Biomechanics of fast-start swimming in fish

Online Hydraulic Stiffness Modulation of a Soft Robotic Fish tail for Improved Thrust and Efficiency

This paper explores online stiffness modulation within a single tail stroke for swimming soft robots. Despite advances in stiffening mechanisms, little attention has been given to dynamically adjusting stiffness in real-time, presenting a challenge in developing mechanisms with the requisite bandwidth to match tail actuation. Achieving an optimal balance between thrust and efficiency in swimming soft robots remains elusive, and the paper addresses this challenge by proposing a novel mechanism for independent stiffness control, leveraging fluid-driven stiffening within a patterned pouch. Inspired by fluidic-driven actuation, this approach exhibits high bandwidth and facilitates significant stiffness changes. We perform experiments to demonstrate how this mechanism enhances both thrust and swimming efficiency. The tail actuation and fluid-driven stiffening can be optimized for a specific combination of thrust and efficiency, tailored to the desired maneuver type. The paper further explores the complex interaction between the soft body and surrounding fluid and provides fluid dynamics insights gained from the vortices created during actuation. Through frequency modulation and online stiffening, the study extends the Pareto front of achievable thrust generation and swimming efficiency.

A bio-inspired robotic fish utilizes the snap-through buckling of its spine to generate accelerations of more than 20g

Inspired by the fastest observed live fishes, we have designed, built and tested a robotic fish that emulates the fast-start maneuver of these fishes and generates acceleration and velocity magnitudes comparable to those of the live fishes within the same time scale. We have designed the robotic fish such that it uses the snap-through bucking of its spine to generate the fast-start response. We have used a dynamic snap-through buckling model and a series of experiments on a beam under snap-through buckling to describe the robotic fish's motion. Our under-actuated robot relies on passive dynamics of a continuous beam to generate organic waveforms. In its transient fast-start maneuver, our robotic fish produces mode shapes very similar to those observed in live fishes, by going through a snap-through bifurcation. We have also used a nonlinear structural model subjected to a non-conservative eccentric compressive force, which is constrained to act tangential to the structure at all times, coupled with a simple fluid dynamic model to approximate the transient behavior of the robot. We relate the numerical results from our nonlinear model to the dynamics observed in the live system proposing an updated kinematic model to understand the mode shapes observed in the fast-start maneuver of the live fishes. We also report on deploying the robotic fish in a river.

Disc starts: the pectoral disc of stingrays promotes omnidirectional fast starts across the substrate

We explored how the flattened and rounded pectoral disc of the ocellate river stingray (Potamotrygon motoro (Müller and Henle, 1841)) enables them to use the benthic plane during fast-start escape. Escape responses were elicited via prodding different locations around the pectoral disc and were recorded using video. Modulation of pectoral-fin movements that power swimming enabled omnidirectional escape across the substrate, with similar performance in all directions of escape. Hence, translation of the body did not necessarily have to follow the orientation of the head, overcoming the constraint of a rigid body axis. An increase in prod speed was associated with an increase in initial translational speed and acceleration away from the prod. As prod location shifted towards the snout, yaw rotation increased, eventually reorienting the fish into a forward swimming position away from the prod. Furthermore, P. motoro yawed with essentially zero turning radius, allowing reorientation of the head with simultaneous rapid translation away from the prod, and yaw rate during escape was substantially greater than reported during routine swimming for stingrays. We conclude that stingrays employ a distinctive approach to escape along the substrate, which we have termed disc starts, that results in effective manoeuvrability across the benthic environment despite limited longitudinal flexibility of the body and that challenges the concept of manoeuvrability typically used for fishes.

Environmental oestrogens cause predation-induced population decline in a freshwater fish

Understanding population-level effects of environmental stressors on aquatic biota requires knowledge of the direct adverse effects of pollutants on individuals and species interactions that relate to survival and reproduction. Here, we connect behavioural assays with survival trials and a modelling approach to quantify changes in antipredator escape performance of a larval freshwater fish following exposure to an environmental oestrogen, and predict changes in population abundance. We quantified the effects of short-term (21 days) exposure to 17β-oestradiol (E2) on the antipredator escape performance of larval fathead minnows (Pimephales promelas) and the probability of predation by a natural predator, the bluegill sunfish (Lepomis macrochirus). Compared with unexposed minnows, minnows exposed to environmentally relevant concentrations of E2 that approach total oestrogenic activity of wastewater-dominated environments (38 and 103 ng l-1) had delayed response times and slower escape speeds, and were more susceptible to predation. Incorporating these data into a stage-structured population model demonstrated that enhanced predation mortality at the larval stage can result in population declines. These results indicate that subtle, sub-lethal shifts in the behaviour of individuals due to human-mediated environmental change can impact species interactions with measurable population-level effects. Such changes have the potential to alter higher-order trophic interactions and disrupt aquatic communities.

Zebrafish swimming in the flow: a particle image velocimetry study

Zebrafish is emerging as a species of choice for the study of a number of biomechanics problems, including balance development, schooling, and neuromuscular transmission. The precise quantification of the flow physics around swimming zebrafish is critical toward a mechanistic understanding of the complex swimming style of this fresh-water species. Although previous studies have elucidated the vortical structures in the wake of zebrafish swimming in placid water, the flow physics of zebrafish swimming against a water current remains unexplored. In an effort to illuminate zebrafish swimming in a dynamic environment reminiscent of its natural habitat, we experimentally investigated the locomotion and hydrodynamics of a single zebrafish swimming in a miniature water tunnel using particle image velocimetry. Our results on zebrafish locomotion detail the role of flow speed on tail beat undulations, heading direction, and swimming speed. Our findings on zebrafish hydrodynamics offer a precise quantification of vortex shedding during zebrafish swimming and demonstrate that locomotory patterns play a central role on the flow physics. This knowledge may help clarify the evolutionary advantage of burst and cruise swimming movements in zebrafish.

Median fin function during the escape response of bluegill sunfish (Lepomis macrochirus). I: Fin-ray orientation and movement

The fast-start escape response is critically important to avoid predation, and axial movements driving it have been studied intensively. Large median dorsal and anal fins located near the tail have been hypothesized to increase acceleration away from the threat, yet the contribution of flexible median fins remains undescribed. To investigate the role of median fins, C-start escape responses of bluegill sunfish (Lepomis macrochirus) were recorded by three high-speed, high-resolution cameras at 500 frames s(-1) and the 3-D kinematics of individual dorsal and anal fin rays were analyzed. Movement and orientation of the fin rays relative to the body axis were calculated throughout the duration of the C-start. We found that: (1) timing and magnitude of angular displacement varied among fin rays based on position within the fin and (2) kinematic patterns support the prediction that fin rays are actively resisting hydrodynamic forces and transmitting momentum into the water. We suggest that regions within the fins have different roles. Anterior regions of the fins are rapidly elevated to increase the volume of water that the fish may interact with and transmit force into, thus generating greater total momentum. The movement pattern of all the fin rays creates traveling waves that move posteriorly along the length of the fin, moving water as they do so. Flexible posterior regions ultimately act to accelerate this water towards the tail, potentially interacting with vortices generated by the caudal fin during the C-start. Despite their simple appearance, median fins are highly complex and versatile control surfaces that modulate locomotor performance.

Median fin function during the escape response of bluegill sunfish (Lepomis macrochirus). II: Fin-ray curvature

Although kinematic analysis of individual fin rays provides valuable insight into the contribution of median fins to C-start performance, it paints an incomplete picture of the complex movements and deformation of the flexible fin surface. To expand our analysis of median fin function during the escape response of bluegill sunfish (Lepomis macrochirus), patterns of spanwise and chordwise curvature of the soft dorsal and anal fin surfaces were examined from the same video sequences previously used in analysis of fin-ray movement and orientation. We found that both the span and chord undergo undulation, starting in the anterior region of either fin. Initiated early in Stage 1 of the C-start, the undulation travels in a postero-distal direction, reaching the trailing edge of the fins during early Stage 2. Maximum spanwise curvature typically occurred among the more flexible posterior fin rays, though there was no consistent correlation between maximum curvature and fin-ray position. Undulatory patterns suggest different mechanisms of action for the fin regions. In the anterior fin region, where the fin rays are oriented dorsoventrally, undulation is directed primarily chordwise, initiating a transfer of momentum into the water to overcome the inertia of the flow and direct the water posteriorly. Within the posterior region, where the fin rays are oriented caudally, undulation is predominantly directed spanwise; thus, the posterior fin region acts to ultimately accelerate this water towards the tail to increase thrust forces. Treatment of median fins as appendages with uniform properties does not do justice to their complexity and effectiveness as control surfaces.

Acceleration performance of individual European sea bass Dicentrarchus labrax measured with a sprint performance chamber: comparison with high-speed cinematography and correlates with ecological performance

Locomotor performance can influence the ecological and evolutionary success of a species. For fish, favorable outcomes of predator-prey encounters are often presumably due to robust acceleration ability. Although escape-response or "fast-start" studies utilizing high-speed cinematography are prevalent, little is known about the contribution of relative acceleration performance to ecological or evolutionary success in a species. This dearth of knowledge may be due to the time-consuming nature of analyzing film, which imposes a practical limit on sample sizes. Herein, we present a high-throughput potential alternative for measuring fish acceleration performance using a sprint performance chamber (SPC). The acceleration performance of a large number of juvenile European sea bass (Dicentrarchus labrax) from two populations was analyzed. Animals from both hatchery and natural ontogenies were assessed, and animals of known acceleration ability had their ecological performance measured in a mesocosm environment. Individuals from one population also had their acceleration performance assessed by both high-speed cinematography and an SPC. Acceleration performance measured in an SPC was lower than that measured by classical high-speed video techniques. However, short-term repeatability and interindividual variation of acceleration performance were similar between the two techniques, and the SPC recorded higher sprint swimming velocities. Wild fish were quicker to accelerate in an SPC and had significantly greater accelerations than all groups of hatchery-raised fish. Acceleration performance had no significant effect on ecological performance (as assessed through animal growth and survival in the mesocosms). However, it is worth noting that wild animals did survive predation in the mesocosm better than farmed ones. Moreover, the hatchery-originated fish that survived the mesocosm experiment, when no predators were present, displayed significantly increased acceleration performance during their 6 mo in the mesocosm; this performance was found to be inversely proportional to growth rate.

Muscle function and swimming in sharks

The locomotor system in sharks has been investigated for many decades, starting with the earliest kinematic studies by Sir James Gray in the 1930s. Early work on axial muscle anatomy also included sharks, and the first demonstration of the functional significance of red and white muscle fibre types was made on spinal preparations in sharks. Nevertheless, studies on teleosts dominate the literature on fish swimming. The purpose of this article is to review the current knowledge of muscle function and swimming in sharks, by considering their morphological features related to swimming, the anatomy and physiology of the axial musculature, kinematics and muscle dynamics, and special features of warm-bodied lamnids. In addition, new data are presented on muscle activation in fast-starts. Finally, recent developments in tracking technology that provide insights into shark swimming performance in their natural environment are highlighted.

A fast-starting mechanical fish that accelerates at 40 m s(-2)

We have built a simple mechanical system to emulate the fast-start performance of fish. The system consists of a thin metal beam covered by a urethane rubber, the fish body and an appropriately shaped tail. The body form of the mechanical fish was modeled after a pike species and selected because it is a widely-studied fast-start specialist. The mechanical fish was held in curvature and hung in water by two restraining lines, which were simultaneously released by a pneumatic cutting mechanism. The potential energy in the beam was transferred into the fluid, thereby accelerating the fish. We measured the resulting acceleration, and calculated the efficiency of propulsion for the mechanical fish model, defined as the ratio of the final kinetic energy of the fish and the initially stored potential energy in the body beam. We also ran a series of flow visualization tests to observe the resulting flow patterns. The maximum start-up acceleration was measured to be around 40 m s(-2), with the maximum final velocity around 1.2 m s(-1). The form of the measured acceleration signal as function of time is quite similar to that of type I fast-start motions studied by Harper and Blake (1991 J. Exp. Biol. 155 175-92). The hydrodynamic efficiency of the fish was found to be around 10%. Flow visualization of the mechanical fast-start wake was also analyzed, showing that the acceleration peaks are associated with the shedding of two vortex rings in near-lateral directions.

Pacific lamprey climbing behavior

New lamprey-friendly fishways feature inclined ramps that facilitate passage of Pacific lampreys ( Lampetra tridentata (Richardson, 1836)) over Bonneville Dam on the Columbia River, USA. We observed the lampreys moving against water at two flow volumes and on two ramps of 45° and 18° angles relative to horizontal. We documented climbing movements using high-speed video (125 frames/s). Lampreys advanced on the ramps by repeated cycles of attaching to the ramps by their sucker mouths (resting phase), bending their bodies into a W shape (stage II), and then, rapidly straightening the body to propel themselves up the ramp, with simultaneous brief (20–140 ms) release of suction (stage III). We inferred that lampreys were using burst swimming to propel themselves up the ramp, because we observed inflection points in the body curvature traveling toward the posterior of the body and the center of mass moving up, during stage III. This climbing behavior is not described for any other fish species. Vertical motion, relative to the ground, during each cycle of movement was greatest in the 45° ramp – low water flow volume treatment (mean of 0.07 L/cycle), but the movement upstream along the ramp plane was greatest on the 18° ramp, regardless of flow volume. These findings can be used to develop ramp designs that maximize lamprey climbing performance.

The tactile-stimulated startle response of tadpoles: acceleration performance and its relationship to the anatomy of wood frog (Rana sylvatica), bullfrog (Rana catesbeiana), and American toad (Bufo americanus) tadpoles

I described the tactile-stimulated startle response (TSR) of wood frog (Rana sylvatica), bullfrog (Rana catesbeiana), and American toad (Bufo americanus) tadpoles. One purpose was to rank species in terms of maximum acceleration performance. Also, I tested whether anatomical indicators of performance potential were predictive of realized performance. TSRs were elicited in a laboratory setting, filmed at 250 Hz, and digitally analyzed. TSRs began with two, initial body curls during which tadpoles showed a broad spectrum of movement patterns. TSR performance was quantified by maximum linear acceleration and maximum rotational acceleration of the head/body, both of which tended to occur immediately upon initiation of motion (< 0.012 sec into the response). Bullfrog tadpoles had higher maximum acceleration than the other species, but other interspecific differences were not significant. The species' rank order for the anatomical indicator of linear acceleration potential was bullfrog > wood frog > American toad. The species' rank order for the anatomical indicator of rotational acceleration potential was bullfrog > wood frog = American toad. Thus, the anatomical indicators roughly predicted the rank order of interspecific average performance. However, the anatomical indicators did not correlate with individual tadpole performance. Variability in behavioral patterns may obscure the connection between anatomy and performance. This is seen in the current lack of intraspecific correlation between a morphological indicator of acceleration capacity and acceleration performance.

Fast-start muscle dynamics in the rainbow trout Oncorhynchus mykiss: phase relationship of white muscle shortening and body curvature

Muscle length changes of the lateral myotomal fast fibers of rainbow trout (Oncorhynchus mykiss) were measured using sonomicrometry during induced fast-starts. Simultaneous high-speed videography allowed for the analysis of midline kinematics to estimate the degree of muscle strain that occurs during body deformation. Comparison of these data was used to examine the phase relationship between local muscle shortening and local body bending during unsteady, large amplitude maneuvers. Our analysis finds that muscle shortening is temporally decoupled from body bending, probably due to the influence of hydrodynamic forces. The phase shift was such that midline curvature lagged behind muscle shortening at both the anterior (0.4 L, where L is fork length) and posterior (0.7 L) axial positions. Stronger escape responses were correlated with high peak strains and rapid strain-wave velocities, but not faster curvature-wave velocities. Under these conditions of high strain, the phase shift at the posterior position is significantly increased, whereas the anterior position fails to be affected. Curvature lag was still observed at both axial locations under conditions of low strain, suggesting that hydrodynamic forces are still significant during weaker escape responses. These data support a previous model that suggests fast-start body bending is determined by the interaction between muscle torque and hydrodynamic resistance along the body.