Marine Mussels. Elizabeth Gosling
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.
Gills
Filter feeding is believed to have evolved in some group of early protobranch molluscs, giving rise to the Autobranchia, the dominant subclass of modern bivalves. These feed by filtering the incoming current as a source of food, the gills having replaced the palps as the feeding organs. One important development in the evolution of filter feeding was movement of the site of water intake to the posterior of the animal (see Chapter 1).
Structure
The gills, often referred to as ctenidia, are two large, curtain‐like structures that are suspended from the gill axis, which is fused along the dorsal margin of the mantle (Figure 2.8A). Within the gill axis are the branchial nerves and afferent and efferent branchial haemolymph (blood) vessels. Each gill is made up of numerous W‐shaped (or double‐V) ciliated filaments and an internal skeletal rod rich in collagen strengthens each filament. Each V is known as a demibranch and each arm is called a lamella, giving an inner descending and outer ascending lamella (Figure 2.8Bi). In the space between the descending and ascending lamellae is the exhalant chamber, connected to the exhalant area of the mantle edge; the space ventral to the filaments is the inhalant chamber, connected to the inhalant area of the mantle edge (Figure 2.8B). In mussels, the gills follow the curvature of the shell margin, with the maximum possible surface exposed to the inhalant water flow (Figure 2.6).
Figure 2.8 (A) Section of a lamellibranch gill showing the ctenidial axis and four W‐shaped filaments. For greater clarity, the descending and ascending lamellae of each demibranch have been separated. Solid arrows indicate direction of water flow through the filaments from inhalant (INH) to exhalant (EXH) chambers and broken arrows indicate path of particle transport to the food grooves. (B) (i) section of a fillibranch gill in the mussel, Mytilus edulis. Adjacent filaments are joined together by ciliary junctions. (ii) Transverse section through one fillibranch gill filament (shaded in Bi), showing pattern of ciliation. Source: (A) From Barnes et al. (1993), with permission from John Wiley & Sons; (B) From Pechenik (2010). Reproduced with permission from the McGraw–Hill Companies
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When the individual filaments are free or loosely attached to one another through interlocking clumps of cilia, this is known as the fillibranch condition (Figure 2.8Bi); it is seen in mussels and scallops. In more advanced bivalves, neighbouring filaments are joined to each other at regular intervals by tissue connections, leaving narrow openings or ostia between them. This gill type, termed eulamellibranch, is a solid structure and is found in the majority of bivalves. In oysters, tissue connections are less extensive than in most eulamellibranch species, so the gills are often referred to as pseudoeulamellibranch. Also, when filaments are similar, as in mussels, the gill is termed homorhabdic, and when there are different types of filaments, through folding of the gill area, it is called heterorhabdic. The filibranch homorhabdic gill in adult mussels is regarded as the ancestral condition from which the other gill types evolved (Beninger & Dufour 2000).
Functions
Cilia on the gill filaments have specific arrangements and functions (Figure