Department Biology II, Zoology, Ludwig-Maximilians-University, Großhaderner Strasse 2, 82152, Planegg-Martinsried, Germany.
Department of Behavioural Biology, University of Vienna, Althanstrasse 14, 1090, Vienna, Austria.
Biol Rev Camb Philos Soc. 2019 Apr;94(2):457-482. doi: 10.1111/brv.12463. Epub 2018 Sep 21.
Otoliths in bony fishes play an important role in the senses of balance and hearing. Otolith mass and shape are, among others, likely to be decisive factors influencing otolith motion and thus ear functioning. Yet our knowledge of how exactly these factors influence otolith motion is incomplete. In addition, experimental studies directly investigating the function of otoliths in the inner ear are scarce and yield partly conflicting results. Herein, we discuss questions and hypotheses on how otolith mass and shape, and the relationship between the sensory epithelium and overlying otolith, influence otolith motion. We discuss (i) the state-of-the-art knowledge regarding otolith function, (ii) gaps in knowledge that remain to be filled, and (iii) future approaches that may improve our understanding of the role of otoliths in ear functioning. We further link these functional questions to the evolution of solid teleost otoliths instead of numerous tiny otoconia as found in most other vertebrates. Until now, the selective forces and/or constraints driving the evolution of solid calcareous otoliths and their diversity in shape in teleosts are largely unknown. Based on a data set on the structure of otoliths and otoconia in more than 160 species covering the main vertebrate groups, we present a hypothetical framework for teleost otolith evolution. We suggest that the advent of solid otoliths may have initially been a selectively neutral 'by-product' of other key innovations during teleost evolution. The teleost-specific genome duplication event may have paved the way for diversification in otolith shape. Otolith shapes may have evolved along with the considerable diversity of, and improvements in, auditory abilities in teleost fishes. However, phenotypic plasticity may also play an important role in the creation of different otolith types, and different portions of the otolith may show different degrees of phenotypic plasticity. Future studies should thus adopt a phylogenetic perspective and apply comparative and methodologically integrative approaches, including fossil otoliths, when investigating otoconia/otolith evolution and their function in the inner ear.
硬骨鱼类的耳石在平衡和听觉感知中起着重要作用。耳石的质量和形状等因素可能是影响耳石运动从而影响耳朵功能的决定性因素。然而，我们对于这些因素如何确切地影响耳石运动的认识并不完整。此外，直接研究耳石在内耳中功能的实验研究很少，并且得出的结果部分相互矛盾。在此，我们讨论了耳石质量和形状以及感觉上皮和覆盖的耳石之间的关系如何影响耳石运动的问题和假设。我们讨论了（i）关于耳石功能的最新知识，（ii）仍然存在的知识空白，以及（iii）可能改善我们对耳石在耳朵功能中的作用的理解的未来方法。我们进一步将这些功能问题与固体硬骨鱼类耳石的进化联系起来，而不是与大多数其他脊椎动物中发现的许多微小的耳石相联系。到目前为止，驱动固体钙质耳石的进化以及硬骨鱼类耳石形状多样性的选择压力和/或约束在很大程度上是未知的。基于一个涵盖主要脊椎动物类群的 160 多种物种的耳石和耳石结构数据集，我们提出了一个硬骨鱼类耳石进化的假设框架。我们认为，固体耳石的出现最初可能是硬骨鱼类进化过程中其他关键创新的选择性中性“副产品”。硬骨鱼类特异性的基因组加倍事件可能为耳石形状的多样化铺平了道路。耳石形状可能随着硬骨鱼类听觉能力的显著多样性和改善而进化。然而，表型可塑性也可能在不同耳石类型的形成中发挥重要作用，并且耳石的不同部分可能表现出不同程度的表型可塑性。因此，未来的研究应该采用系统发育的观点，并应用比较和方法上综合的方法，包括化石耳石，以研究耳石/耳石的进化及其在内耳中的功能。
