Sagittal otoliths, typically composed of aragonite, are frequently laid down rather as vaterite during growth in hatchery-reared fish populations. Sagittal vateritisation is believed to impair individual hearing/balancing abilities, but the causal mechanism remains unclear. Here we experimentally demonstrated that rearing in Sr-rich water induces sagittal vateritisation in the HdrR-II1 inbred strain of the Japanese rice fish, Oryzias latipes. Both sagittae were partly vateritised in 70% of individuals subjected to the Sr²⁺ treatment (n = 10), whereas fish reared in normal tap water showed no sagittal vateritisation (n = 8). Our result is consistent with the theoretical prediction that vaterite becomes thermodynamically more stable than aragonite as the Sr²⁺ concentration in solution increases. A vateritic layer develops surrounding the original aragonitic sagitta in vateritised otoliths, some of which take on a comma-like shape. Electron probe microanalysis demonstrates that the vateritised phase is characterised by lower Sr²⁺ and higher Mg²⁺ concentrations than the aragonitic phase. It is unlikely that increased environmental Sr²⁺ is responsible for the sagittal vateritisation in farmed fish. However, our findings likely help to establish an in vivo assay using O. latipes to understand the physiological process underlying the sagittal vateritisation in farmed fish.

Concentrations of some divalent cation species were sclerochronologically quantified from the core to the edge of each otolith, because the vateritic phase is characterised by both lower strontium (Sr) and higher magnesium (Mg) concentrations than the aragonitic phase. Prior to the analysis, the upper surface of the specimens was coated with platinum-palladium for 60 s in an ion-beam sputter coater (Hitachi E-1030) to enhance electrical conductivity. Chemical composition of the otoliths was obtained using a JEOL JXA-8230 electron probe microanalyser (EPMA) at the Atmosphere and Ocean Research Institute, the University of Tokyo. The wavelength-dispersive spectrometry (WDS) method was used to measure Ca, Mg, and Sr concentrations with the electron beam of 5 µm in diameter with 5 µm intervals. Exposure times were set at 10 s for peaks and 5 s for background measurements with an accelerating voltage of 15 kV and a beam current of 12 nA. Wollastonite (CaSiO₃), magnesium oxide (MgO), and strontium titanate (SrTiO₃) were used as standard materials for the ZAF correction. Otoliths collected from all 18 individuals (n = 36) were subjected to the EPMA analysis.