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Concept
Byssus
Summary
A byssus ('bIs@s) 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.
Byssus filaments are created by certain kinds of marine and freshwater bivalve mollusks, which use the byssus to attach themselves to rocks, substrates, or seabeds. In edible mussels, 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 including the Arcidae, Mytilidae, Anomiidae, Pinnidae, Pectinidae, Dreissenidae, and Unionidae.
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.
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