rel="nofollow" href="#ulink_61e6d58c-6a68-57b0-b093-330c60ffe04a">Figure 2.1a). The valves are hinged together at the anterior by means of a ligament. This area of the shell is called the umbo. A series of interlocking teeth and sockets along the hinge line prevent the valves from sliding against one another. The interior of the shell is white with a broad border of purple or dark blue, while in Perna viridis the shell’s interior has a pale blue sheen. The border is called the pallial line and is the part of the shell along which the mantle is attached when empty shells are examined (Figure 2.1b). On the inside of each valve are two muscle scars, the attachment points for the large posterior adductor muscle and the much‐reduced anterior adductor muscle. All Perna species lack an anterior adductor muscle, so the shell anterior has only one muscle scar. Anterior and posterior retractor muscles are also attached to the inner shell; these control the movement of the foot (see later). The foot in turn secretes a byssus, a bundle of tough threads of tanned protein. These threads emerge through the ventral part of the shell and serve as mooring lines for attachment of the mussel to the substrate and to other mussels. Details of shell characteristics in selected mussel species are presented in Table 2.1. In summary, most mussel species have an equivalve, oblong, dark‐coloured shell with a smooth or ribbed surface marked with concentric growth lines; the inner shell is often iridescent white to purple, with two adductor scars and a variable number of hinge teeth.
Figure 2.1 (a) External and (b) internal shell features of the mussel Mytilus edulis.
Source: Photograph and permission to reproduce from Craig Burton.
Table 2.1 Details of shell characteristics in selected mussel species. (www.fao.org/fishery/species/search/en). Sources: Adapted from information from species databases: Centre for Agriculture and Biosciences International (www.cabi.org), Encyclopedia of Life (eol.org), Invasive Species Specialist Group (www.issg.org) Sanctuary Integrated Monitoring Network (SIMoN; www.sanctuarymonitoring.org) and FAO Aquatic Species Fact Lists (www.fao.org/fishery/species/search/en).
| Species | Shell exterior | Shell interior | Maximum shell length |
|---|---|---|---|
| Arcuatula senhousia | Olive‐green to brown with darker radial lines or zig‐zag markings; umbones subterminal | Lustrous purple‐grey; no hinge teeth | 35 mm |
| Aulacomya atra | Brown ribbed eqivalve shell, which darkens to black with age; pointed umbo; concentric growth grooves | Nacreous; hinge with a single tooth that fits in the groove of the opposed shell | 200 mm |
| Bathymodiolus childressi | Thin, fragile equivalve shell; dark brown to straw‐yellow | Nacreous, off‐white colour; no hinge teeth in individuals >11 mm shell length; muscle scars and pallial line indistinct | 120 mm |
| Geukensia demissa | Solid, oblong shell with blunt umbones; glossy yellow or brown‐black with grooved or ribbed surface | Iridescent, white with purple tints; no hinge teeth | 100 mm |
| Modiolus modiolus | Purple or dark blue; annual growth lines clear with fine sculpturing of concentric grooves and ridges | White with a broad pallial line | 220 mm |
| Mytella charruana | Mostly dark brown to black exhibiting a wavy dark pattern on a lighter background; concentric growth rings evident on shell surface | Iridescent purple | 40 mm |
| Mytilus californianus | Brown to dark purple, bluish or black and often eroded; teardrop shaped inequivalve shell with several strong concentric grooves and weak radial lines | Blue‐gray; may be slightly iridescent | 250 mm |
| Mytilus galloprovincialis | Black‐violet colour; equivalve shell, nearly quadrangular, with one valve pointed with slightly bent umbo and other rounded | Mantle edge blue or purple | 120 mm |
| Perna viridis | Equivalve shell; vivid green to dark brownish‐green near the dorsal edge and olive‐green near the umbo, which has a downturned beak | Surface iridescent blue‐green hue; three interlocking hinge teeth; wavy pallial line and large kidney‐shaped adductor muscle, both diagnostic for the species | 150 mm |
Structure
The main component of the shell is calcium carbonate, formed by the deposition of crystals of this salt in an organic matrix composed of polysaccharide β‐chitin, a hydrophobic silk protein and a complex assemblage of hydrophilic proteins, many rich in aspartic acid (Addadi et al. 2006). The mineral component represents more than 95% of the shell weight, while the organic matrix represents 1–5%. Despite this, the organic component increases the fracture toughness of the shell by two or three orders of magnitude (Kamat et al. 2000). Three layers make up the shell: (1) a thin outer periostracum of conchiolin, a tough insoluble protein forming the organic matrix of the shell, within which calcium carbonate is deposited – this layer is often a different colour to the underlying shell layers; (2) a middle prismatic layer composed of fibres of aragonite or calcite – both crystalline forms of calcium carbonate (Figure 2.2); and (3) an inner nacreous layer composed of tablets of aragonite arranged in layers separated by sheets of an organic matrix of elastic biopolymers composed of chitin and silk‐like proteins – the nacre is either of a dull texture or iridescent mother‐of‐pearl, depending on the species.
Figure 2.2 Calcitic fibrous layers