Marine Mussels. Elizabeth Gosling
is probably the factor that has been most often cited as influencing mussel attachment (references in Garner & Litvaitis 2013b). Babarro & Carrington (2011) compared byssus tenacity (attachment) and associated features in mussels at an exposed and a sheltered site in the Ría de Vigo (NW Spain). They found that mussels inhabiting the rougher outer Ría secreted stronger and stiffer threads and had a higher potential to form crosslinks or metal chelation in the byssal collagen in order to gain structural integrity when needed (Figure 2.10B). Their results from reciprocal transplants indicated that mussels have the potential to change byssus diameter and mechanical properties in order to increase strength in stressful abiotic conditions, and can reallocate energy for vital activities such as gonadal and soft tissue growth in more benign environments (see also Babarro & Carrington 2013). However, Moeser & Carrington (2006) and Moeser et al. (2006) suggest that seasonal variations in material properties of the byssus play an even more significant role than wave action in determining mussel attachment strength. They found that thread strength and extensibility increase after autumn and winter, leading to the strongest attachment occurring during the spring, at which point energetic resources switch their focus toward gamete production (see also Zardi et al. 2007). This shift in energetic allocation, combined with increased thread decay, decreases attachment strength throughout the summer, leading to the weakest attachment strength occurring in the autumn. Hawkins & Bayne (1985) estimate that byssus production can consume up to 8% of a mussel’s monthly energy expenditure.
The mechanical properties of byssal threads vary depending on the species. An examination of the material and structural properties of the threads of M. californianus, M. galloprovincialis and M. trossulus indicated that while the material properties of the threads were similar among species, the distal portion of the threads of M. californianus extended further before breaking, leading to a stronger attachment strength (8–17% increase) relative to the other two species. This may be a factor in the domination of M. californianus on wave‐exposed shores on the Pacific coast of North America (Bell & Gosline 1996). The preCOLs in this species are more divergent from those of the other two than the preCOLs of M. galloprovincialis and M. trossulus are from each other. The single most influential factor in the tensile superiority of M. californianus is the greater abundance of silk‐like alanine‐rich sequences in the flanking domains of preCOLs (Harrington & Waite 2007). See Bell & Gosline (1997), Lucas et al. (2002), Brazee & Carrington (2006), Pearce & LaBarbera (2009), Bouhlel et al. (2017), George et al. (2018) and Newcomb et al. (2019) and references therein for additional comparative studies on mytilid thread properties.
As seen 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