Function of the caudal fin during locomotion in fishes: Kinematics, flow visualization, and evolutionary patterns

One of the most prominent characteristics of early vertebrates is the elong ate caudal fin bearing fin rays. The caudal fin represents a fundamental de sign feature of vertebrates that predates the origin of jaws and is found i n both agnathans and gnathostomes. The caudal fin also represents the most posterior region of the vertebrate axis and is the location where fluid, ac celerated by movement of the body anteriorly, is shed into the surrounding medium. Despite the extensive fossil record of the caudal fin, the use of c audal characters for systematic studies, and the importance of tail functio n for understanding locomotor dynamics in fishes, few experimental studies have been undertaken of caudal fin function. In this paper I review two exp erimental approaches which promise to provide new insights into the functio n and evolution of the caudal fin: three-dimensional kinematic analysis, an d quantitative flow measurements in the wake of freely-swimming fishes usin g digital particle image velocimetry (DPIV). These methods are then applied to the function of the caudal fin during steady swimming in fishes with he terocercal and homocercal morphologies: chondrichthyians (leopard sharks) a nd ray-fined fishes (sturgeon and bluegill sunfish). The caudal fin of leop ard sharks functions in a manner consistent with the classical model of het erocercal 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 fin s. An alternative model in which the shark tail produces a reactive force t hat acts through the center of mass Is not supported. The sturgeon heteroce rcal tail is extremely flexible and the upper tail lobe trails the lower du ring the fin beat cycle. The sturgeon tail does not function according to t he classical model of the heterocercal tail, and is hypothesized to generat e reactive forces oriented near the center of mass of the body which is til ted at an angle to the flow during steady locomotion. Functional analysis o f the homocercal tail of bluegill shows that the dorsal and ventral lobes d o 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 a ngle to the horizontal plane suggesting that the homocercal tail of bluegil l generates lift during steady swimming. These movements are actively gener ated by the hypochordal longitudinalis muscle within the tail. This result, combined with DPIV flow visualization data, suggest a new hypothesis for t he function of the homocercal tail: the homocercal tall generates tilted an d linked vortex rings with a central jet inclined postero-ventrally, produc ing an anterodorsal reactive force on the body which generates lift and tor que 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 fi shes reveals a previously unknown diversity of homocercal and heterocercal tail function, and that morphological characterizations of caudal fins do n ot accurately reflect ill vivo function.