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 2.8Bii). Lateral cilia are set along the sides of the filaments in fillibranch gills and in the ostia of eulamellibranch gills. These cilia are responsible for drawing water into the mantle cavity and passing it through the gill filaments or the ostia, and then upward to the exhalant chamber and on to the exhalant opening. Lying between the lateral and frontal cilia (see later) are the large feather‐like latero‐frontal cilia, which are unique to bivalves. When the incoming current hits the gill surface, these cilia flick particles from the water and convey them to the frontal cilia. The frontal cilia, which are abundantly distributed on the free outer surface of the gill facing the incoming current, convey particles aggregated in mucous – secreted by the filaments – downward toward the ciliated food grooves on the ventral side of each lamella. The movement of cilia is under nervous control. Each gill axis is supplied with a branchial nerve from a visceral ganglion, which subdivides to innervate individual groups of filaments. The general architecture and fine structure of the gill vary little from one mussel species to the next, even when rock (e.g. Lithophaga lithophaga; Akşit & Falakali Mutaf 2014) and sediment (e.g. Mytella falcate; David & Fontanetti 2005) burrowing species are considered. See Chapter 4 for a detailed description of the role of the gill in water pumping and particle capture.
In bivalves, the gills have a respiratory as well as a feeding role. Their large surface area and rich haemolymph supply make them well suited for gas exchange. Deoxygenated haemolymph is carried from the kidneys to the gills by way of the afferent gill vein. Each filament receives a small branch of this vein. The filaments are essentially hollow tubes within which the haemolymph circulates. Gas exchange takes place across the thin walls of the filaments. The oxygenated haemolymph from each filament is collected into the efferent gill vein, which goes to the kidney and on to the heart. It is likely that gas exchange also occurs over the general mantle surface.
The gills perform an additional function in hydrothermal vents mussels, which depend almost entirely on endosymbiont chemosynthetic bacteria in the gill filaments as an energy source. The bacteria use the energy obtained from the oxidation of reduced sulphur compounds and methane from hydrothermal fluid for the fixation of the CO2 required for primary production (Duperron et al. 2016 and references therein; see also Chapter 4).
Due to their dominant role in ingestion and respiration, the gills are among the main target organs in the bioaccumulation of pesticides, soluble heavy metals and hydrocarbons. Complex mixtures of heavy metals and polycyclic aromatic hydrocarbons (PAHs) cause morphological changes in the gill epithelium of Mytella falcata, leading to an increase in the number of gill mucous cells, haemocytes and cell turnover processes. These are possible mechanisms to compensate for cell injury and prevent entry of pollutants from gill filaments into the entire
