mercury over a 24‐day period caused an initial deterioration in neural and epithelial cells, increased interstitial cell oedema and reduced ciliation in Perna perna. However, after a metal‐free recovery period, gill filament morphology returned to near normal (Gregory et al. 2002). This is not unexpected, as metallothioneins (MTs) – metal‐binding, heat‐stable, low‐molecular‐weight proteins – play an important role in detoxifying trace metals in bivalves and are widely distributed in gill and digestive gland tissue. Consequently, mussel MT levels are increasingly being used as a biomarker of heavy metal contamination in coastal ecosystems (Khati et al. 2012; see Chapter 8).
Foot
The foot first appears when bivalve larvae are about 200 μm in length, and becomes functional in crawling and attachment at ~260 μm shell length. This is the pediveliger stage of development, which immediately precedes settlement and metamorphosis (see Figure 5.10). The ciliated foot is proportionately very large and sock shaped, and is made up of layers of circular and longitudinal muscles surrounding a capacious haemolymph space. A byssal duct opens at the ‘heel’ of the foot, and a byssal pedal groove extends forward along the ‘sole’ from this opening. The groove is embedded in secretory tissue, which produces the different byssal thread components (see later). While swimming, the foot is fully extended, and periodically the velum (larval swimming organ) is withdrawn and the larva sinks to the bottom and begins to crawl. If the substrate is unsuitable (i.e. does not stimulate the secretion of byssus), the foot is withdrawn and the larva once again swims off (Lutz & Kennish 1992). This cycle can be repeated many times over a period of a few days. In Mytilus, when a suitable substrate is found, the larva continues to crawl for some time, gradually ceases movement, protrudes the foot and quickly secretes a single byssal thread. In the newly attached mussel larva, this thread can be repeatedly broken and reformed before final settlement takes place. As the mussel grows in length, more and more attachment threads are secreted; this is not surprising, as larger individuals are subject to greater mechanical stress than smaller ones. To resist dislodgement, mussels cluster their threads in the direction of applied forces (e.g. facing ebb and flow of tide). The adhesive in mussel larvae differs from that of adults, resembling the mucous secreted by other benthic marine species at the larval stage (Petrone et al. 2008). The green crenella, Musculus discors, is unusual in that the byssus threads that are used to fix it to the substrate are woven into a nest or cage surrounding the shell, similar to a ball of twine. Eggs in mucous strings are retained within this nest, which may incorporate a variety of macroalgae (Merrill & Turner 1963).
Byssus Composition
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
