The multifaceted diversification of the sagitta otolith across the fish tree of life

Abstract Otoliths of actinopterygians are calcified structures playing a key role in hearing and equilibrium functions. To understand their morphological diversification, we quantified the shape of otoliths in both lateral and dorsal view from 697 and 323 species, respectively, using geometric morphometrics. We then combined form (i.e. size and shape) information with ecological data and phylogenetically informed comparative methods to test our hypotheses. Initially, the exploration of morphospaces revealed that the main variations are related to sulcus acusticus shape, elongation and lateral curvature. We also found strong integration between otolith and sulcus shape, suggesting that they are closely mirroring each other, reinforcing a shape-dependent mechanism crucial for otolith motion relative to its epithelium and validating the functional significance of otolith morphology in auditory and vestibular processes. After revealing that otolith shape and size retained a low phylogenetic signal, we showed that the disparity of otolith size and shape is decoupled from order age and from the level of functional diversity across clades. Finally, some traits in otolith disparity are correlated with their morphological evolutionary rate and the order speciation rate. Overall, we observed that the pattern of diversification of otoliths across the fish tree of life is highly complex and likely to be multifactorial.