Abstract :
[en] The otoliths, which are calcified structures found in the inner ear, play a crucial role in hearing and equilibrium of fishes. Saccular otoliths are widely used in fields such as palaeontology and fishery science owing to their species-specific morphology and lifelong recording of environmental conditions, offering a window into virtually every moment of the fish life. Nevertheless, the factors explaining their outstanding morphological diversity remain unclear. The primary aim of this study was to gain a deeper understanding of the drivers that have shaped the current diversity of saccular otoliths in fish.
To achieve this, I pursued two main objectives: (1) to conduct comprehensive broad scale comparative analyses to disentangle the main drivers of morphological variation in otoliths accross fishes and (2) to perform an experimental study to assess the influence of the vestibular function of the sagitta and demonstrate the adaptability of this hard structure in response to environmental conditions.
The diversification of otolith shape was assessed across actinopterygians, encompassing hundreds of species. Geometric morphometric analyses applied to both lateral and dorsal views revealed a strong integration between the overall otolith morphology and sulcus shape, supporting the close functional link between otolith in the inner ear functions. Otolith disparity is primarily driven by rostro-caudal elongation, sulcus acusticus shape variation, and differences in curvature. Otolith shape and size exhibited a low phylogenetic signal and were decoupled from order age and functional diversity. However, there was evidence linking certain aspects of otolith disparity to evolutionary rates and order-level speciation rates, suggesting complex evolutionary dynamics.
Using the same large taxonomic dataset, I then tested the relationships between seven functionally relevant ecological traits and otolith shape. By combining multivariate analyses with phylogenetically informed linear models, I demonstrated that light/dark environment, water column use, social behaviour, and acoustic communication were the strongest predictors of otolith shape. Species living in photic environments and gregarious taxa exhibited thinner, elongated, and curved otoliths with a relatively large sensory area implying a functional feedback of fish lifestyle on otolith morphology. Interestingly, acoustic-related traits had lower influence than expected on lateral and dorsal shape variation compared to ecological factors related to the vestibular function.
A third comparative work focused on the ecologically diverse family of Labridae (wrasses). This study showed that otolith shape resulted from a complex interplay between phylogenetic and ecological constraints. I detected a high level of morphological conservatism within tribes. However, model-fitting approaches revealed that adaptation to various habitats and feeding ecologies was associated with divergence in the complexity of the otolith contour. Benthic feeders possessed more lobate and crenate otoliths than pelagic feeders, likely enhancing vestibular function to accommodate the smoother manoeuvrability required in benthic environments.
Finally, beyond comparative works, an original experimental setup tested the plasticity of otolith morphology in response to environmental complexity. Nile tilapia raised in structurally enriched environments developed larger, thicker, and more voluminous sagittae while their auditory sensitivity remained unchanged. This suggests that the observed morphological variation primarily affected vestibular function rather than hearing capabilities, providing empirical support for the dual function of the sagittae and reinforcing our previous ecomorphological findings.
The approach developed along this PhD thesis and its associated results reaffirm the dual auditory and vestibular influences on sagittae morphology. This study underscores the multifaceted nature of otolith evolution and demonstrates how the combination of genetic inheritance, clade-specific life-history traits, ecological pressures, and phenotypic plasticity had driven the tremendous morphological diversity of otoliths. By combining large-scale phylogenetic analyses with controlled experiments, this work provides new insights into the selective forces driving the diversification of this key sensory structure, encouraging future research to adopt a more integrative approach when studying the evolution of fish inner ear features.
