Ontogenetic and sex-related variations in otolith morphology and structure of thinlip gray mullet from a transitional environment in northern Italy

Otoliths are calcified structures present in the inner ear of fish involved in hearing and balance. Notwithstanding that their shape is species-specific, which is helpful for taxonomic identification, the intraspecific variability of shape and structures may reflect differences in the auditory system and the capability of detecting different frequencies between males and females and between adults and juveniles. Moreover, as calcified structures are able to store information, otoliths allow us to understand details about an organism’s behavioral and adaptive methods in relation to its environment. A multiscale approach that includes biometric analysis, X-ray powder diffraction, Fourier transform infrared spectroscopy, and thermogravimetric analysis was used in this study to characterize the sagittal otolith of the thinlip gray mullet (Chelon ramada) in the transitional water at the mouth of the Reno River, in the North of Italy. The aim of this study was to investigate otolith morphological characteristics using biometric parameters (length, width, perimeter, and area) and shape descriptors (circularity, roundness, aspect ratio, and solidity), otolith structure, mineral composition, and organic matrix content in relation to biological growth and sexual differentiation among three groups: not determined, males, and females. Differences were observed between juveniles and adults considering some morphological characteristics, suggesting a different and faster growth of the otolith in the first stages of life, consistent with the fastest growth of juvenile individuals. Furthermore, sexual dimorphism was detected through the analysis of shape descriptors characterizing the increment of irregularities along the perimeter, highlighting an intraspecific divergence. The observed variations in shape complexity suggest a difference in the abundance and organization of otolith protuberances, which potentially play a crucial role in acoustic sensitivity and detecting frequencies between males and females, reflecting different needs.