Abstract:Most fish otoliths are composed of aragonite, but occasionally they form a mosaic with its polymorph, vaterite. We found an unusual mosaic structure of extremely low strontium to calcium (Sr/Ca) ratio in the aragonite otoliths of European eels Anguilla anguilla. The mosaic crystal structure in the eel otolith appeared opaque under reflected light after EDTA etching and was confirmed to be a vaterite crystal by Raman spectroscopy. Analysis by laser ablation inductively coupled plasma mass spectrometry indicated that the elements sodium (Na), strontium (Sr) and barium (Ba) were lower in abundance and magnesium (Mg) and manganese (Mn) were higher in the vaterite than in the aragonite. Otoliths with vaterite inclusions were found in 48.1% of the 108 eels examined. If the mosaic vaterite in otoliths is not identified and avoided, the migratory environmental history of fish could be misidentified when otolith elemental signatures are used as biological tracers.