et al. 2012). Distinct areas of the outer epithelium secrete prismatic and nacre layers. Also, the epithelium contains membrane pumps and channels for extruding the inorganic precursors of calcium carbonate such as calcium (Ca2+) and bicarbonate (HCO3 −). Calcium and bicarbonate ions are taken up from food, from water filtration activity or by passive diffusion over the body surface; bicarbonate may also come from hydration of metabolic carbon dioxide.
The outer mantle epithelium is not in direct contact with the shell but is separated from it by the minute fluid‐filled extrapallial space (ES). Calcium and bicarbonate ions are transported in the haemolymph to the calcifying epithelium, where they are stored as granules and pumped into the ES when needed. In addition to these precursor ions, the fluid contains other inorganic ions, minor elements, proteins and glycosaminoglycans (GAGs) (Marin et al. 2012 and references therein). As the extrapallial fluid is supersaturated, macromolecules, especially acidic proteins and GAGs, maintain calcium in solution by inhibiting the precipitation of calcium carbonate and by allowing it to precipitate where needed.
The edge of the mantle has three folds (Figure 2.3). The ridge between the outer and middle fold is called the periostracal groove; here, specialised cells secrete the periostracum and prismatic layers, while the inner nacreous layer is secreted by the general mantle surface. The myostracum is a very thin mineral layer covering the nacre, where muscles attach to the shell. The shell grows in circumference by the addition of material from the edge of the mantle and grows in thickness by deposition from the general mantle surface. The functions of the periostracum are to act as a first support for calcium carbonate crystals, to protect the shell and to serve as a seal of the ES in such a way that supersaturation conditions – a prerequisite for crystal formation – can be attained (Marin & Luquet 2004). In adults, the periostracum is often much reduced due to mechanical abrasion, fouling organisms, parasites or disease.
Figure 2.3 Schematic representation of the mytilid shell margin.
Source: After Taylor et al. (1969). From Génio et al. (2012). Reproduced with permission from Elsevier.
While the organic fraction (matrix) of the shell is small (1–5%), it is comprised of a mixture of extracellular macromolecules that play an important role in the mineralisation process. A wide variety of shell matrix proteins (SMPs) have been identified in molluscs, including mussels, but to date their specific functions have not been elucidated (Marin et al. 2012; Gao et al. 2015; Yarra et al. 2016). However, several candidate genes coding for nacreous and prismatic layer proteins have been identified in bivalves (Inoue et al. 2010; Jackson et al. 2010; Kinoshita et al. 2011; Hüning et al. 2016; Bjärnmark et al. 2016 and references therein). A wide range of enzyme proteins are expressed during the formation of the shell, including carbonic anhydrase, alkaline phosphatase, DOPA‐oxidase (tyrosinase)/peroxidase and chitin synthase. After proteins, polysaccharides are the next most important component of the organic matrix; for example, chitin is a long‐chain insoluble polymer that plays a key role by defining the interlamellar matrix between nacre tablets (Addadi et al. 2006). Soluble acidic polysaccharides, many of which are bound to protein, are also present in the matrix, but their characterisation in mussels is still in its infancy.
External Characteristics
Shell colour in mussels is controlled by one (Gantsevich & Tyunnikova 2005) or several (Innes & Haley 1977; Newkirk 1980) genes, and also varies depending on the age and location of the animal. In the intertidal zone, M. edulis has a blue‐black and heavy shell, while in the sublittoral region, where mussels are continuously submerged, the shell is thin and brown with dark brown to purple radial markings. In Perna perna, the shell is red‐maroon with irregular patches of light brown and green. Juvenile green mussels, P. viridis, have bright green or blue‐green shells, but older individuals tend to have more brown (Siddall 1980).
Presence of concentric rings on the shell exterior has been extensively used in age determination. In scallops and clams, these rings are annual in origin and therefore can be used as a reliable estimate of age, but in mussels there are few geographic locations where they provide an accurate estimate (Lutz 1976). Age, however, can be determined by examining annual growth bands in sections of the prismatic and inner nacreous shell layers or in other parts of the shell umbo, hinge plate, pallial line scar or posterior adductor muscle scar (see Chapter 6).
The principal parameters used in shell measurement are: height, the maximum distance from the hinge to the shell margin; length, the widest part of the shell at 90° to the height; and width, the thickest part of the two shell valves (Figure 2.4). Under optimal conditions, such as in the sublittoral zone, M. edulis and the Mediterranean mussel, M. galloprovincialis, attain a shell length of 100–130 mm, whereas in marginal conditions, such as the high intertidal zone on an exposed shore, mussels may measure as little as 20–30 mm, even after 15–20 years (Seed 1976). This is not however a universal pattern. In South Africa, the native mussel Perna perna is largest on more exposed shores whereas the invasive mussel M. galloprovincialis is largest at intermediate levels of shore exposure (McQuaid et al. 2000; Hammond & Griffiths 2004). Shell shape is also very variable in these two mussel species. The shells of densely packed mussels have higher length to height ratios than those from less crowded conditions. This is most extreme in older mussels and ensures that they can more readily exploit posterior feeding currents, since they are effectively elevated above younger mussels in the same clump (Seed & Suchanek 1992). Density also has a negative effect on shell thickness in the intertidal mussel Perumytilus purpuratus (Briones et al. 2014). Shell morphology can also be correlated with wave exposure; on the west coast of Canada, both juvenile and adult M. trossulus at wave‐exposed sites show a lower shell height to width ratio and a thicker shell than mussels from sheltered locations (Akester & Martel 2000). Shell shape, as well as internal features of the shell, have also been used to differentiate both within and between various mytilid species (Innes & Bates 1999; Aguirre et al. 2006; Krapivka et al. 2007; Gardner & Thompson 2009; Valladares et al. 2010; Bonel et al. 2013; Van der Molen et al. 2013; Lajus et al. 2015; Katolikova et al. 2016). Figure 2.5 illustrates 18 shell characters which when used in combination give good separation of M. edulis, M. galloprovincialis and M. trossulus in the Northern Hemisphere (McDonald et al. 1991). See Chapter 6 for more on the methods used in shell shape analysis.
Figure 2.4 The convention used for the main external shell parameters in bivalves.
Source: Sandra Noel, http://www.noeldesigninterp.com. Reproduced with permission.
The shells of mussels, and other bivalves, are increasingly being used as potential bioarchives of proxies for physicochemical changes in their marine habitats. In marine mussels, isotopes (δ18O, δ13C) and rare earth element values, along with Mg/Ca and Sr/C ratios in shells, have been used to reconstruct changes in surface water salinity (Hahn et al. 2012), temperature (Cusack et al. 2008; Bau et al. 2010; Ford et al. 2010) and freshwater inputs to the ocean (McConnaughey
