LAUDER, George V.*; DRUCKER, Eliot G.: Experimental Hydrodynamics and Evolution: Function of Median Fins in Fishes

Two key structural trends characterizing the evolution of median fins in teleost fishes are (1) the transformation of the caudal fin from the plesiomorphic heterocercal condition to the derived homocercal morphology, and (2) the elaboration of the dorsal fin into anterior spiny and posterior soft-rayed portions from the plesiomorphic structure with soft rays only. In addition, many fishes possess two dorsal fins and there is considerable diversity in dorsal fin structure among ray-finned fishes. Using the technique of Digital Particle Image Velocimetry (DPIV) on a phylogenetically diverse array of freely-swimming fishes has allowed us to quantify the function of the caudal fin, dorsal fin, and interactions between these two propulsors. In this paper we review the structural diversity of ray-finned fish dorsal and caudal fins, and present DPIV results on the function of these median fins. Experimental hydrodynamic data from sharks, sturgeon, trout, bluegill sunfish, and mackerel illustrate the diversity of median fin force production and versatility during steady swimming and maneuvering. Heterocercal tails of sharks and sturgeon differ considerably in function despite a generally similar external morphology. Homocercal tails of trout, bluegill, and mackerel produce a chain of linked vortex rings with a reverse von Karman wake. The dorsal fin of bluegill generates a thrust wake during both steady swimming and maneuvering and thus augments locomotor force generated by the tail and pectoral fins. Vortices shed from the bluegill dorsal fin also may increase circulation around the tail and enhance thrust at the caudal fin. These data demonstrate the functional versatility of fins in fishes, and the importance of in vivo experimental hydrodynamics for elucidating the function of fins.