Studies on chitin and calcification in the inner layers of the shell of Anodonta cygnea

Summary An orthorhombic structure -chitin, probably in the form of a chitin-protein complex, was identified in the matrix of the shell of Anodonta cygnea by X-ray diffraction. Aragonite crystals of pseudohexagonal symmetry were also found by a Lauegram on the nacreous layer of the shell. The orthorhombic structure of these two compounds together with the identical reticular spacing d₁₁₀ corroborate, in Anodonta cygnea, the indirect chitin-aragonite relationships already suggested for molluscan shells.Observations with SEM in the inner surface of the shell showed CaCO₃ crystals with irregular geometrical shapes in spring and summer and regular geometrical shapes in autumn and winter. The more elaborate aspect appearing in winter corresponds to an accurate hexagonal shape. This suggests that the observed variability may depend on the balance between calcium and hydrogen ions in the extrapallial fluid.Abbreviations OME outer mantle epithelium - SEM scanning electron microscopy