Structural and Chemical Characterization of Gastroliths and Calcium Carbonate Stability in Narrow-Clawed Crayfish (Pontastacus leptodactylus (Eschscholtz, 1823)) from Atikhisar Reservoir, Türkiye

Narrow-clawed crayfish (Pontastacus leptodactylus) is an economically important freshwater species in inland fisheries. During molting, gastroliths function as temporary calcium carbonate storage structures and contribute to calcium mobilization for subsequent cuticle mineralization. In this study, the structural, chemical, and morphological properties of white and blue gastroliths extracted from mature P. leptodactylus individuals collected from Atikhisar Reservoir, Çanakkale, Türkiye, were investigated. Raman spectroscopy, X-ray diffraction (XRD), inductively coupled plasma-optical emission spectroscopy (ICP-OES), and scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS) were used for complementary characterization. Raman spectra confirmed the presence of carbonate-related vibrational bands in both gastrolith types, while XRD patterns indicated differences in crystallinity between white and blue samples. Blue gastroliths exhibited sharper and more pronounced diffraction peaks, suggesting a higher crystalline calcium carbonate fraction, whereas white gastroliths showed broader features consistent with a greater amorphous component. ICP-OES results showed that calcium concentration was higher in blue gastroliths, while phosphorus and magnesium concentrations were relatively higher in white gastroliths. When expressed as calculated calcium carbonate equivalent, the gastroliths contained approximately 75-78% CaCO3. SEM-EDS observations revealed layered microstructures and fibrous organic-associated features, with Ca, O, C, P, and Mg as the dominant detected elements. Overall, the results suggest that color variation in P. leptodactylus gastroliths is associated with differences in elemental composition, crystallinity, and the relative contribution of amorphous and crystalline calcium carbonate phases. The higher phosphorus content in white gastroliths may contribute to the stabilization of amorphous calcium carbonate, whereas the higher calcium content and sharper XRD peaks in blue gastroliths indicate a more crystalline mineral structure. These findings provide baseline data for understanding gastrolith biomineralization in Turkish populations of P. leptodactylus and may contribute to future studies on molting physiology, calcium metabolism, and freshwater crayfish aquaculture.