DRUCKER, E.G.*; LAUDER, G.V.; University of California, Irvine; Harvard University, Cambridge, MA: Locomotor function of the dorsal fin in teleost fishes: propulsor design and wake dynamics

A major evolutionary transformation of the locomotor system of teleost fishes is the morphological elaboration of the dorsal fin. In basal teleost clades, the dorsal fin typically is a single midline structure supported by soft, flexible fin rays. In its derived condition, the fin is commonly comprised of two anatomically distinct portions: an anterior section supported by stiff spines, and a posterior section that is soft-rayed. The goal of this study was to investigate the hydrodynamic significance of such anatomical variation in dorsal fin design. Using digital particle image velocimetry to visualize wake flow, we examined the locomotor functions served by the dorsal fin in its plesiomorphic and derived conditions (represented by rainbow trout Oncorhynchus mykiss and bluegill sunfish Lepomis macrochirus, respectively) during both steady swimming and unsteady maneuvering. Regardless of its design, the soft dorsal fin can undulate independently of the body to generate momentum flow and fluid force. However, the single dorsal fin of trout has a greatly limited capacity to exert thrust for forward swimming, as compared to the dual-fin configuration of sunfish (stroke-averaged force lower in trout by a factor of 8 on average). The plesiomorphic dorsal fin instead primarily plays the role of body stabilizer, generating predominately laterally oriented wake force during rectilinear swimming as well as during turning (mean wake jet angle 85-92 degrees). We propose that the locomotor repertoire of the dorsal fin within Teleostei is dictated in part by the proximity of the propulsor to the body�s center of mass and by the presence or absence of an anterior stabilizing spiny section of the fin.