.
The range of distribution for this family is world-wide. It is at its most divers ...
.
Mechanics
The byssus, or byssal complex, is composed of multiple extracellular collagenous threads that are placed radially by the mussel from a central stem. Each thread is composed of three regions: a corrugated proximal region close to the mussel body, a longer, smooth distal region connecting the proximal region to the ending plaque, and the adhesive plaque itself, which anchors the mussel to the surface.
The proximal region consists of a corrugated sheath enveloping loosely-arranged coiled fibers; these coils can unravel to extend the fiber under an applied force. The distal region is more ordered, consisting of aligned collagenous fiber bundles that give the fiber stiffness. The plaque consists of collagen-like fibers over a spongy matrix, in which the adhesive protein is deposited and hardens.
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 ageing 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 20-60; 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 and 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
A plunger, force cup, plumber's friend or plumber's helper is a tool used to clear blockages in drains and pipes. It consists of a rubber suction cup attached to a stick (''shaft'') usually made of wood or plastic. A different bellows-like des ...
unclogging a drain. The byssus, which is made of
keratin
Keratin () is one of a family of structural fibrous proteins also known as ''scleroproteins''. Alpha-keratin (α-keratin) is a type of keratin found in vertebrates. It is the key structural material making up scales, hair, nails, feathers, ho ...
,
quinone
The quinones are a class of organic compounds that are formally "derived from aromatic compounds uch as benzene or naphthaleneby conversion of an even number of –CH= groups into –C(=O)– groups with any necessary rearrangement of double ...
-tanned proteins (
polyphenolic proteins), and other proteins, is spewed into this chamber in liquid form similar to
injection moulding in polymer processing, 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.
The attachment dynamics of the plaque are studied both to imitate the strong adhesive and to create coatings to which the plaque cannot adhere. Foul release strategies such as fluoropolymer paints and lubricant-infused coatings are an active research area important to preventing the fouling of marine structures by invasive mussel species such as the zebra and quagga mussel.
Biomimetics
Byssus is a remarkable adhesive, one that is neither degraded nor deformed by water as many synthetic adhesives are.
The remarkable properties of this adhesive, specifically the mussel foot proteins (Mfps), has spurred many attempts to imitate the excellent adhesive capacity that mussels show, either by producing Mfps via other organisms or by creating synthetic polymers with similar properties. For instance,
genetic engineers have inserted mussel
DNA into
yeast
Yeasts are eukaryotic, single-celled microorganisms classified as members of the fungus kingdom. The first yeast originated hundreds of millions of years ago, and at least 1,500 species are currently recognized. They are estimated to constitut ...
cells to translate the genes into the appropriate proteins.
Synthetic approaches generally utilize
catechol
Catechol ( or ), also known as pyrocatechol or 1,2-dihydroxybenzene, is a toxic organic compound with the molecular formula . It is the ''ortho'' isomer of the three isomeric benzenediols. This colorless compound occurs naturally in trace amoun ...
as a
cross-link
In chemistry and biology a cross-link is a bond or a short sequence of bonds that links one polymer chain to another. These links may take the form of covalent bonds or ionic bonds and the polymers can be either synthetic polymers or natural ...
ing agent to produce wear-resistant polymer networks. Imitation of Mfp-3 to induce
coacervation Coacervate ( or ) is an aqueous phase rich in macromolecules such as synthetic polymers, proteins or nucleic acids. It forms through liquid-liquid phase separation (LLPS), leading to a dense phase in thermodynamic equilibrium with a dilute phase. Th ...
is another key property, as this protects the material from partial dissolution in saltwater.
Applications of biomimetic byssus adhesive include biomedical adhesives,
therapeutic applications,
and anti-fouling coatings.
Historical uses
Byssus often refers to the long, fine, silky threads secreted by the large
Mediterranean
The Mediterranean Sea is a sea connected to the Atlantic Ocean, surrounded by the Mediterranean Basin and almost completely enclosed by land: on the north by Western and Southern Europe and Anatolia, on the south by North Africa, and on the e ...
pen shell, ''
Pinna nobilis
''Pinna nobilis'', common name the noble pen shell or fan mussel, is a large species of Mediterranean clam, a marine bivalve mollusc in the family Pinnidae, the pen shells. *''Pinna gigas'' Chemnitz
It reaches up to of shell length.Zavodn ...
''. The byssus threads from this ''
Pinna'' species can be up to in length and have historically been made into cloth.
Byssus cloth
Sea silk is an extremely fine, rare, and valuable fabric that is made from the long silky filaments or byssus secreted by a gland in the foot of pen shells (in particular '' Pinna nobilis''). The byssus is used by the clam to attach itself to ...
is a rare fabric, also known as ''sea silk'', that is made using the byssus of pen shells as the fiber source.
References
External links
*
{{Fibers
Bivalve anatomy
Mollusc products