Marine Mussels. Elizabeth Gosling

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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).

Mantle

      Structure

In bivalves, the mantle consists of two lobes of tissue that completely enclose the animal within the shell (Figure 2.6). Between the mantle and the internal organs is a capacious mantle cavity. Unlike in other marine bivalves, the mantle in mussels contains most of the gonad. Gametes proliferate within the mantle and are carried along ciliated channels to paired gonoducts that discharge through the exalant opening of the mantle (see Chapter 5). The colour of the mantle varies from a creamy white to pink, brown or orange, depending on the stage of gametogenesis. After mussels have released their gametes, the mantle is thin and transparent. The mantle consists of connective tissue with haemolymph (‘blood’) vessels, nerves and muscles that are particularly well developed near the mantle margins. The mantle edge is usually darkly pigmented, which may give protection from the harmful effects of solar radiation. Cilia on the inner surface of the mantle play an important role in directing particles on to the gills and in deflecting heavier material along rejection tracts toward the inhalant opening, the entry point on the mantle for incoming water (see Chapter 4). Periodically, the rejected material is discharged by sudden and forceful closure of the shell valves; this is sufficient to blow the rejected material out of the mantle cavity through the inhalant opening.

      Function

      The mantle plays a crucial role in the formation of the shell, a process that has already been covered in some detail. Also, the mantle is the site of gametogenesis and the main location for the storage of nutrient reserves, especially glycogen. In M. edulis, reserves are laid down in summer and utilised in autumn and winter in the formation of gametes (see Chapter 5). For a full discussion of energy metabolism in the mantle and other tissues, see de Zwann & Mathieu (1992).

The mantle is also involved in pearl formation. Mussels (Mytilus spp.) produce pearls in response to infection by the larva of a small parasitic flatworm (Gymnophallus spp.). If the larva gets between the mantle epithelium and the shell, the bivalve, in self‐defence, encapsulates it with a pearly (nacreous) coat produced by the outermost layer of the mantle. Pearling can be extremely damaging as it affects the potential for the development of growing mussels for the live – most profitable – market (Wilcox 2013). The problem can be eliminated by avoiding areas where pearl formation occurs or by growing mussels on ropes and marketing them before any pearls reach a detectable size (Morse & Rice 2010). The mantle is also host to various nonpathogenic viruses, potentially pathogenic protozoans, commensal cnidarians and parasitic flatworms. The parasitic flatworm, Proctoeces maculatus, seriously reduces glycogen energy reserves in heavily infected mussels. This can lead to disturbances of gametogenesis and possible castration and death (Bower 2009). Additional information on diseases, parasites and pests of mussel mantle is presented in Chapter 11.