Ultrastructure of lamellae, mineral and matrix components of fish otolith twinned aragonite crystals: implications for estimating age in fish

Abstract

Atomic force microscopy (AFM) of the crystalline ultrastructure of otoliths fromPagrus major(Sparidae),Macruronus novaezelandiae(Merlucciidae),Oncorhynchus tshawytscha(Salmonidae),Sebastes alutus(Scorpaenidae), andHoplostethus atlanticus(Trachichthyidae) showed regular deposition of lamellae in the size range 13–490 nm. The orientation of lamellae in the {010} plane was the same as lamellae in crystals of mineral aragonite. Lamellae in mineral aragonite were in the size range 15–45 nm. Lamellae observed in the otolith ofM. novaezelandiaeby transmission electron microscopy showed a range of widths (25–225 nm) similar to lamellae observed by AFM. The observed lamella widths were in the size range that has been described for sub-daily and daily microincrements in otoliths. Observed lamellae widths were also in the size range of α-recoil trajectories of222Rn and provide a potential diffusion sink correction for the222Rn losses in radionuclide method of ageing otoliths. Comparison of the orientations of lamellae to templates based on the Bragg unit cell structure of twinned aragonite indicated that the lamellae resulted from polysynthetic twinning on the {010} aragonite crystal face. Additional cyclic twinning occurred on the {110} face of the aragonite crystal and sometimes led to pseudohexagonal crystals, whose sizes were orders of magnitude larger than lamellae. The organic matrix of the otolith was visible by atomic force and transmission electron microscopy at the nanometer level of resolution, but the organic matrix was confined to the {110} twinning plane of symmetry of the otolith crystal.