Fig. 5
From: Crystallographic and geochemical responses of giant clams on turbid reefs
Conceptual representation of biomineralization in giant clams. Not to scale. Shell mineralization pathways for each shell layer in relation to the inner shell mantle (IM) and outer shell mantle (OM) shown. Formation of the shell (Ca2 + HCO3− → CaCO3 + H+) takes place in an extracellular privileged space, known as the extrapallial fluid (EPF) (e.g.57). The inner shell layer (IL), with a complex crossed lamellar microstructure (CCL) (a) forms from the inner EPF (iEPF) and the outer shell layer (OL), with a crossed lamellar microstructure (CL) (b) from the outer EPF (oPF). The IL and OL are demarcated by the pallial line (PL). Transport of Ca to the EPF occurs at the mantle epithelium through active (e.g., Ca2+ ATP-ases)52 and passive (e.g., paracellular diffusion) transport mechanisms3. Organic and Mg rich amorphous calcium carbonate (ACC) vesicles cross the epithelial cell membrane and are transported to the calcification front. ACC particle attachment and subsequent ion attachment occur at the biomineral growth front and crystallize into aragonite (e.g.,58). Zooxanthellae are housed extracellularly in Z-tubules (not shown) within the inner fold (IF) of the OM and carry out photosynthesis from sunlight to provide nutrition to the clam through an autotrophic feeding pathway54. The EPF is partially open to surrounding seawater but is chemically different from seawater due to ‘vital effects’ tightly controlled by the organism17. The IL is situated between the IM and OL, in close contact with the iEPF and physically further away from seawater (a). The OL is in direct contact with seawater, where OM specific mechanisms are carried out and the clam undergoes light-dependent physiological phenomena18. In the case of Mg and Sr, different predominant pathways are responsible for transportation to the site of calcification (a). Mg is associated with the organic or ACC phase because its ionic radius is not favorably substituted in the orthorhombic crystal lattice of aragonite44,55. Sr is substituted for Ca within the aragonite lattice due to similar ionic radii and may predominantly enter the EPF through active transport mechanisms such as Ca2+ ATPase (e.g.,18).
