Shellfish that live in salt water environments have an easily accessible source of the calcium and carbonate ions needed to build new shells. In contrast, those that live in freshwater environments, such as the common pond snail, need to develop clever mechanisms for obtaining those resources since the availability of dissolved calcium and carbonate ions is significantly less than that of their marine cousins. When resources of calcium ions are particularly low, the organism maintains critical calcium requirements for new shell formation by cycling internal sources from previously formed shell. Cells create a driving force for the uptake of calcium ions by utilizing the hydrogen ions generated from dissolved carbon dioxide. The hydrogen ions essentially exit the cell through a revolving protein door through which calcium ions summarily enter the cell.
"The system for Ca2+ and carbonate acquisition in the freshwater common pond snail Lymnaea stagnalis exhibited sensitivity to several pharmacological agents, which indicate that both Ca2+ uptake from the environment as well as endogenous HCO3– and/or CO32– production work together directly and indirectly for the post-metamorphic embryo to acquire the necessary ions for calcification in freshwater...In times of shell accretion, Ca2+ is taken up from the surrounding medium or from food and into the blood for transport to fresh tissues, particularly the mantle, for deposition as CaCO3 after combining with endogenously produced HCO3–. Internal production of HCO3– by the hydration of CO2, also produces H+ ions that are released from the tissues, transported by the blood to excretion sites on the epithelial membrane, and can be exchanged for Ca2+. During times of Ca2+ depletion, Ca2+ can be acquired from the shell, taken into the soft tissues and blood for transport to sites in need to Ca2+ supplement. Thus in adult Lymnaea, Ca2+ is transported between ambient media, fresh tissues, and shell via the blood...In the absence of external HCO3–, Lymnaea stagnalis embryos grow, form shell, develop normally, and hatch in the same time intervals as controls. These observations point to an internal source of HCO3–/CO32– for shell formation via the hydration of endogenous CO2. For this hydration reaction to allow for CO32– accumulation within the organism and for shell formation, hydrogen ions resulting from the hydration reaction must be eliminated. Thus the H+ pump plays a significant role in Ca2+ flux in post- metamorphic stages. By electrogenic excretion of H+ ions, the electrical potential across the apical membrane, and thereby the electrochemical gradient for Ca2+ uptake is maintained." (Ebanks et al. 2010:4095-4097)