Although otoliths are widely used as archives to infer life history traits and habitat use in fishes, their biomineralization process remains poorly understood. This lack of knowledge is problematic as it can lead to misinterpretation of the different types of signals (e.g. optical or chemical) that provide basic data for research in fish ecology, fisheries management and species conservation. Otolith calcification relies on a complex system involving a pericrystalline fluid, the endolymph, which organic and inorganic compositions are spatially heterogeneous for some constituents. This property stems from the particular structure of the calcifying saccular epithelium. In this study, we explored the spatial heterogeneity of elemental incorporation in otoliths for two species of high economic interest, European hake Merluccius merluccius (L. 1758) and European sea bass Dicentrarchus labrax (L. 1758). Two-dimensional mappings of chemical elements were obtained by UV-fs LA-HR-ICPMS analyses on transverse sections of sagittae. Results highlighted a clear asymmetry between proximal (sulcus) and distal (antisulcus) concentrations for elements such as magnesium Mg, phosphorus P, manganese Mn and potassium K with concentration gradient directions that varied depending on the element. Strontium Sr and barium Ba did not show a proximo-distal gradient. These results are discussed in the light of current knowledge on the endolymph composition and the mechanisms that lead to its compartmentalization, highlighting the need for further research on otolith biomineralization. Operational implications for studies based on otolith chemical composition are also discussed with emphasis on advice for sampling strategies in order to avoid analytical biases, and the need for in-depth analyses of analytical settings before comparing otolith signatures between species or geographical areas.