earlier, marine mussels use a natural adhesive to adhere to a wide variety of substrates in an aqueous environment, and to date there are no synthetic glues that are as strong, versatile and unaffected by water as mussel glue. It is not surprising therefore that mussel adhesive proteins are attractive targets for biomimetic technology, which entails using designs from nature to solve problems in engineering, materials science, medicine and other fields. So far, more than a dozen adhesive proteins have been identified and characterised, and recombinant DNA technology has been used to obtain them in large amounts for conventional adhesion tests and practical applications. Two commercial mussel adhesive products are available on the market: Cell‐Tak, a naturally extracted adhesive consisting of fp‐1 and fp‐2, and MAP, which contains only fp‐1 (Cha et al. 2008). Alternatively, the exceptional adhesive properties exhibited by the native proteins can be captured in synthetic polymer systems (see Lee et al. 2011 for review). These have potential use as coatings for a wide range of organic and inorganic materials. For example, they are used for adhesion and sealing in foetal membrane rupture, corneal tissue sutures, surgical repair of nerves and cancer drug delivery (Kaushik et al. 2015). They are also used in antifouling coatings (Lee et al. 2011), to create hydrogels for drug delivery (Lee & Konst 2014) and to anchor nanoparticles on to a variety of surfaces (Zhu & Pan 2014).
Labial Palps
Each gill terminates within a pair of triangular‐shaped palps that are situated on either side of the mouth (Figure 2.6) and extend posteriorly about one‐third of the length of the mantle cavity (Morton 1992). The inner surface of each palp faces the gill and is folded into numerous ridges and grooves that carry a complicated series of ciliary tracts. The outer surfaces of the palps are smooth, and between the inner and outer surfaces there is muscular connective tissue (see Figures 4.15 and 4.16).
The main function of the labial palps is to continually remove material from the food tracts on the gills in order to prevent gill saturation. In dense suspensions, sorting and rejection tracts on the palps channel most of the filtered material away from the mouth and deposit it as pseudofaeces so that the animal can continue to filter and ingest at an optimum rate. The pseudofaeces is carried along rejectory tracts on the mantle to the inhalant opening and periodically forcefully ejected through it. When the ingestive capacity is not exceeded, particles from the gill move along acceptance tracts on the labial palps toward the mouth (see Chapter 4).
Alimentary Canal
Stomach and Digestive Gland
The mouth is ciliated and leads into a narrow ciliated oesophagus. Ciliary movement helps to propel material toward the anterior part of the stomach. Indeed, this method of moving material is found throughout the length of the alimentary canal, primarily because it lacks a muscular wall. The stomach is large and oval‐shaped and lies completely embedded in the digestive gland, which opens into it by several ducts. A semi‐transparent gelatinous, tapering rod, the crystalline style, originates in a style sac at the posterior end and projects forward and dorsally across the cavity of the stomach to rest against the gastric shield, a thickened area of the stomach wall (Figure 2.12; details in Morton 1992). The projecting anterior end of the style is rotated against the gastric shield by the style sac cilia, and in the process the style end is abraded and dissolved, releasing a range of carbohydrate‐, fat‐ and protein‐splitting enzymes in the process (see Chapter 4). This loss is made good by continual additions by the style sac to the base of the style. Some digestive enzymes are actually synthesised in the digestive gland and transported to the style sac, where they are secreted (Sakamoto et al. 2008). Rotation of the style also aids in pulling a food‐laden mucous strand through the mouth into the stomach.
Figure 2.12 (A) The bivalve digestive system. Redrawn from Langdon & Newell (1996), after Galtsoff (1964).
Reprinted with permission from Maryland Sea Grant.
(B) Bivalve stomach showing rotation of crystalline style and winding of food string. Rejectory groove on floor of stomach not shown.
Source: From Pechenik (1991). Reproduced with permission from the McGraw‐Hill Companies.
The length of the style is correlated with shell length; in M. edulis and M. galloprovincialis, the length is approximately 50% of shell length (Alyakrinskaya 2001). Style length changes with the season, with maximum length in spring when food intake is high. Also, the length exhibits a tidal cycle, with maximum length when the animal is submerged and feeding; in the absence of water, the style shortens by approximately 25% in M. edulis (Alyakrinskaya 2001). While the style has an ephemeral existence in most bivalves, in Perna canaliculus it is a firm, robust, 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.
