Figure 7.

Proposed pathways of the shell calcification and biomineralisation in Achatina fulica. The primary source was the release of CO₂ from the carbohydrate metabolism (green arrow) and transport to the extrapallial fluid space (EFS) (grey dashed arrow). The CO₂ was converted to HCO₃⁻ by several intracellular carbonic anhydrases (red arrow), and the bicarbonate ions were actively transported to the EFS through SLC26A transporters or HCO₃⁻/Cl⁻ exchangers (dark blue oval and dark brown arrows) and SLC4A transporters or HCO₃⁻/Na⁺ exchangers (orange oval and dark brown arrow). Excess hydrogen ions produced by the formation of calcium carbonate and from carbonic anhydrase were eliminated through the Na⁺/H⁺ exchanger (dark red oval and dark red arrow) out of the EFS. Calcium intake from food could reach the extrapallial fluid space passively by diffusion through Ca²⁺ channels (purple oval and dark green arrow) and gap junctions (green dash arrow) or actively by the Ca²⁺-pumping ATPase (pink oval and dark green arrow). The CaCO₃ then formed the structure of prismatic and nacreous layers making up the A. fulica shell with the assistance of a collagen protein network as the scaffold (red line).