Eyes and ears: A comparative approach linking the chemical composition of cod otoliths and eye lenses

Fish eye lenses are a protein-based chronological recorder of microchemical constituents that are a potentially useful tool for interpretations of environmental, ecological and life-history experienced by fish. Here, we present the first study with data on the chemical composition of eye lenses from Baltic cod examined using laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) and compare these spatially resolved data to otoliths from the same fish also analysed by LA-ICPMS, measuring the isotopes ²⁷ Al, ¹³⁷ Ba, ⁴³ Ca, ⁵² Cr, ⁶⁵ Cu, ⁵⁷ Fe, ³⁹ K, ⁷ Li, ²⁵ Mg, ⁵⁵ Mn, ³¹ P, ²⁰⁸ Pb, ⁸⁵ Rb, ⁴⁵ Sc, ²⁹ Si, ⁸⁸ Sr, ⁴⁷ Ti, ⁵⁰ V, ¹⁴⁹ Yb, ⁶⁶ Zn and ⁹⁰ Zr. Comparison of the variation in element concentrations between eye lenses and otoliths from the same individuals showed minor similarities, suggesting a different governance in the uptake processes. A strong overlap between the concentric growth rings in the eye lenses and the otolith Sr periodicity was observed, where each consecutive minima in the chemical profile with high accuracy correspond to the width of each lens ring. No comparable trends were seen between growth rings and all other elements measured from both lenses and otoliths. The characteristic rings observed in cod eye lenses do not seem to represent seasonal fluctuation nor are they found to be directly linked to age. With this research, we provide a baseline study identifying elements in corresponding eye lenses and otoliths that show potential for unravelling the environmental and biological conditions experienced by fish.