Marine Mussels. Elizabeth Gosling

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Figure 2.14 Schematic representation of the nervous system in the mussel Mytilus edulis. Dashed line indicates the outline of the two shell valves. (a) Cerebral ganglia. (b) Pedal ganglia. (c) Visceral ganglia. (d) Cerebrovisceral pedal connective. (e) Cerebropedal connective. (f) Cerebrovisceral connective. (g) Pedal nerve. (h) Byssal retractor nerve. (i) Branchial nerve. (j) Posterior pallial nerve. (k) Anterior pallial nerve. (l) Buccal nerve. (m) Cerebral commissure. (n) Visceral commissure. (o) Circumpallial nerve. Ganglia are small, their longest dimension never exceeding 2 mm, but each contains numerous neurosecretory cells.

      Source: From de Zwann & Mathieu (1992). Reproduced with permission

      from Elsevier.

      The ganglia also have a major neurosecretory role in bivalves. Several different types of neurosecretory cells have been identified in mussels, and most of these are located in the cerebral ganglia (de Zwann & Mathieu 1992). These cells produce peptides that are released into the circulatory system. At least four different neuropeptides have been identified that mediate reproductive‐related events in bivalves (Morishita et al. 2010). Insulin‐related peptides have also been identified in neurosecretory cells of several bivalve species and have been shown to be involved in growth regulation by stimulating protein synthesis in mantle edge cells involved in shell and soft tissue growth. More details on the role of neurohormones in gametogenesis and growth are presented in Chapters 5 and 6, respectively.

      During the evolution of bivalves, with loss of a distinct head, most of the sense organs withdrew from the anterior end and came to lie at the edge of the mantle. Most sensory receptors are located on the middle fold of the mantle. This fold is thick and bears a large number of pallial tentacles, their length and number varying with the species (Figure 2.7). The tentacles are covered in epithelial tactile cells that are sensitive to touch. A slight tactile stimulus elicits local contraction of the mantle or siphon musculature. This is a reflex action and is not under the control of the central nervous system (CNS). A strong stimulus produces a coordinated retraction of the whole animal into its shell. This more general and clearly adaptive type of contraction is under the control of the visceral ganglion. The mantle also contains sensory cells (chemoreceptors) that are stimulated by waterborne chemicals. These cells are capable of detecting the presence of gametes in the water column and provide a powerful chemical stimulus for ripe mussels to release their gametes, thereby enhancing the chances of fertilisation. Chemoreceptors also help juvenile mussels to select a suitable substrate for settlement (see Chapter 5).

      Ocelli, which can detect sudden changes in light intensity, may also be present on the middle fold of the mantle or siphons. These may take the form of invaginated eyecups lined with pigmented sensory cells and filled with a mucoid substance that acts as a ‘lens’, or they may be very well developed structures that produce a low‐contrast image as in scallops. Research shows that swimming scallops tend to have better vision than sessile scallops, and this suggests that mobile scallops may visually detect preferred habitats (Speiser & Johnsen 2008a). Also, during shell gaping, when eyes on the mantle edges are exposed, scallops may visually detect the size and speed of moving particles and use this information to help identify favourable feeding conditions (Speiser & Johnsen 2008b). For more information on the visual physiology of scallops, see Speiser & Wilkens (2016).

Sensory receptors called osphradia are well known in gastropods, but in bivalves they are difficult to detect, either because of their small size or because they are in fact absent in some species. These receptors, usually paired, consist of a pigmented epidermal patch of sensory and secretory cells located in the gill axis and enervated by the branchial nerve. There has been some debate as to the role of these structures in bivalves (reviewed by Haszprunar 1987), but the general consensus is that they have a dual function: reception of chemical spawning cues by the sensory cells and synchronisation of gamete emission by the secretory cells through the release of serotonin, a powerful stimulant for spawning in bivalves (Beninger et al. 1995). While these sensory structures are no doubt real anatomical features, Lindberg & Sigwart (2015) question the assumption that there is a single molluscan osphradium and propose that at least two distinct classes of epithelial sensory structures have been identified as such.

      Another type of sensory receptor is the statocyst, which lies in the foot near the pedal ganglia and is innervated by the cerebral ganglia. The structure of the statocyst, while varying between invertebrate species, typically consists of a dense mass known as the statolith or of multiple smaller statoconia, in a fluid‐filled chamber lined by sensory hair cells (Budelmann 1988). The solid concretion(s) interact with the cilia and convey information to the mussel on its orientation in space. Anthropogenic activities in the oceans, such as pile driving, involve direct contact with the seabed, creating radiating particle motion waves. Sensitivity of M. edulis to substrate‐borne vibration has been quantified by exposure of the species to vibration under controlled conditions (Roberts et al. 2015). The results indicate that vibration is likely to impact overall fitness in this species due to disruption of natural valve gape periodicity. It is likely that vibration travelling through the mussel body may stimulate movement of the statocyst system, as observed in other invertebrates (Roberts et al. 2015 and references therein).

      Finally, another sensory receptor, the abdominal sense organ (ASO), first described in scallops but now reported from at least 19 bivalve families (Haszprunar 1983), is a small piece of tissue situated on the adductor muscle near the anus. The receptor epithelium contains ciliated sensory cells and mucocytes, the former making up 90% of the total cell number (details in Zhadan et al. 2004). While some believe that the ASO functions in the regulation of water flow within the pallial cavity (Beninger & Le Pennec 2006), others have suggested that it may be involved in the perception of waterborne vibrations – similar in some respects to the acoustic lateral line system in fish – and Zhadan (2005) has indeed shown that the scallops Mizuhopecten yessoensis and Chlamys swifti are sensitive to ultrasonic vibrations in the range 30–1000 Hz. This type of sensory input could give warning of the approach of a predator.

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