Marine Mussels. Elizabeth Gosling

Чтение книги онлайн.

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.