Marine Mussels. Elizabeth Gosling
permanent structure. The greater proportion of high‐molecular‐weight mucin‐like proteins (>500 kDa) in the P. canaliculus style suggests that these proteins may be important in the formation of the hard, permanent nature of the style, perhaps through interaction with medium‐molecular‐weight proteins (MacKenzie & Marshall 2014).
The style has additional functions in the digestive process. The low pH of the stomach facilitates the dislodgement of particles from the mucous string. These particles are then mixed with the other contents of the stomach, including the liberated enzymes from the style. The rotation of the style helps the mixing process. While all the mixing and extracellular digestion is taking place, the stomach contents come under the influence of ciliary tracts that cover all areas of the stomach except those occupied by the gastric shield. These ciliated tracts have fine ridges and grooves and act as sorting areas in much the same way as the labial palps. Finer particles and digested matter are kept in suspension by cilia at the crests of the ridges, and this material is continually swept toward the digestive gland duct openings. Larger particles segregate out and are channelled into the intestine along a deep rejectory groove on the floor of the stomach (see Chapter 4).
The digestive gland, which is brown or black and consists of blind‐ending tubules that connect to the stomach by several ciliated ducts, is the major site of intracellular digestion. The epithelium of the tubules is composed of two cell types, digestive cells and basophil (secretory) cells. The former are the most abundant type and are responsible for intracellular digestion of food. Digestion takes place within large vesicles called lysosomes that contain hydrolytic enzymes. The end products of digestion are released directly into the haemolymph system and waste products are contained in residual bodies within the digestive cells. The cells eventually rupture and the waste material enclosed in excretory spheres is swept along the ciliated secondary and primary ducts of the digestive gland toward the stomach, and ultimately to the intestine. The intestine terminates in an anus, and faeces in the form of faecal pellets are swept away through the exhalant opening. The basophil cells display a highly developed rough endoplasmic reticulum and numerous secretory granules, and carry out extensive protein synthesis and probably secrete digestive enzymes (Dimitriadis et al. 2004; Beninger & Le Pennec 2016). A more detailed description of the morphology and role of these two cell types is presented in Chapter 4.
Gonads
Mussels are dioecious (i.e. the sexes are separate), and there are usually equal numbers of males and females. The gonads extend throughout most parts of the body, except the gills, muscles and foot. Most of the gonad is in the mantle (Figure 2.6), thus accounting for its unusual thickness in mussels. The colour of the reproductive tissue is not a good indicator of the sex of an individual. Each gonad consists of a converging system of ducts leading to a gonopore on the tip of the genital papilla, which is located in the exhalant portion of the mantle cavity. Gametes are shed through the exhalant opening of the mantle and fertilisation takes place in the water column. After mussels have released their gametes, the mantle is thin and transparent. See Chapter 5 for a full description of mussel reproduction.
Heart and Haemolymph Vessels
The heart lies in the mid‐dorsal region of the body, close to the hinge line of the shell, in a space called the pericardial cavity, which surrounds the heart dorsally and a portion of the intestine ventrally. The wall of the cavity, a thin, translucent layer, is called the pericardium. The heart consists of a single, muscular ventricle and two thin‐walled auricles. Haemolymph – a colourless fluid of haemocytes and plasma (cell‐free haemolymph) – flows from the auricles into the ventricle, which contracts to drive it into a single vessel, the anterior aorta. The aorta divides into many arteries, the most important of which are the pallial arteries from the posterior aorta, which supply the mantle with haemolymph, and the visceral arteries (gastro‐intestinal, hepatic and terminal) from the anterior aorta, which supply the stomach, intestine, shell muscles and foot (Figure 2.13). While a posterior aorta is present in Guekensia spp., it is absent in Mytilus spp. The arteries break up into a network of vessels in all tissues and then join to form veins, which empty into three extensive spaces, the pallial, pedal and median ventral sinuses. The circulatory system is therefore an open system with haemolymph in the sinuses bathing the tissues directly. One consequence of an open circulatory system is that the animal is continually exposed to fluctuations in environmental factors, such as temperature, salinity and contaminants. From the sinuses, haemolymph is carried to the kidneys for purification. In Mytilus, some of the haemolymph from the kidney network enters the gills, discharging into the afferent gill vein, which gives off a branch to each gill filament, descending on one side and ascending on the other. The ascending vessels join to form an efferent gill vein that passes back to the kidneys. The haemolymph from the kidneys returns to the auricles of the heart. In other bivalves (e.g. Pecten spp.), haemolymph from the gills does not return to the kidney but flows directly from the gills to the heart (Figure 2.13). In all bivalves, there are well‐developed circulatory pathways through the mantle, which therefore serves as an additional site of oxygenation. See Field (1922) for a very detailed description of the arterial and venous systems in M. edulis.
Figure 2.13 Circulatory system of a typical bivalve. The shaded areas indicate the route of oxygenated haemolymph. While the bivalve heart has two auricles, only one of these is illustrated. Source:
From Pechenik (2010). Reproduced with permission from the McGraw‐Hill Companies.
Haemolymph plays a number of important roles in bivalve physiology. These include gas exchange, osmoregulation (see Chapter 7), nutrient distribution, waste elimination and internal defence (see Chapter 11). Because haemolymph constitutes 40–60% of the fresh tissue weight, it also serves as a fluid skeleton, giving temporary rigidity to such organs as the labial palps, foot and mantle edges. The haemolymph contains cells called haemocytes, which float in a colourless plasma. Most bivalves lack circulating respiratory pigments, probably because their sedentary lifestyle and large exposed surfaces (for oxygen uptake) preclude the need for such pigments. However, haemocyanin, the typical molluscan respiratory pigment, is found in some protobranch bivalves, while haemoglobin has been reported in several bivalve families (references in Giribet 2008). Haemocytes are not confined to the haemolymph system but move freely out of the sinuses into surrounding connective tissue, the mantle cavity and gut lumen. Therefore, it is not surprising that these cells play an important role in physiological processes such as nutrient digestion and transport, excretion, tissue repair, heavy metal metabolism and internal defence. See Chapter 7 for details on haemocyte types and their functions.
Excretory Organs
There are two types of excretory organs in bivalves, the pericardial glands and the paired nephridia or kidneys. In Mytilus, the reddish‐brown elongate kidneys lie ventral to the pericardial cavity surrounding the heart and dorsal to the gill axis, and in fact extend the complete length of the gill axis from the labial palps to the posterior adductor muscle (Figure