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2 Functional Morphology
Introduction
In this chapter, the approach is to use much studied representative species of marine mussels to describe the general morphology and functions of the shell, mantle, foot, gill, alimentary canal, gonad, heart, kidney and nervous tissue. Additional information on their particular roles in feeding, reproduction, circulation, excretion, osmoregulation and contaminant accumulation is presented in Chapters 4, 5, 7 and 8.
Shell
Mussels have two shell valves that are hinged dorsally and connected by an elastic ligament. Adductor muscles hold the valves together, and relaxation of the ligament and contraction of these muscles open and close the shell, respectively. In Mytilus spp., the two shell valves are similar in size, and are roughly triangular in shape (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 |
|---|
