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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 & Gillikin 2008) (see further details in Chapter 3). This type of research is particularly relevant in the light of observed and potential climate change impacts on marine organisms (see Chapter 3). Another use of shells is in trace elemental fingerprinting, which assesses larval origins or trajectories based on the elemental composition of their shells, which in turn reflects the chemistry of the water in which they were formed (see Chapter 5). The technique has also been used to identify population sources and thus redistribution potentials in adult M. edulis (Sorte et al. 2013).

Figure 2.5 Shell morphological characters used to distinguish between different Mytilus taxa. (1) Length of the anterior adductor muscle scar. (2) Distance between the anterior end of the posterior retractor muscle scar and the dorsal shell margin. (3) Width of the anterior adductor muscle scar. (4) Length of the hinge plate. (5) Shell height. (6) Length of the anterior retractor muscle scar. (7) Distance between the umbo and the posterior end of the ligament. (8) Length of the posterior retractor muscle scar. (9) Length of the posterior adductor muscle scar. (10) Distance between the anterior edge of the posterior adductor muscle scar and the posterior shell margin. (11) Distance between the ventral edge of the posterior adductor muscle scar and the ventral shell margin. (12) Distance between the pallial line and the ventral shell margin midway along the shell. (13) Distance between the posterior edge of the posterior adductor muscle scar and the posterior shell margin. (14) Number of major teeth on the hinge plate, excluding any small crenulations, which may be present, especially on the posterior ventral face of the hinge plate. (15) Distance between the umbo and the posterior end of the anterior retractor scar. (16) Distance between the ventral edge of the posterior retractor muscle scar and the dorsal shell margin. (17) Shell width. (18) Width of the posterior retractor muscle scar.

Source: From McDonald et al. (1991). Reproduced with permission from Springer.

Finally, mussel shells are extensively used to assess environmental contamination (Bellotto & Miekeley 2007; Pereira et al. 2012; see Chapter 8). Radionucleotides (e.g. uranium) and metals such as Cu, Cd, Cr, Pb, U, V and Zn will be highly concentrated in contaminated shells (Widdows & Donkin 1992; Boisson et al. 1998; Avelar et al. 2000; Richardson et al. 2001).