Evidences of formation of predominant calcite polymorphs in otoliths of the fishes of riverine environment

Fish otoliths record biological and environmental history through a metabolically driven biomineralization process. The otolith's crystalline structure is due to the presence of calcium carbonate polymorphs that may influence trace element chemistry; thus, understanding otolith polymorphs is essential. This study employs confocal micro-Raman spectroscopy to investigate the crystalline polymorphs in otoliths of three freshwater fishes (Heteropneustes fossilis, Channa punctata, and Labeo bata) from the River Ganga. In all three fishes, Raman signals showed the strong calcite band intensities of v₁ stretching mode at 1087 cm⁻¹ and weak signals of lattice mode at 154.9 cm⁻¹, aragonite band intensities of v₄ vibrational mode at 707 cm⁻¹ and lattice mode at 208 cm⁻¹. The vaterite band intensities of the lattice mode were recorded in L. bata and C. punctata. The mean baseline-corrected Raman intensities were highest for calcite in all three fishes. The results suggested that calcite is the predominant polymorph in the selected fish species. The replacement of calcium by barium was mapped as witherite (BaCO₃) in H. fossilis and C. punctata. The Kruskal-Wallis and post hoc Dunn’s test revealed significant differences among nearly all polymorph intensities. The findings can help to understand the biological processes involved in otolith formation and growth. Further detailed research is warranted to decipher the ecological factors that could have led to the relative abundance of calcite polymorphs in the target fish species.