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 2.6). One arm of each kidney is glandular and opens into the pericardium, and the other end is a thin‐walled bladder that opens through a nephridiopore and empties into the exhalant chamber of the mantle cavity. See Pirie & George (1979) for a more detailed description of the excretory system in M. edulis.
The brown‐coloured pericardial glands, sometimes referred to as Keber’s organs, develop from the epithelial lining of the pericardium and come to lie over the auricular walls of the heart. Waste accumulates in certain cells of the pericardial glands and is periodically discharged into the pericardial cavity, and from there is eliminated via the kidneys. Other cells of the pericardial glands are involved in filtering the haemolymph, the first stage of urine formation. The filtrate then flows to the glandular part of the kidney, where the processes of secretion and reabsorption of ions occur. The end result is urine that has a high concentration of ammonia and smaller amounts of amino acids and creatine. Most aquatic invertebrates excrete ammonia as the end product of protein metabolism. Ammonia is highly toxic but its small molecular size and high solubility in water ensure that it very rapidly diffuses away from the animal.
While the kidneys and pericardial glands are the major excretory organs, excretory products are probably also lost across the general body surface and particularly across the gills (see Chapter 7 for details on excretion and osmoregulation). The kidney also plays a very important role in the storage and elimination of radionuclides and heavy metals such as silver, cobalt, mercury, manganese, lead and zinc (Metian et al. 2011 and references therein; Pouil et al. 2015). In scallops, Metian et al. (2009) have shown that several of these metals are sequestered in renal concretions, mostly of calcium carbonate, before being eliminated in the urine.
Nerves and Sensory Receptors
The nervous system of mussels is fundamentally simple. It is bilaterally symmetrical and consists of three pairs of ganglia and several pairs of nerves (Figure 2.14). The cerebral ganglia are joined by a short commissure dorsal to the oesophagus. From each cerebral ganglion, two pairs of nerve cords extend to the posterior of the animal. One pair extends directly back to the visceral ganglion, which is located on the surface of the posterior adductor muscle. The second pair extends posteriorly
