The otoliths of teleosts are part of the inner ear. They are related to the balance system and are also mechanical components of sound transmission, responsible for hearing in fish. Their morphology is determined by the function they perform and is also influenced by environmental factors. In this context, our goal was to: analyse the possible morphological changes of sagitta otoliths of Micropogonias furnieri (Desmarest, 1823) during its ontogeny, based on six biometric indexes. We analysed 162 sagittae otoliths of Micropogonias furnieri collected in the Rio de la Plata estuary. The variation of the indexes was associated with the different size groups. Rectangularity and aspect ratio [otolith length/total fish length (OL/TL) and sulcus area/otolith area (SA/OA)] were the indexes that contributed the most to the discriminant function. Biometric index differences were related to diverse juvenile environments. The observed shift in the SA/OA relationship could be attributable to a need to respond to a change in the perception of sound (pulse and frequency) experienced by individuals larger than 20 cm that begin to frequent the same areas as spawning adults. In this sense, the ontogenetic morphological change of sagitta otoliths plays an important role in perception and acoustic communication for this species.