Exploring Early Life Transitions in <i>Micropogonias furnieri</i> Otoliths Through Elemental Profiles and Crystallographic Analysis

ABSTRACT Understanding early ontogenetic transitions in fish is essential for reconstructing life histories and to refine age and growth assessments. In this study, we combined microchemical and crystallographic analyses of sagittal otoliths to investigate early developmental stages in Micropogonias furnieri , a key estuarine species in the Southwestern Atlantic. Raman spectroscopy revealed that otoliths remain crystallographically stable throughout early development, with aragonite as the sole calcium carbonate polymorph even in regions of structural reorganisation. Core‐to‐edge Sr/Ca, Ba/Ca and Mn/Ca profiles were analysed by LA‐ICP‐MS on sagittal otoliths sectioned to expose the core. Elemental profiles exhibited trends aligned with key physiological and environmental transitions: a high Sr/Ca signal in the core (pre‐hatch and larval stage) in most samples, a Mn/Ca peak during metamorphosis and a progressive increase in Ba/Ca spanning the metamorphic and juvenile stages. Interindividual variability and partial misalignment with microstructural markers were observed, likely due to the complexity of the region of the otolith formed during metamorphosis and probably due to environmental heterogeneity in the Río de la Plata Estuary. These findings suggest that combining otolith elemental ratios may serve as biological “timers” to retrospectively identify early life stages, enhancing age and growth studies in estuarine fish with complex life cycles.