Carangiform swimmers typically have a narrow caudal peduncle, which probably allows them to separate thrust from drag both spatially and temporally. Eels seem to lack this separation, which may explain why they produce a wake with little downstream momentum while carangiform swimmers produce a wake with a clear thrust signature.

Animal movements result from a complex balance of many different forces. Muscles produce force to move the body; the body has inertial, elastic, and damping properties that may aid or oppose the muscle force; and the environment produces reaction forces back on the body. The actual motion is an emergent property of these interactions. To examine the roles of body stiffness, muscle activation, and fluid environment for swimming animals, a computational model of a lamprey was developed. The model uses an immersed boundary framework that fully couples the NavierStokes equations of fluid dynamics with an actuated, elastic body model. This is the first model at a Reynolds number appropriate for a swimming fish that captures the complete fluid-structure interaction, in which the body deforms according to both internal muscular forces and external fluid forces. Results indicate that identical muscle activation patterns can produce different kinematics depending on body stiffness, and the optimal value of stiffness for maximum acceleration is different from that for maximum steady swimming speed. Additionally, negative muscle work, observed in many fishes, emerges at higher tail beat frequencies without sensory input and may contribute to energy efficiency. Swimming fishes that can tune their body stiffness by appropriately timed muscle contractions may therefore be able to optimize the passive dynamics of their bodies to maximize peak acceleration or swimming speed.computational fluid dynamics | elasticity | locomotion

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SUMMARYEscape responses of fishes are one of the best characterized vertebrate behaviors, with extensive previous research on both the neural control and biomechanics of startle response performance. However, very little is known about the hydrodynamics of escape responses, despite the fact that understanding fluid flow patterns during the escape is critical for evaluating how body movement transfers power to the fluid, for defining the time course of power generation, and for characterizing the wake signature left by escaping fishes, which may provide information to predators. In this paper, we present an experimental hydrodynamic analysis of the C-start escape response in bluegill sunfish (Lepomis macrochirus). We used time-resolved digital particle image velocimetry at 1000 frames s -1 (fps) to image flow patterns during the escape response. We analyzed flow patterns generated by the body separately from those generated by the dorsal and anal fins to assess the contribution of these median fins to escape momentum. Each escape response produced three distinct jets of fluid. Summing the components of fluid momentum in the jets provided an estimate of fish momentum that did not differ significantly from momentum measured from the escaping fish body. In contrast to conclusions drawn from previous kinematic analyses and theoretical models, the caudal fin generated momentum that opposes the escape during stage one, whereas the body bending during stage one contributed substantial propulsive momentum. Additionally, the dorsal and anal fins each contributed substantial momentum. The results underscore the importance of the dorsal and anal fins as propulsors and suggest that the size and placement of these fins may be a key determinant of fast start performance. Supplementary material available online at

Simultaneous swimming kinematics and hydrodynamics are presented for American eels, Anguilla rostrata, swimming at speeds from 0.5 to 2 L s(-1). Body outlines and particle image velocimetry (PIV) data were collected using two synchronized high-speed cameras, and an empirical relationship between swimming motions and fluid flow is described. Lateral impulse in the wake is estimated assuming that the flow field represents a slice through small core vortex rings and is shown to be significantly larger than forces estimated from the kinematics via elongated body theory (EBT) and via quasi-steady resistive drag forces. These simple kinematic models predict only 50% of the measured wake impulse, indicating that unsteady effects are important in undulatory force production. EBT does, however, correctly predict both the magnitude and time course of the power shed into the wake. Other wake flow structures are also examined relative to the swimming motions. At all speeds, the wake contains almost entirely lateral jets of fluid, separated by an unstable shear layer that rapidly breaks down into two vortices. The jet's mean velocity grows with swimming speed, but jet diameter varies only weakly with swimming speed. Instead, it follows the body wavelength, which changes more among individuals than at different speeds. Circulation of the stop-start vortex, shed each time the tail changes direction, can also be predicted at low speeds by the integral of squared tail velocity over half of a tail beat. At high speeds, these kinematics predict more circulation than is actually present in the stop-start vortex. Finally, the cost of producing the wake, one component of the total cost of transport, increases with swimming speed to the 1.48 power, lower than would be expected if the power coefficient remained constant over the speed range examined.