Function of the Caudal Fin During Locomotion in Fishes: Kinematics, Flow Visualization, and Evolutionary Patterns

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Abstract
One of the most prominent characteristics of early vertebrates is the elongate caudal fin bearing fin rays. The caudal fin represents a fundamental design feature of vertebrates that predates the origin of jaws and is found in both agnathans and gnathostomes. The caudal fin also represents the most posterior region of the vertebrate axis and is the location where fluid, accelerated by movement of the body anteriorly, is shed into the surrounding medium. Despite the extensive fossil record of the caudal fin, the use of caudal characters for systematic studies, and the importance of tail function for understanding locomotor dynamics in fishes, few experimental studies have been undertaken of caudal fin function. In this paper I review two experimental approaches which promise to provide new insights into the function and evolution of the caudal fin: three-dimensional kinematic analysis, and quantitative flow measurements in the wake of freely-swimming fishes using digital particle image velocimetry (DPIV). These methods are then applied to the function of the caudal fin during steady swimming in fishes with heterocercal and homocercal morphologies: chondrichthyians (leopard sharks) and ray-fined fishes (sturgeon and bluegill sunfish). The caudal fin of leopard sharks functions in a manner consistent with the classical model of heterocercal tail function in which the caudal surface moves at an acute angle to the horizontal plane, and hence is expected to generate lift forces and torques which must be counteracted anteriorly by the body and pectoral fins. An alternative model in which the shark tail produces a reactive force that acts through the center of mass is not supported. The sturgeon heterocercal tail is extremely flexible and the upper tail lobe trails the lower during the fin beat cycle. The sturgeon tail does not function according to the classical model of the heterocercal tail, and is hypothesized to generate reactive forces oriented near the center of mass of the body which is tilted at an angle to the flow during steady locomotion. Functional analysis of the homocercal tail of bluegill shows that the dorsal and ventral lobes do not function symmetrically as expected. Rather, the dorsal lobe undergoes greater lateral excursions and moves at higher velocities than the ventral lobe. The surface of the dorsal lobe also achieves a significantly acute angle to the horizontal plane suggesting that the homocercal tail of bluegill generates lift during steady swimming. These movements are actively generated by the hypochordal longitudinalis muscle within the tail. This result, combined with DPIV flow visualization data, suggest a new hypothesis for the function of the homocercal tail: the homocercal tail generates tilted and linked vortex rings with a central jet inclined posteroventrally, producing an anterodorsal reactive force on the body which generates lift and torque in the manner expected of a heterocercal tail. These results show that the application of new techniques to the study of caudal fin function in fishes reveals a previously unknown diversity of homocercal and heterocercal tail function, and that morphological characterizations of caudal fins do not accurately reflect in vivo function.