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There are four distinct regions in the mussel byssus: root, stem, thread and plaque (Figure 2.9). The root is embedded in the muscular tissue at the base of the foot. The stem is divided into sections, each with a thread attached; each thread ends in a plaque at which attachment to the substrate takes place. In M. edulis and M. galloprovincialis, byssal threads are a few centimetres in length and less than 0.1 mm in diameter (Waite et al. 2002). Each thread is further divided into a smooth and stiff distal region and a soft and wavy proximal region (Figure 2.10A); in terms of mechanical properties, the latter resembles soft rubber (elasticity of 200%) and the former behaves like rigid nylon with a high tensile strength (Young’s modulus = 500 MPa) (Waite et al. 2002). The overall mechanical properties of threads reflect those of elastomers, withstanding significant deformations without rupture and returning to their original state when the stress is removed (Hagenau et al. 2009). Each thread has a flexible, collagenous inner core covered by a tough, durable cuticle (Holten‐Andersen et al. 2009), which has a thickness of 2–5 μm and is composed of the protein mefp‐1, rich in dihydroxyphenylalanine (DOPA) residues. Metal crosslinking of DOPA residues further enhances the toughness of the byssal threads (Arnold et al. 2010). The collagenous proteins that make up the core are known as preCOLs (prepepsinised collagens). The proximal region of the thread contains preCOL‐P, a protein with remarkable extensibility and toughness due to the coiled nature of the collagen fibres (Figure 2.10B). The distal region of the thread contains preCOL‐D, arranged in straight bundles which provide the stiff properties of the thread–plaque interface. A third protein, preCOL‐NG, is distributed throughout the core and is believed to mediate the function between the elastic proximal and stiff distal regions of the thread (Figure 2.11). PreCols extend into the thread–plaque interface. Another component of the core is the thread matrix proteins (TMPs; Figure 2.11), which are distributed throughout the thread and provide a viscoelastic matrix around the collagen fibres (Figure 2.10A). Apparently, TMPs lubricate the fibres and help in reforming byssal threads following deformation from tensile loads (Sagert & Waite 2009). The cuticle, which covers the thread and plaque regions, is about fivefold harder than the thread core. Six different families of foot proteins have been classified from M. edulis byssal threads: Mefp‐1, Mefp‐2, and so on (Figure 2.11). One distinguishing characteristic of all Mefps is the presence of the post‐translational modification of tyrosine to 3,4‐dihydroxyphenyl‐l‐alanine (DOPA) as well as organic ions, notably iron (Fe³⁺). Although DOPA is found throughout the byssus, it is present in higher amounts in the Mefps found at the plaque–substrate interface and is believed to be an important component in wet adhesion (Anderson et al. 2010). The plaque has a solid foam structure and is commonly only ~0.15 mm in diameter where it meets the thread and ~2–3 mm at the substrate interface. The six foot proteins are utilised in plaque formation: fp‐3 and fp‐5 are adhesive and adhere to the substrate; fp‐6 is cohesive and binds the former two proteins together; fp‐2 is also cohesive and binds most of the plaque matrix; fp‐4 is cohesive and connects the plaque to the thread; and fp‐1 is cohesive and forms the plaque and thread outer sheath. Fifteen additional foot proteins have been identified through transcriptomic analysis of the foot of the mussel M. californianus (DeMartini et al. 2017). The discovery of these new Mfps sets the groundwork for future biochemical investigations to build a more complete model of byssus structure and function in bioadhesion. Mussels concentrate metals like iron, zinc, copper and manganese more than 100 000‐fold higher in the adhesive plaque compared to their quantities in the sea, which suggests that metal ions play an important role in adhesion (Bandara et al. 2013; Merten 2013). How the mussel processes all these proteins and transition metals into a high‐strength, robust, mechanical adhesive is still not understood (Ornes 2013). For additional information on the fine structure of byssal thread and plaque, see Silverman & Roberto (2011).

Figure 2.9 Anatomy of the byssus in Mytilus edulis.

Source: From Silverman & Roberto (2007). Reproduced with permission from Springer Nature.

Figure 2.10 Model of the hierarchical arrangement of a mussel byssal thread. (A) Originating from a stem, individual threads show a gradual transition from an elastic proximal to a stiff distal portion ending in an adhesive plaque. (B) Triple helix arrangement of the underlying collagen proteins (preCOLs). Crosslinking for lateral and longitudinal assembly of triple helices is accomplished through lateral His–metal chelate complexes (Me⁺), disulphide bridges (S‐S) or linearly after oxidising two individual DOPA side chains. Deviations from a repeat motif in preCOL‐D can lead to kinks in the characteristically rod‐like collagen morphology.

Source: From Hagenau et al. (2009). Reproduced with permission from John Wiley & Sons.

Figure 2.11 Localisation of adhesive proteins in the byssal thread and plaque of Mytilus.

Source: From Silverman & Roberto (2011). Reproduced with permission from Springer Nature.