10:30 AM - 10:45 AM
[R4-06] Unique structures and chemical compositions found around the center of fish otoliths
[Presentation award entry]
Keywords:biomineralization, calcium carbonate, vaterite, polymorph, TEM
Otoliths, which form in the inner ear of fish, are thought to serve as sensors detecting tilt, acceleration, and sound, made of a block of calcium carbonate (CaCO3). In general, teleost fish have three types of otoliths known as lapillus, sagitta, and asteriscus. The lapillus and sagitta are made of aragonite and the asteriscus is made of vaterite. However, it remains unclear how these polymorphs are selected. In this study, each otolith in the initial formation phase of juvenile fish are collected and the structure and chemical composition near the center of the otolith, which is the starting point for crystal growth, were analyzed by transmission electron microscopes to elucidate the polymorph selection mechanism in fish otoliths.
As samples, lapillus, sagitta, and asteriscus, each about 20 to 50 μm in diameter, were collected from juvenile goldfish (Carassius auratus), thin films containing their centers were prepared by focused ion beam and observed with transmission electron microscopes. As a result, in the center of the lapillus and sagitta, a structure in which multiple spherical substances had aggregated was observed (Figures 1, 2) and electron diffraction patterns explainable by single-crystal calcite are obtained from them. Around these substances, fine aragonite crystals were growing radially. On the other hand, the center of the asteriscus contained a mixture of a low-density substance (probably organic) and almost single-crystal vaterite (Figure 3), and the crystals growing around it were mosaic crystals of vaterite with roughly the same orientation as this. Furthermore, an energy-dispersive X-ray detector mounted on a scanning transmission electron microscope was used to examine the elemental distribution in the centers of these otoliths. In the lapillus and sagitta, Mn was localized in the spherical substances and F was localized around them (Figures 4, 5), while in the asteriscus, almost no Mn was detected, and only F was localized (Figure 6). Thus, the structure and chemical composition near the center of the otoliths are completely different between aragonite lapillus and sagitta and vaterite asteriscus, suggesting that the selection of these polymorphs may be due to the differences in the substances that serve as the starting point of the crystal.
As samples, lapillus, sagitta, and asteriscus, each about 20 to 50 μm in diameter, were collected from juvenile goldfish (Carassius auratus), thin films containing their centers were prepared by focused ion beam and observed with transmission electron microscopes. As a result, in the center of the lapillus and sagitta, a structure in which multiple spherical substances had aggregated was observed (Figures 1, 2) and electron diffraction patterns explainable by single-crystal calcite are obtained from them. Around these substances, fine aragonite crystals were growing radially. On the other hand, the center of the asteriscus contained a mixture of a low-density substance (probably organic) and almost single-crystal vaterite (Figure 3), and the crystals growing around it were mosaic crystals of vaterite with roughly the same orientation as this. Furthermore, an energy-dispersive X-ray detector mounted on a scanning transmission electron microscope was used to examine the elemental distribution in the centers of these otoliths. In the lapillus and sagitta, Mn was localized in the spherical substances and F was localized around them (Figures 4, 5), while in the asteriscus, almost no Mn was detected, and only F was localized (Figure 6). Thus, the structure and chemical composition near the center of the otoliths are completely different between aragonite lapillus and sagitta and vaterite asteriscus, suggesting that the selection of these polymorphs may be due to the differences in the substances that serve as the starting point of the crystal.