CALCIUM CARBONATE POLYMORPHS IN FISH OTOLITHS: TOWARDS UNDERSTANDING ONTOGENETIC, ENVIRONMENTAL, AND PHYLOGENETIC CONTROLS

Fish otoliths, or ear bones, are comprised of the CaCO₃ polymorphs (aragonite, calcite and vaterite), which can occur either alone or in combination. Otoliths are frequently used in environmental studies to infer temporally-explicit environmental conditions or fish life history events such as migrations based on concentrations of trace elements within daily-to-annual growth rings. Incorrect characterization of the mineralogical composition and microsctructure of fish otoliths can impact the interpretation of trace elements. The polymorph phase abundance in an otolith depends on, as yet, unexplained genetic and environmental factors. Moreover, the occurrence of multiple polymorphs in an otolith is not well understood. Most fish otoliths are comprised of the densest CaCO₃ polymorph, aragonite, and concentric growth rings are the common microstructure, but these notions are potentially outdated with many recent literature reports showing otherwise. We have been employing a variety of materials science methods ( Polarized Light Microscopy, X-ray Diffraction, Neutron Diffraction, X-ray Tomography, Raman Spectroscopy. Neutron Vibrational Spectroscopy, Synchrotron XRF, Gas Pycnometry, Crystal Growth Screens) to characterize the CaCO₃ polymorphs distributions and microstructures of fish otoliths to understand their ontogenetic, environmental, and phylogenetic controls. Our work suggests that examination of the CaCO₃ polymorph composition of otoliths should become more common particularly in studies where results will or may inform fisheries management decisions. Future research should work to attribute controls on otolith CaCO₃ polymorph expression using a combination of -omics and material characterization approaches to enrich the life history and environmental information output from otoliths. Research conducted at ORNL's High Flux Isotope Reactor and Spallation Neutron Source was sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy.