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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 organism (David & Fontanetti 2005; David et al. 2008). Exposure to mercury over a 24‐day period caused an initial deterioration in neural and epithelial cells, increased interstitial cell oedema and reduced ciliation in Perna perna. However, after a metal‐free recovery period, gill filament morphology returned to near normal (Gregory et al. 2002). This is not unexpected, as metallothioneins (MTs) – metal‐binding, heat‐stable, low‐molecular‐weight proteins – play an important role in detoxifying trace metals in bivalves and are widely distributed in gill and digestive gland tissue. Consequently, mussel MT levels are increasingly being used as a biomarker of heavy metal contamination in coastal ecosystems (Khati et al. 2012; see Chapter 8).