Maia Anabela, Lauder George V, Wilga Cheryl D
Department of Biological Sciences, College of the Environmental and Life Sciences, University of Rhode Island, 120 Flagg Road, Kingston, RI 02881-0816, USA
Department of Organismic and Evolutionary Biology, Harvard University, Cambridge, MA 02138, USA.
J Exp Biol. 2017 Nov 1;220(Pt 21):3967-3975. doi: 10.1242/jeb.152215. Epub 2017 Sep 7.
A key feature of fish functional design is the presence of multiple fins that allow thrust vectoring and redirection of fluid momentum to contribute to both steady swimming and maneuvering. A number of previous studies have analyzed the function of dorsal fins in teleost fishes in this context, but the hydrodynamic function of dorsal fins in freely swimming sharks has not been analyzed, despite the potential for differential functional roles between the anterior and posterior dorsal fins. Previous anatomical research has suggested a primarily stabilizing role for shark dorsal fins. We evaluated the generality of this hypothesis by using time-resolved particle image velocimetry to record water flow patterns in the wake of both the anterior and posterior dorsal fins in two species of freely swimming sharks: bamboo sharks () and spiny dogfish (). Cross-correlation analysis of consecutive images was used to calculate stroke-averaged mean longitudinal and lateral velocity components, and vorticity. In spiny dogfish, we observed a velocity deficit in the wake of the first dorsal fin and flow acceleration behind the second dorsal fin, indicating that the first dorsal fin experiences net drag while the second dorsal fin can aid in propulsion. In contrast, the wake of both dorsal fins in bamboo sharks displayed increased net flow velocity in the majority of trials, reflecting a thrust contribution to steady swimming. In bamboo sharks, fluid flow in the wake of the second dorsal fin had higher absolute average velocity than that for first dorsal fin, and this may result from a positive vortex interaction between the first and second dorsal fins. These data suggest that the first dorsal fin in spiny dogfish has primarily a stabilizing function, while the second dorsal fin has a propulsive function. In bamboo sharks, both dorsal fins can contribute thrust and should be considered as propulsive adjuncts to the body during steady swimming. The function of shark dorsal fins can thus differ considerably among fins and species, and is not limited to a stabilizing role.
鱼类功能设计的一个关键特征是存在多个鳍，这些鳍能够实现推力矢量控制以及流体动量的重新定向，从而有助于稳定游动和灵活转向。此前已有多项研究在此背景下分析了硬骨鱼类背鳍的功能，但对于自由游动的鲨鱼背鳍的流体动力学功能尚未进行分析，尽管前后背鳍可能具有不同的功能作用。先前的解剖学研究表明鲨鱼背鳍主要起稳定作用。我们通过使用时间分辨粒子图像测速技术来记录两种自由游动鲨鱼——条纹鲨（）和白斑角鲨（）前后背鳍尾流中的水流模式，评估了这一假设的普遍性。利用连续图像的互相关分析来计算行程平均纵向和横向速度分量以及涡度。在白斑角鲨中，我们观察到第一背鳍尾流中存在速度亏缺，而第二背鳍后方水流加速，这表明第一背鳍受到净阻力，而第二背鳍有助于推进。相比之下，在大多数试验中，条纹鲨两个背鳍的尾流都显示出净流速增加，这反映出对稳定游动有推力贡献。在条纹鲨中，第二背鳍尾流中的流体流动绝对平均速度高于第一背鳍，这可能是由于第一和第二背鳍之间存在正向涡旋相互作用。这些数据表明，白斑角鲨的第一背鳍主要起稳定作用，而第二背鳍具有推进功能。在条纹鲨中，两个背鳍都能提供推力，在稳定游动时应被视为身体推进的辅助结构。因此，鲨鱼背鳍的功能在不同鳍和不同物种之间可能有很大差异，并不局限于稳定作用。
