Convergence of undulatory swimming kinematics across a diversity of fishes

Fishes exhibit an astounding diversity of locomotor behaviors, from classic swimming with their body and fins to jumping, flying, walking, and burrowing. Fishes that use their body and caudal fin (BCF) during undulatory swimming have been traditionally divided into modes based on the length of the propulsive body wave and the ratio of head:tail oscillation amplitude: anguilliform, sub-carangiform, carangiform and thunniform. This classification was first proposed based on key morphological traits, such as body stiffness and elongation, to group fishes based on their expected swimming mechanics. Here, we present a comparative study of 44 diverse species quantifying kinematics and morphology of BCF-swimming fishes. Our results reveal that most species we studied share similar oscillation amplitude during steady locomotion that can be modeled using a second-degree order polynomial. The length of the propulsive body wave was shorter for species classified as anguilliform and longer for those classified as thunniform, although substantial variability existed both within and among species. Moreover, there was no decrease in head:tail amplitude from anguilliform to thunniform mode of locomotion as we expected from the traditional classification. While the expected swimming modes correlated with morphological traits, they did not accurately represent the kinematics of BCF locomotion. These results indicate that even fish species differing as substantially in morphology as tuna and eel exhibit statistically similar two-dimensional midline kinematics and point toward unifying locomotor hydrodynamic mechanisms that can serve as the basis for understanding aquatic locomotion and controlling biomimetic aquatic robots.鱼类展现出令人惊叹的运动行为多样性，从依靠躯体与鳍完成的经典游泳方式，到跳跃、飞行、爬行与穴居等运动模式。传统上，针对采用躯干与尾鳍（body and caudal fin, BCF）进行波动游泳的鱼类，研究者会基于推进体波长度以及头部与尾部摆动振幅之比，将其划分为鳗鲡型、亚鲹型、鲹型与金枪鱼型四类运动模式。该分类体系最初基于躯体硬度、躯体延长度等关键形态学特征提出，旨在根据鱼类预期的游泳力学机制进行归类。本研究针对44种不同鱼类展开对比分析，量化研究了采用BCF游泳模式的鱼类的运动学与形态学特征。研究结果显示，所调研的多数鱼类在稳态运动过程中的摆动振幅较为相似，且可通过二阶多项式进行建模。尽管种内与种间均存在显著变异，但被划分为鳗鲡型的鱼类其推进体波长度更短，而金枪鱼型鱼类的推进体波长度则更长。此外，与传统分类体系的预期相悖，从鳗鲡型到金枪鱼型运动模式，鱼类的头部与尾部摆动振幅并未呈现递减趋势。尽管预期的游泳模式与形态学特征存在相关性，但它们无法准确反映BCF运动的运动学特征。本研究结果表明，即便如金枪鱼与鳗鱼这类形态差异极大的鱼类，其二维中线运动学特征在统计学上也极为相似；同时，本研究揭示了具有普适性的运动流体力学机制，可为水生运动的机理研究以及仿生水下机器人的控制提供理论基础。