Locomotor function of the dorsal fin in teleost fishes: experimental analysis of wake forces in sunfish

A key evolutionary transformation of the locomotor system of ray-finned fis hes is the morphological elaboration of the dorsal fin. Within Teleostei, t he dorsal fin primitively is a single midline structure supported by soft, flexible fin rays. In its derived condition, the fin is made up of two anat omically distinct portions: an anterior section supported by spines, and a posterior section that is soft-rayed. We have a very limited understanding of the functional significance of this evolutionary variation in dorsal fin design. To initiate empirical hydrodynamic study of dorsal fin function in teleost fishes, we analyzed the wake created by the soft dorsal fin of blu egill sunfish (Lepomis macrochirus) during both steady swimming and unstead y turning maneuvers. Digital particle image velocimetry was used to visuali ze wake structures and to calculate in vivo locomotor forces. Study of the vortices generated simultaneously by the soft dorsal and caudal fins during locomotion allowed experimental characterization of median-fin wake intera ctions. During high-speed swimming (i.e. above the gait transition from pectoral- t o median-fin locomotion), the soft dorsal fin undergoes regular oscillatory motion which, in comparison with analogous movement by the tail, is phase- advanced (by 30 % of the cycle period) and of lower sweep amplitude (by 1.0 cm). Undulations of the soft dorsal fin during steady swimming at 1.1 body length s(-1) generate a reverse von Karman vortex street wake that contrib utes 12 % of total thrust. During low-speed turns, the soft dorsal fin prod uces discrete pairs of counterrotating vortices with a central region of hi gh-velocity jet flow. This vortex wake, generated in the latter stage of th e turn and posterior to the center of mass of the body, counteracts torque generated earlier in the turn by the anteriorly positioned pectoral fins an d thereby corrects the heading of the fish as it begins to translate forwar d away from the turning stimulus. One-third of the laterally directed fluid force measured during turning is developed by the soft dorsal fin. For ste ady swimming, we present empirical evidence that vortex structures generate d by the soft dorsal fin upstream can constructively interact with those pr oduced by the caudal fin downstream. Reinforcement of circulation around th e tail through interception of the dorsal fin's vortices is proposed as a m echanism for augmenting wake energy and enhancing thrust. Swimming in fishes involves the partitioning of locomotor force among sever al independent fin systems. Coordinated use of the pectoral fins, caudal fi n and soft dorsal fin to increase wake momentum, as documented for L. macro chirus, highlights the ability of teleost fishes to employ multiple propuls ors simultaneously for controlling complex swimming behaviors.