Structure and Nanostructure of Sturgeon Otoliths and Implications for Microchemistry Studies

Otoliths, calcium carbonate (CaCO₃) earbones, are among the most commonly used age and growth structures of fishes. Most fish otoliths are comprised of the most dense and energetically stable CaCO₃ polymorph, aragonite. Sturgeon otoliths, on the other hand, have been characterized as the rare and enigmatic polymorph, vaterite—a brittle and unstable form of CaCO₃. Neither vaterite nor sturgeon otoliths have been well-characterized; however, studies of habitat use throughout sturgeon life history sometimes use otolith microchemistry techniques whose results can be highly influenced by otolith nanostructure. We therefore sought to characterize the structure and nanostructure of sturgeon otoliths using light micrographs and neutron diffraction. We found that while sturgeon otoliths are primarily composed of vaterite, they also contain the more dense CaCO₃ polymorph, calcite. Our data also suggest that grinding the otoliths—a common procedure for some types of microchemical analyses—can lead to conversion of vaterite to calcite. Our findings suggest that careful documentation of methods and the otolith materials themselves is important for interpreting microchemistry data of sturgeon and other fishes that may contain more than one phase of CaCO₃.