Tomita Taketeru, Sato Keiichi, Suda Kenta, Kawauchi Junro, Nakaya Kazuhiro
Department of Earth and Planetary Science, The University of Tokyo, Tokyo, Japan.
J Morphol. 2011 May;272(5):513-24. doi: 10.1002/jmor.10905. Epub 2011 Mar 4.
Studies of the megamouth shark, one of three planktivorous sharks, can provide information about their evolutionary history. Megamouth shark feeding has never been observed in life animals, but two alternative hypotheses on biomechanics suggest either feeding, i.e., ram feeding or suction feeding. In this study, the second moment of area of the ceratohyal cartilages, which is an indicator of the flexural stiffness of the cartilages, is calculated for 21 species of ram- and suction-feeding sharks using computed tomography. The results indicate that suction-feeding sharks have ceratohyal cartilages with a larger second moment of area than ram-feeding sharks. The result also indicates that the ram-suction index, which is an indicator of relative contribution of ram and suction behavior, is also correlated with the second moment of area of the ceratohyal. Considering that large bending stresses are expected to be applied to the ceratohyal cartilage during suction, the larger second moment of area of the ceratohyal of suction-feeding sharks can be interpreted as an adaptation for suction feeding. Based on the small second moment of area of the ceratohyal cartilage of the megamouth shark, the feeding mode of the megamouth shark is considered to be ram feeding, similar to the planktivorous basking shark. From these results, an evolutionary scenario of feeding mechanics of three species of planktivorous sharks can be suggested. In this scenario, the planktivorous whale shark evolved ram feeding from a benthic suction-feeding ancestor. Ram feeding in the planktivorous megamouth shark and the basking shark evolved from ram feeding swimming-type ancestors and that both developed their unique filtering system to capture small-sized prey.
巨口鲨是三种滤食性鲨鱼之一，对其进行的研究可以提供有关它们进化历史的信息。从未在活体动物中观察到巨口鲨的进食情况，但关于生物力学的两种替代假说表明其进食方式要么是冲撞式进食，要么是吸食式进食。在本研究中，利用计算机断层扫描技术计算了21种采用冲撞式和吸食式进食的鲨鱼的角舌软骨的面积惯性矩，该指标可反映软骨的抗弯刚度。结果表明，吸食式进食的鲨鱼的角舌软骨面积惯性矩比冲撞式进食的鲨鱼大。结果还表明，冲撞 - 吸食指数（即冲撞和吸食行为相对贡献的指标）也与角舌软骨的面积惯性矩相关。考虑到在吸食过程中角舌软骨预计会承受较大的弯曲应力，吸食式进食鲨鱼的角舌软骨较大的面积惯性矩可被解释为对吸食式进食的一种适应。基于巨口鲨角舌软骨较小的面积惯性矩，其进食方式被认为是冲撞式进食，类似于滤食性的姥鲨。根据这些结果，可以提出三种滤食性鲨鱼进食机制的进化情景。在这种情景中，滤食性的鲸鲨从底栖吸食式进食的祖先进化出冲撞式进食。滤食性的巨口鲨和姥鲨的冲撞式进食是从冲撞式游泳型祖先进化而来，并且两者都发展出了独特的过滤系统来捕获小型猎物。
