Diversity of Inner Ears in Fishes: Possible Contribution Towards Hearing Improvements and Evolutionary Considerations

Fishes have evolved the largest diversity of inner ears among vertebrates. While G. Retzius introduced us to the diversity of the gross morphology of fish ears in the late nineteenth century, it was A. N. Popper who unraveled the large variety of the fine structure during the last four decades. Modifications of the basic inner ear structure-consisting of three semicircular canals and their sensory epithelia, the cristae and three otolithic end organs (utricle, saccule, lagena) including the maculae-mainly relate to the saccule and lagena and the respective sensory epithelia, the macula sacculi and macula lagenae. Despite the profound morphological knowledge of inner ears and the morphological variability, the functional significance of this diversity is still largely unknown. The aims of this review are therefore twofold. First it provides an update of the state of the art of inner ear diversity in bony fishes. Second it summarizes and discusses hypotheses on the evolution of this diversity as well as formulates open questions and promising approaches to tackle these issues.