Identification of calcitic and aragonitic otoconia by selective staining methods

This report concerns whether the otoconia of the utricle and saccule removed from the inner ear of amphibia, especially of the frog Rana nigromaculata, are composed of calcite or aragonite. These compounds were identified by selective staining with alizarine red S, Harris' hematoxylin and Feigl's solution. The result showed that both the saccular and the utricular otoconia were stained red and purple with alizarine red S and Harris' hematoxylin, respectively, while only the saccular otoconia were stained by black precipitates with Feigl's solution which stains aragonite. Furthermore, mineral aragonite, which is an inorganic substance, was stained with Feigl's solution for the purpose of comparison with the saccular otoconia, which contain organic substances. Thus, the saccular otoconia and the mineral aragonite had the same staining appearance. These observations provide evidence that the saccular otoconia consist of aragonite, the utricular of calcite. The present, selective staining methods may be useful for studies, such as those focused on phylogenetic development, of calcitic and aragonitic otoconia in otolithic organs.