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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.

Figure 2.6 Inner anatomy of Mytilus edulis. The white posterior adductor muscle is visible in the upper image but has been cut in the lower image to allow the valves to open fully.

Source: Photograph by Rainer Zenz. (See colour plate section for colour representation of this figure).

The mantle margins are thrown into three folds (Figure 2.3): the outer one, next to the shell, is concerned with shell secretion (see earlier); the middle one has a sensory function; and the inner one is muscular and controls water flow in the mantle cavity. The ES separates the mantle from the shell, except in the regions of muscle attachment. As already seen, the calcareous and organic materials for shell secretion are deposited into this space. The mantle is attached to the shell by pallial muscle fibres in the inner fold; the line of attachment, the pallial line, runs in a semicircle a short distance from the edge of the shell. In mussels, the exhalant opening is small, smooth and conical, and the inhalant aperture is wider and fringed by sensory papillae (Figure 2.7). The middle fold has assumed a sensory role in the evolution of the bivalve form from the ancestral mollusc – a change that involved the loss of the head and associated sense organs. The middle fold is frequently drawn out into short tentacles that contain tactile and chemoreceptor cells. Both of these cell types play an important role in predator detection and avoidance. Ocelli, which are sensitive to sudden changes in light intensity, may also be present on the middle fold. In mussels, these ‘eyes’ are simple invaginations lined with pigment cells and filled with a mucoid substance or ‘lens’, whereas in scallops they are highly developed, with a cornea, lens and retina that produce a low‐contrast image (Colicchia et al. 2009). The inner mantle fold, or velum, the largest of the three folds, has small sensory tentacles or papillae that usually fringe the fold. There is also a large muscular component, especially on the inhalant opening. The velum plays an important role in controlling the flow of water into and out of the mantle cavity.

Figure 2.7 Exhalant (white and smooth) and inhalant (fringed with tentacles) openings in the mantle of the mussel Mytilus edulis.

Source: Photo courtesy of John Costelloe, Aquafact International Services Ltd., Galway, Ireland.