Quantitative assessment of animal swimming performance is essential to gaining an understanding the ability of aquatic species to compete in and withstand changes in their environment. A thorough understanding of swimming performance requires quantifying both the motion of the propulsors and the resulting fluid flow. For the myriad aquatic animals that use them, the ability to quantify simultaneously fluid flows produced by their various propulsors is constrained by the current methodological approaches that measure flow in only two dimensions. In this project, the investigators propose a novel 3D approach for studying swimming animals. They will focus on the two separate, but coordinated, propulsive systems of squid (jets and fins) as follows: (1) collect 3D data of the complete fluid flow (wake) generated by swimming squid (both fin and jet wakes simultaneously) and 3D kinematic data of the swimming motion; (2) apply new mathematical tools to quantitatively distinguish between hydrodynamic and kinematic patterns (i.e., gaits) based on their physical features; and (3) evaluate the propulsive performance (i.e., thrust and efficiency) associated with gaits identified in step 2. This quantitative approach will illuminate the selective pressures driving the structure, mechanics, and dynamics of the musculoskeletal system that powers and supports the propulsors. This research holds great promise for developing a universal framework for gait identification in any swimmer or flyer, especially those employing multiple propulsors, and thus may potentially transform current methods for studying locomotion. Beyond the field of biology, this quantitative, 3D approach could provide a valuable framework for engineers of bioinspired propulsion systems, who may be seeking improved propulsive performance in compact designs similar to what nature offers. Finally, the collaborative interdisciplinary nature of this project will allow undergraduate and graduate students with diverse backgrounds in physiology, biomechanics, and engineering to interact and acquire training in cutting edge technologies.

Project Report

To fully understand swimming in aquatic animals that use coordinated appendages (or systems), 3D visualization/measurement tools and mathematical approaches for making sense of complex datasets are required. In particular, it is important to (1) simultaneously measure the 3D body movements and accompanying 3D momentum consequences of these movements, i.e., fluid wake behind the animal, and (2) identify and distinguish fundamentally distinct hydrodynamic features and body patterns. This is not trivial and requires new technologies and approaches. This project focused on developing an integrated 3D technique for recording swimming behavior and quantitatively assessing speed-specific locomotive patterns in aquatic animals. The test animal used in our approach was a squid, which embodies complexities appropriate for such an analysis, including the use of coordinated fin action and jetting and the ability to swim in different orientations (e.g., arms-first and tail-first). A 3D velocimetry system and 3D high-speed camera array were successfully configured and used to study squid while they swam steadily against currents or completed turning maneuvers. Computer programs to calculate hydrodynamic properties were developed and mathematical tools, including proper orthogonal decomposition and critical point analysis, were successfully applied to identify key fin motions and distinguish wake flows. Our results show that squid produce different wakes, body motions, and propulsive characteristics as they swim steadily at different speeds and complete maneuvers. Isolated vortex ring structures were present in both the jet and fin wakes at low speeds. At intermediate speeds, the greatest wake complexity was observed, with wake structures ranging from isolated vortex rings to interconnected vortex ring complexes involving both fin and jet flows. At high speeds the fins contributed minimally to propulsion while the jet produced either rapidly pulsed vortex rings (arms-first swimming) or elongated regions of concentrated vorticity with ring elements (tail-first swimming). In general, the jet played the most significant propulsive role, with the fins playing a more subordinate role during steady swimming, though the fins were important for lift and body stabilization at low and intermediate speeds. During arms-first swimming the fins produced more pronounced undulations and overall greater motion complexity, which resulted in more intricate wake patterns and higher thrust production relative to tail-first fin motions. Propulsive efficiency increased with speed and some squid achieved propulsive efficiencies as high as 95%, which is impressive for jet-propelled swimming. Squid exhibited remarkable turning performance and used a suite of hydrodynamic approaches to complete turns, including rapid pulsed jetting, asymmetric fin force production, and coordinated fin and jet production. This flexibility contributes to their high maneuverability, as squid were found to turn quickly and more tightly than any aquatic animal examined to date. This project provided training for both undergraduate and graduate students, some of whom belonged to underrepresented groups in the sciences. Results from this study were disseminated at national conferences and public tours, and the PIs worked closely with students in a young scholars program at a local public middle school (94% minority enrollment), with the goal of getting 6-7th graders excited about squids and other marine creatures. Project results are currently being used to inform the design of bioinspired underwater vehicles both in academe and private industry. Furthermore, this project provides a 3D framework for quantitatively identifying gaits and other locomotive patterns with speed-specific performance benefits in swimming and flying animals, especially those employing multiple systems/propulsors for locomotion.

National Science Foundation (NSF)
Division of Integrative Organismal Systems (IOS)
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Steven Ellis
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Old Dominion University Research Foundation
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