Fine-Scale Oscillatory Banding in Otoliths from Arctic Charr (Salvelinus Alpinus) and Pike (Esox Lucius)

Abstract

AbstractTransmission electron microscopy of otoliths from the inner ear of arctic charr and pike has revealed the presence of fine banding on the scale of several nanometers. The thickness of the bands was observed to vary in different portions of the sample, and some areas were not banded. EDS analysis could not detect chemical differences within the bands, but electron diffraction showed that the crystallographic orientation of the bands is related by a lattice mismatch indicative of a twinning relationship. Previously, banding on the scale of 50 to 100 microns was observed by SEM in otoliths from arctic charr and was attributed to seasonal variations in growth. The fine-scale twinning observed in this study, however, is unlikely to be nucleated through daily variations. Electron diffraction from the pike samples shows that the material is composed of CaCO3 having the both the vaterite and aragonite structure, and hydrous CaCO3 was also observed.The interaction of the electron beam with the sample material was investigated by conducting several electron-irradiation experiments. The electron beam was observed to interact strongly with the sample and caused the precipitation of cubic CaO from the calcium carbonate matrix. Bright-field imaging showed the development of fine grained (∼ 5 nm) randomly-oriented crystallites which accumulated with increasing electron dose. These initial results suggest that the precipitation of GaO is not driven by electron-beam heating. Previously, a similar phase-change phenomenon has been observed in hydroxyapatite from dental enamel. Other Ca-bearing biominerals may therefore also be expected to be sensitive to electron irradiation.