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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. 

Abstract To what extent do modifications in the nervous system and peripheral effectors contribute to novel behaviors? Using a combination of morphometric analysis, neuroanatomical tract‐tracing, and intracellular neuronal recording, we address this question in a sound‐producing and a weakly electric species of synodontid catfish,Synodontis grandiops, andSynodontis nigriventris, respectively. The same peripheral mechanism, a bilateral pair of protractor muscles associated with vertebral processes (elastic spring mechanism), is involved in both signaling systems. Although there were dramatic species differences in several morphometric measures, electromyograms provided strong evidence that simultaneous activation of paired protractor muscles accounts for an individual sound and electric discharge pulse. While the general architecture of the neural network and the intrinsic properties of the motoneuron population driving each target was largely similar, differences could contribute to species‐specific patterns in electromyograms and the associated pulse repetition rate of sounds and electric discharges. Together, the results suggest that adaptive changes in both peripheral and central characters underlie the transition from an ancestral sound to a derived electric discharge producing system, and thus the evolution of a novel communication channel among synodontid catfish. Similarities with characters in other sonic and weakly electric teleost fish provide a striking example of convergent evolution in functional adaptations underlying the evolution of the two signaling systems among distantly related taxa.