Arcuatula senhousia
Structure
The main component of the shell is calcium carbonate, formed by the deposition of crystals of this salt in an organic matrix composed of polysaccharide β‐chitin, a hydrophobic silk protein and a complex assemblage of hydrophilic proteins, many rich in aspartic acid (Addadi et al. 2006). The mineral component represents more than 95% of the shell weight, while the organic matrix represents 1–5%. Despite this, the organic component increases the fracture toughness of the shell by two or three orders of magnitude (Kamat et al. 2000). Three layers make up the shell: (1) a thin outer periostracum of conchiolin, a tough insoluble protein forming the organic matrix of the shell, within which calcium carbonate is deposited – this layer is often a different colour to the underlying shell layers; (2) a middle prismatic layer composed of fibres of aragonite or calcite – both crystalline forms of calcium carbonate (Figure 2.2); and (3) an inner nacreous layer composed of tablets of aragonite arranged in layers separated by sheets of an organic matrix of elastic biopolymers composed of chitin and silk‐like proteins – the nacre is either of a dull texture or iridescent mother‐of‐pearl, depending on the species.
Figure 2.2 Calcitic fibrous layers of adult mytilids. (a–d) Fibrous layer of Bathymodiolus azoricus. (a) General view showing high irregularity in size of fibres. (b–d) Details of fibres. The arrows point to (b) newly formed, (c) twisting or (b,d) splitting fibres. (e,f) Choromytilus chorus. (e) General view of the layer, showing high regularity in size and distribution. (f) Detail of the same layer close to the shell margin. The arrow points to bending fibres. (g,h) Mytilus edulis platensis (now accepted as M. platensis by World Register of Marine Species, WoRMS; www.marinespecies.org). General view and detail of the fibrous layer. Note the even orientation of rhombohedral faces in (h). (i) Mytilus californianus. Fibres are characterised by irregular outlines and endings. The arrow indicates a bent fibre.
Source: From Checa et al. (2014). Reproduced with permission from Schweizerbart Science Publishers.
A range of techniques, such as scanning electron microscopy (SEM), transmission electron microscopy (TEM), X‐ray diffraction (XRD), electron backscatter diffraction (EBSD) and atomic force microscopy (AFM), have played an important role in clarifying the microstructure of the different shell layers (Chateigner et al. 2000; Furuhashi et al. 2009; Checa et al. 2014; Nakamura Filho et al. 2014), while analytical methods such as secondary ion mass spectrometry (SIMS; Shirai et al. 2008), electron probe microanalysis (EPMA; Jacob et al. 2008) and laser ablation inductively coupled plasma mass spectrometry (LA‐ICP‐MS; Jacob et al. 2008) have been used to elucidate chemical composition of the shell in several species. In addition, the NanoSIMS microprobe allows trace element imaging and quantification in bivalve shells (Shirai et al. 2008).
The construction of the shell begins very early in larval development. An area of the ectoderm thickens in the dorsal region of the developing embryo. The area invaginates to become a shell gland, which forms a groove, which eventually becomes the future ligament between the two shell valves (Marin & Luquet 2004). The peripheral cells of the shell gland produce an extracellular lamella, the future periostracum, which will serve as a scaffold for the developing shell. Subsequently, the shell gland everts and the shell field spreads by flattening of the cells and mitotic divisions, thus becoming the calcifying mantle. Between the periostracum and the cells of the shell field, primary mineralisation takes place. The first larval shell is the prodissoconch I stage, followed by the prodissoconch II stage and then the dissoconch stage after metamorphosis (details in Chapter 5). At the prodissoconch I stage, the mineral produced is usually amorphous calcium carbonate, followed by either aragonite or calcite at prodissoconch II (Marin & Luquet 2004). The sequence can vary. For example, in the oyster, Ostrea edulis, the first mineral deposited is calcite, followed by aragonite at the prodissoconch II stage; at the dissoconch stage, the fraction of calcite rapidly increases and that of aragonite decreases (Medaković et al. 1997). By the time the juvenile stage is reached, the shell is heavily calcified and has different pigmentation and more conspicuous sculpturing than the larval shell.
Shell formation in juveniles and adults involves three separate elements: the mantle and its outer epithelium, the periostracum and the interface between the outer epithelium, the periostracum
