DRUCKER, E.G.*; LAUDER, G.V.: Wake dynamics and locomotor function in fishes: interpreting evolutionary patterns in paired fin design

Documenting phylogenetic patterns in the structure of the paired fins of fishes, and interpreting the functional significance of such patterns, has been the subject of ongoing study by functional morphologists and hydrodynamicists. The evolution of ray-finned fishes is characterized by a suite of structural transformations of the paired pectoral fins, including migration of the fins from a ventral to mid-dorsal body position, and rotation of the fin base from a primarily horizontal to vertical orientation. In spite of these well-known patterns, however, we presently understand very little about the relationship between paired fin morphology and fluid force generated during locomotion. This paper presents experimental tests of hypotheses regarding the functional significance of evolutionary variation in pectoral fin design. In taxonomically diverse lineages of ray-finned fishes, we measured patterns of fluid flow in the wake of the pectoral fins during steady swimming and unsteady maneuvering. Using digital particle image velocimetry, we estimated locomotor forces from rates of momentum transfer. By studying species with the plesiomorphic and apomorphic fin design (including trout, sunfish and surfperch), we addressed a central question: does the location and orientation of the pectoral fin constrain the range of directions in which force may be applied on the fluid during locomotion? Our empirical analyses revealed that species with the apomorphic fin design have a greater capacity for generating thrust and for directing force through the body�s center of mass to minimize destabilizing pitching moments. Continued experimental study of fluid flow in the wake of swimming fishes will further facilitate functional interpretation of evolutionary trends in propulsor design.