Presentation Information
[R4P-01]Ultrastructure of the core region of otoliths in goldfish (Carassius auratus) and medaka (Oryzias latipes)
*Gen TAKAHASHI1, Toshihiro Kogure1, Micho Suzuki2, Taiga Okumura1 (1. Waseda Univ. Edu., 2. Univ. Tokyo Agri.)
Keywords:
biomineral,calcium carbonate,polymorph,vaterite,TEM
Otoliths in the fish inner ear are CaCO3 biominerals involved in sensing body tilt, acceleration, and sound. Most teleosts have three otoliths: the lapillus, sagitta, and asteriscus. The lapillus and sagitta consist of aragonite, whereas the asteriscus consists of vaterite. Although this selective formation of CaCO3 polymorphs is common among teleosts, its mechanism remains unclear. Here, juvenile otoliths of goldfish (Carassius auratus) and medaka (Oryzias latipes) were examined to compare the structure and chemistry of the core region, the starting point of crystal growth.
Lapilli, sagittae, and asterisci about 20–50 μm in diameter were collected, and thin films including the core were prepared by focused ion beam. TEM showed that the lapillus and sagitta cores in both species contained aggregates of spherical calcite, from which fine aragonite crystals grew outward (Figs. 1 and 2). In contrast, the asteriscus cores contained intergrown vaterite and low-electron-density material, probably organic matter (Figs. 3 and 4). This vaterite shared nearly the same crystallographic orientation as the surrounding crystals and grew continuously as a mosaic crystal. STEM-EDS mapping showed that Mn and Mg were concentrated in the spherical calcite of the lapilli and sagittae, whereas F was localized around them. Mn and Mg concentrations were absent in the asterisci, and F localization was observed only in the goldfish asteriscus.
The core structure and chemistry thus differ markedly between the aragonitic lapilli/sagittae and the vateritic asterisci. The basic core structure was similar between goldfish and medaka, with only minor differences in crystal size and elemental distribution. Thus, teleost otolith cores possess distinct initial structures corresponding to CaCO3 polymorphs. These differences may be related to crystal-growth initiation site control and CaCO3 polymorph selection during early otolith formation.
Lapilli, sagittae, and asterisci about 20–50 μm in diameter were collected, and thin films including the core were prepared by focused ion beam. TEM showed that the lapillus and sagitta cores in both species contained aggregates of spherical calcite, from which fine aragonite crystals grew outward (Figs. 1 and 2). In contrast, the asteriscus cores contained intergrown vaterite and low-electron-density material, probably organic matter (Figs. 3 and 4). This vaterite shared nearly the same crystallographic orientation as the surrounding crystals and grew continuously as a mosaic crystal. STEM-EDS mapping showed that Mn and Mg were concentrated in the spherical calcite of the lapilli and sagittae, whereas F was localized around them. Mn and Mg concentrations were absent in the asterisci, and F localization was observed only in the goldfish asteriscus.
The core structure and chemistry thus differ markedly between the aragonitic lapilli/sagittae and the vateritic asterisci. The basic core structure was similar between goldfish and medaka, with only minor differences in crystal size and elemental distribution. Thus, teleost otolith cores possess distinct initial structures corresponding to CaCO3 polymorphs. These differences may be related to crystal-growth initiation site control and CaCO3 polymorph selection during early otolith formation.
