Experimental Hydrodynamics and Evolution: function of median fins in ray-finned fishes

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Organisms that live in the water represent a substantial fraction of life's diversity, and understanding how animals move through water, exert forces on their environment, and control their body position in a turbulent environment is critical to developing insights into the diversity of aquatic life. However, attempts to study this question over the past 20 years have met with many difficulties. Chief among these has been the considerable technical difficulty of quantifying the forces exerted by the movement of organisms on the water. On land such measurements are technically easy (stepping on a scale, for example, gives a force measurement of the body on the ground), but our inability to quantify forces exerted by organisms in the water has made it very difficult to understand the functional significance of different body and fin shapes.This study adopts from the field of engineering a new flow visualization technique called Digital Particle Image Velocimetry (DPIV) and modifies it to study fish body and fin motion. This technique provides, for the first time, a means of experimentally quantifying water movement in the wake of freely swimming fishes, and calculating the magnitudes and directions of forces exerted on the water by fins and the body of freely-moving aquatic organisms. Progress on previous NSF grants has demonstrated the ability of DPIV to provide data critical to understanding mechanisms of aquatic locomotion in organisms, and has shown how using this new approach to measuring the motion of water leads to previously unexpected insights into the diversity of aquatic organisms. The general objectives of this research are to study the hydrodynamic function of fins in fishes, focusing on the dorsal and caudal fins in sunfish, trout, and sturgeon with the aim of testing several long-standing hypotheses in the literature regarding the mechanisms by which these fins generate force and allow fishes to maneuver and position themselves in the water. Of special interest is the hypothesis of "wake interception" : that fishes can enhance the force generated by their tail fin by having the tail intercept the hydrodynamic wake shed by the dorsal fin. If this hypothesis is corroborated, it will represent an important new general mechanism by which both organisms and man-made submersibles could increase their propulsive efficiency. This research project contributes to advanced training of undergraduates in new approaches and technologies for the study of animal biomechanics and evolution through individual student research projects in an advanced undergraduate course co-taught by the Principal Investigator (PI), the interdisciplinary training of biology graduate students in engineering approaches to the study of organismal function, and post-doctoral training for the next generation of academic faculty. The research proposed here will have increased breadth of impact through a broad new training program in biomechanics at Harvard University which integrates biomechanics graduate training in biology, chemistry, physics, and engineering. The PI is one of the faculty on this grant, and the research proposed will serve as projects for students undertaking mandatory rotations, thus introducing a wide diversity of chemistry, physics and engineering graduate students to concepts and approaches in organismic functional biology that they would not otherwise be exposed to.