Early mineralization in Biomphalaria glabrata: Microscopic and structural results

The formation of the shell of Biomphalaria glabrata was studied in dissected embryos and egg masses with scanning electron microscopy, electron spectroscopic imaging (ESI), thermogravimetry, synchrotron powder diffraction and Ca K-edge X-ray absorption spectroscopy (EXAFS). The results show that shell formation starts by accumulation of calcium below the periostracum at an age of about 60 h. At first, amorphous calcium carbonate is precipitated. EXAFS studies reveal that an aragonitic structure is already present at a microscopic level in this amorphous precursor phase. The first crystalline phase (calcium carbonate in the form of aragonite) can be detected at an age of 72 h. It is concluded that amorphous calcium carbonate initially present is transformed into crystalline aragonite during embryonic development, without vaterite appearing as a potential intermediate.