Comparative morphology of labial cartilages in sharks (Chondrichthyes, Elasmobranchii)

During the last years, feeding mechanisms of sharks have been documented mainly based on their stomach contents supplemented by video footages of feeding behaviours in some species. Rare kinematic analyses of shark jaws contributed additionally to our knowledge. However, not all structures important to prey capture in sharks have been investigated yet. Here, we summarize the current knowledge about labial cartilages in sharks and compare them on a morphological base. Labial cartilages are considered a plesiomorph character found in extinct and extant species and their number and size varies a lot between species. We identified labial cartilages in representatives of 27 from 36 shark families and detected different numbers, sizes and forms of labial cartilages, as well as differing positioning along the jaws. The morphology of labial cartilages can help to identify suction-feeding in sharks and to estimate the quality of the suction generated by a species. We were able to summon different shark taxa according to their morphology and positioning of labial cartilages into eight groups, which we consider morphological modules. To determine if these modules bare a phylogenetic signal further testing with rigorous methodologies and more shark species as well as batomorphs needs to be done. This study therefore only can represent a first starting point providing future research pathways.

近年来，鲨鱼的取食机制主要基于其胃容物，并辅以部分物种的取食行为影像资料得以研究记录。针对鲨鱼颌骨的少量运动学分析进一步丰富了现有认知。然而，目前尚未对鲨鱼捕食过程中所有关键结构开展系统性研究。本文综述了当前学界关于鲨鱼唇软骨（labial cartilages）的研究成果，并基于形态学维度开展比较分析。唇软骨被认为是存在于灭绝与现生鲨鱼类群中的祖征（plesiomorph character），其数量与大小在不同物种间差异显著。我们在36个鲨鱼科中的27个科的代表物种中识别出了唇软骨，并观测到不同类群的唇软骨在数量、大小、形态以及沿颌骨的排布位置上均存在显著差异。唇软骨的形态特征可用于识别鲨鱼的抽吸式取食（suction-feeding）行为，并可用于评估该物种产生抽吸能力的强弱。我们依据唇软骨的形态特征与排布位置，将不同鲨鱼类群划分为八个类群，并将其视为形态模块（morphological modules）。为验证这些类群是否携带系统发育信号，未来需采用严谨的研究方法，纳入更多鲨鱼物种以及鳐总目（batomorphs）物种开展进一步实验。因此，本研究仅为首个起点，可为后续相关研究提供方向指引。