Wake dynamics and fluid forces of turning maneuvers in sunfish

Historical Field Records Reveal Habitat as an Ecological Correlate of Locomotor Phenotypic Diversity in the Radiation of Neotropical Geophagini Fishes

AbstractPhenotypic macroevolutionary studies provide insight into how ecological processes shape biodiversity. However, the complexity of phenotype-ecology relationships underscores the importance of also validating phenotype-based ecological inference with direct evidence of resource use. Unfortunately, macroevolutionary-scale ecological studies are often hindered by the challenges of acquiring taxonomically and spatially representative ecological data for large and widely distributed clades. The South American cichlid fish tribe Geophagini represents a continentally distributed radiation whose early locomotor morphological divergence suggests habitat as one ecological correlate of diversification, but an association between locomotor traits and habitat preference has not been corroborated. Field notes accumulated over decades of collecting across South America provide firsthand environmental records that can be mined for habitat data in support of macroevolutionary ecological research. In this study, we applied a newly developed method to transform descriptive field note information into quantitative habitat data and used it to assess habitat preference and its relationship to locomotor morphology in Geophagini. Field note-derived data shed light on geophagine habitat use patterns and reinforced habitat as an ecological correlate of locomotor morphological diversity. Our work emphasizes the rich data potential of museum collections, including often-overlooked material such as field notes, for evolutionary and ecological research.

Growing out of the fins: Implications of isometric and allometric scaling of morphology relative to increasing mass in blue sharks (Prionace glauca)

Disproportional changes (i.e. allometry) in shark morphology relative to increasing body size have been attributed to shifts in function associated with niche shifts in life history, such as in habitat and diet. Photographs of blue sharks (Prionace glauca, 26-145 kg) were used to analyze changes in parameters of body and fin morphology with increasing mass that are fundamental to swimming and feeding. We hypothesized that blue sharks would demonstrate proportional changes (i.e. isometry) in morphology with increasing mass because they do not undergo profound changes in prey and habitat type; accordingly, due to geometric scaling laws, we predicted that blue sharks would grow into bodies with greater turning inertias and smaller frontal and surface areas, in addition to smaller spans and areas of the fins relative to mass, which are parameters that are associated with the swimming performance in sharks. Many aspects of morphology increased with isometry. However, blue sharks demonstrated negative allometry in body density, whereas surface area, volume and roll inertia of the body, area, span and aspect ratio of both dorsal fins, span and aspect ratio of the ventral caudal fin, and span, length and area of the mouth increased with positive allometry. The dataset was divided in half based on mass to form two groups: smaller and larger sharks. Besides area of both dorsal fins, relative to mass, larger sharks had bodies with significantly greater turning inertia and smaller frontal and surface areas, in addition to fins with smaller spans and areas, compared to smaller sharks. In conclusion, isometric scaling does not necessarily imply functional similarity, and allometric scaling may sometimes be critical in maintaining, rather than shifting, function relative to mass in animals that swim through the water column.

Vectored jets power arms-first and tail-first turns differently in brief squid with assistance from fins and keeled arms

Squids maneuver to capture prey, elude predators, navigate complex habitats, and deny rivals access to mates. Despite the ecological importance of this essential locomotive function, limited quantitative data on turning performance and wake dynamics of squids are available. To better understand the contribution of the jet, fins, and arms to turns, the role of orientation (i.e., arms-first vs tail-first) in maneuvering, and relationship between jet flow and turning performance, kinematic and 3D velocimetry data were collected in tandem from brief squid Lolliguncula brevis. The pulsed jet, which can be vectored to direct flows, was the primary driver of most turning behaviors, producing flows with the highest impulse magnitude and angular impulse about the main axis of the turn (yaw) and secondary axes (roll and pitch). The fins and keeled arms played subordinate but important roles in turning performance, contributing to angular impulse, stabilizing the maneuver along multiple axes, and/or reducing rotational resistance. Orientation affected turning performance and dynamics, with tail-first turns being associated with greater impulse and angular impulse, longer jet structures, higher jet velocities, and greater angular turning velocities than arms-first turns. Conversely, arms-first turns involved shorter, slower jets with less impulse, but these directed short pulses resulted in lower minimum length-specific turning radii. Although the length-to-diameter ratio (L/D) of ejected jet flow was a useful metric for characterizing vortical flow features, it, by itself, was not a reliable predictor of angular velocity or turning radii, which reflects the complexity of the squid multi-propulsor system.

Knifefish turning control and hydrodynamics during forward swimming

Rapid turning and swimming contribute to ecologically important behaviors in fishes such as predator avoidance, prey capture, mating, and the navigation of complex environments. For riverine species, such as knifefishes, turning behaviors may also be important for navigating locomotive perturbations caused by turbulent flows. Most research on fish maneuvering focuses on fish with traditional fin and body morphologies, which primarily use body bending and the pectoral fins during turning. However, it is uncertain how fishes with uncommon morphologies, are able to achieve sudden and controllable turns. Here we studied the turning performance and the turning hydrodynamics of the Black ghost knifefish (Apteronotus albifrons, N=6) which has an atypical elongated ribbon fin. Fish were filmed while swimming forward at ∼2 BL s-1 and feeding from a fixed feeder (control) and an oscillating feeder (75 Hz) at two different amplitudes. 3D kinematic analysis of the body revealed the highest pitch angles and lowest body bending coefficients occurred during steady swimming. Low pitch angle, high maximum yaw angles and large body bending coefficients were characteristic of small and large turns. Asynchrony in pectoral fin use was low during turning, however ribbon fin wavelength, frequency, and wave speed were greatest during large turns. Digital particle image velocimetry (DPIV) showed larger counter-rotating vortex pairs produced during turning by the ribbon-fin in comparison to vortices rotating in the same direction during steady swimming. Our results highlight the ribbon fin's role in controlled rapid turning through modulation of wavelength, frequency, and wave speed.

Course Control of a Manta Robot Based on Amplitude and Phase Differences

Due to external interference, such as waves, the success of underwater missions depends on the turning performance of the vehicle. Manta rays use two broad pectoral fins for propulsion, which provide better anti-interference ability and turning performance. Inspired by biological yaw modes, we use the phase difference between the pectoral fins to realize fast course adjustment and the amplitude difference to realize precise adjustment. We design a bionic robot with pectoral fins and use phase oscillators to realize rhythmic motion. An expected phase difference transition equation is introduced to realize a fast and smooth transition of the output, and the parameters are adjusted online. We combine the phase difference and amplitude difference yaw modes to realize closed-loop course control. Through course interference and adjustment experiments, it is verified that the combined mode is more effective than a single mode. Finally, a rectangular trajectory swimming experiment demonstrates continuous mobility of the robot under the combined mode.

Form and function of the caudal fin throughout the phylogeny of fishes

Fishes are the longest persisting living vertebrates and as such, display an incredible array of diversity. Variation in the tail, or caudal fin, is often a reflection of a fish's environment, and affects movement, predation, defense, and reproduction. Previous literature has discussed many aspects of caudal fin form and function in particular taxonomic groups; however, no previous work has synthesized these studies in order to detail how the caudal fin is structured, and what purpose this structure serves, throughout the phylogeny of fishes. This review examines the caudal fin throughout the main lineages of fish evolution, and highlights where changes in shape and usage have occurred. Such novelties in form and function tend to have far-reaching evolutionary consequences. Through integration of past and present work, this review creates a coherent picture of caudal fin evolution. Patterns and outliers that demonstrate how form and function of this appendage are intertwined can further inform hypotheses that fill critical gaps in knowledge concerning the caudal fin.

Archerfish coordinate fin maneuvers with their shots

Archerfish down a variety of aerial prey from a range of distances using water jets that they adjust to the size and distance of their prey. We describe here that characteristic rapid fin maneuvers, most notably of the pectoral and pelvic fins, are precisely coordinated with the release of the jet. We discovered these maneuvers in two fish, the jets of which had been characterized in detail, that had been trained to shoot from fixed positions at targets at different heights and that remained stable during their shots. Based on the findings in these individuals, we examined shooting-associated fin movement in 28 further archerfish of two species that could shoot from freely chosen positions at targets at different heights. Slightly before the onset of the water jet, at a time when the shooter remains stable, the pectoral fins of all shooters switched from asynchronous low-amplitude beating to a synchronized rapid forward flap. The onset and duration of the forward and subsequent backward flap were robust across all individuals and shooting angles but depended on target height. The pelvic fins were slowly adducted at the start of the jet and stopped moving after its release. All other fins also showed a characteristic sequence of activation, some starting ∼0.5 s before the shot. Our findings suggest that shooting-related fin maneuvers are needed to stabilize the shooter, and that these maneuvers are an important component in the precise and powerful far-distance shooting in archerfish.

Neuromechanical wave resonance in jellyfish swimming

For organisms to have robust locomotion, their neuromuscular organization must adapt to constantly changing environments. In jellyfish, swimming robustness emerges when marginal pacemakers fire action potentials throughout the bell's motor nerve net, which signals the musculature to contract. The speed of the muscle activation wave is dictated by the passage times of the action potentials. However, passive elastic material properties also influence the emergent kinematics, with time scales independent of neuromuscular organization. In this multimodal study, we examine the interplay between these two time scales during turning. A three-dimensional computational fluid-structure interaction model of a jellyfish was developed to determine the resulting emergent kinematics, using bidirectional muscular activation waves to actuate the bell rim. Activation wave speeds near the material wave speed yielded successful turns, with a 76-fold difference in turning rate between the best and worst performers. Hyperextension of the margin occurred only at activation wave speeds near the material wave speed, suggesting resonance. This hyperextension resulted in a 34-fold asymmetry in the circulation of the vortex ring between the inside and outside of the turn. Experimental recording of the activation speed confirmed that jellyfish actuate within this range, and flow visualization using particle image velocimetry validated the corresponding fluid dynamics of the numerical model. This suggests that neuromechanical wave resonance plays an important role in the robustness of an organism's locomotory system and presents an undiscovered constraint on the evolution of flexible organisms. Understanding these dynamics is essential for developing actuators in soft body robotics and bioengineered pumps.

Swimming in unsteady water flows: is turning in a changing flow an energetically expensive endeavor for fish?

Unsteady, dynamic flow regimes commonly found in shallow marine ecosystems such as coral reefs pose an energetic challenge for mobile organisms that typically depend on station holding for fitness-related activities. The majority of experimental studies, however, have measured energetic costs of locomotion at steady speeds, with only a few studies measuring the effects of oscillatory flows. In this study, we used a bidirectional swimming respirometer to create six oscillatory water flow regimes consisting of three frequency and amplitude combinations for both unidirectional and bidirectional oscillatory flows. Using the goldring surgeonfish, Ctenochaetus strigosus, a pectoral-fin (labriform) swimmer, we quantified the net cost of swimming (swimming metabolic rate minus standard metabolic rate) associated with station-holding under these various conditions. We determined that the swimming costs of station-holding in the bidirectional flow regime increased by 2-fold compared with costs based on swimming over the same range velocities at steady speeds. Furthermore, as we found minimal differences in energetic costs associated with station-holding in the unidirectional, oscillating-flow compared with that predicted from steady swimming costs, we conclude that the added acceleration costs are minimal, while the act of turning is an energetically expensive endeavor for this reef fish species.

Multichannel stroboscopic videography (MSV): a technique for visualizing multiple channels for behavioral measurements

Biologists commonly visualize different features of an organism using distinct sources of illumination. Such multichannel imaging has largely not been applied to behavioral studies because of the challenges posed by a moving subject. We address this challenge with the technique of multichannel stroboscopic videography (MSV), which synchronizes multiple strobe lights with video exposures of a single camera. We illustrate the utility of this approach with kinematic measurements of a walking cockroach (Gromphadorhina portentosa) and calculations of the pressure field around a swimming fish (Danio rerio). In both, transmitted illumination generated high-contrast images of the animal's body in one channel. Other sources of illumination were used to visualize the points of contact for the feet of the cockroach and the water flow around the fish in separate channels. MSV provides an enhanced potential for high-throughput experimentation and the capacity to integrate changes in physiological or environmental conditions in freely-behaving animals.

Structure, composition, mechanics and growth of spines of the dorsal fin of blue tilapia Oreochromis aureus and common carp Cyprinus carpio

The structural, compositional and mechanical properties of the spines of the dorsal fin in mature anosteocytic blue tilapia Oreochromis aureus and osteocytic common carp Cyprinus carpio are described, as well as their temporal growth pattern and regenerative capacities. The three-dimensional architecture of both spines, from macro to sub-micron levels, is shown to be axially oriented and therefore highly anisotropic and the spines of both species are able to regenerate after partial amputation.

Hydrodynamic study of freely swimming shark fish propulsion for marine vehicles using 2D particle image velocimetry

Two-dimensional velocity fields around a freely swimming freshwater black shark fish in longitudinal (XZ) plane and transverse (YZ) plane are measured using digital particle image velocimetry (DPIV). By transferring momentum to the fluid, fishes generate thrust. Thrust is generated not only by its caudal fin, but also using pectoral and anal fins, the contribution of which depends on the fish’s morphology and swimming movements. These fins also act as roll and pitch stabilizers for the swimming fish. In this paper, studies are performed on the flow induced by fins of freely swimming undulatory carangiform swimming fish (freshwater black shark, L = 26 cm) by an experimental hydrodynamic approach based on quantitative flow visualization technique. We used 2D PIV to visualize water flow pattern in the wake of the caudal, pectoral and anal fins of swimming fish at a speed of 0.5–1.5 times of body length per second. The kinematic analysis and pressure distribution of carangiform fish are presented here. The fish body and fin undulations create circular flow patterns (vortices) that travel along with the body waves and change the flow around its tail to increase the swimming efficiency. The wake of different fins of the swimming fish consists of two counter-rotating vortices about the mean path of fish motion. These wakes resemble like reverse von Karman vortex street which is nothing but a thrust-producing wake. The velocity vectors around a C-start (a straight swimming fish bends into C-shape) maneuvering fish are also discussed in this paper. Studying flows around flapping fins will contribute to design of bioinspired propulsors for marine vehicles.

Introduction to the Symposium-Unsteady Aquatic Locomotion with Respect to Eco-Design and Mechanics

The importance of unsteadiness in the aquatic environment has come to the forefront in understanding locomotor mechanics in nature. The impact of unsteadiness, starting with control of posture and trajectories during aquatic locomotion, is ultimately expressed in energy costs, morphology, and fitness. Unsteadiness from both internal and external perturbations for aquatic animals is important at scales ranging from micro to macro to global.

Streamwise vortices destabilize swimming bluegill sunfish (Lepomis macrochirus)

In their natural environment, fish must swim stably through unsteady flows and vortices, including vertical vortices, typically shed by posts in a flow, horizontal cross-flow vortices, often produced by a step or a waterfall in a stream, and streamwise vortices, where the axis of rotation is aligned with the direction of the flow. Streamwise vortices are commonly shed by bluff bodies in streams and by ships' propellers and axial turbines, but we know little about their effects on fish. Here, we describe how bluegill sunfish use more energy and are destabilized more often in flow with strong streamwise vorticity. The vortices were created inside a sealed flow tank by an array of four turbines with similar diameter to the experimental fish. We measured oxygen consumption for seven sunfish swimming at 1.5 body lengths (BL) s−1 with the turbines rotating at 2 Hz and with the turbines off (control). Simultaneously, we filmed the fish ventrally and recorded the fraction of time spent maneuvering side-to-side and accelerating forward. Separately, we also recorded lateral and ventral video for a combination of swimming speeds (0.5, 1.5 and 2.5 BL s−1) and turbine speeds (0, 1, 2 and 3 Hz), immediately after turning the turbines on and 10 min later to test for accommodation. Bluegill sunfish are negatively affected by streamwise vorticity. Spills (loss of heading), maneuvers and accelerations were more frequent when the turbines were on than in the control treatment. These unsteady behaviors, particularly acceleration, correlated with an increase in oxygen consumption in the vortex flow. Bluegill sunfish are generally fast to recover from roll perturbations and do so by moving their pectoral fins. The frequency of spills decreased after the turbines had run for 10 min, but was still markedly higher than in the control, showing that fish partially adapt to streamwise vorticity, but not completely. Coping with streamwise vorticity may be an important energetic cost for stream fishes or migratory fishes.

Center of mass motion in swimming fish: effects of speed and locomotor mode during undulatory propulsion

Studies of center of mass (COM) motion are fundamental to understanding the dynamics of animal movement, and have been carried out extensively for terrestrial and aerial locomotion. But despite a large amount of literature describing different body movement patterns in fishes, analyses of how the center of mass moves during undulatory propulsion are not available. These data would be valuable for understanding the dynamics of different body movement patterns and the effect of differing body shapes on locomotor force production. In the present study, we analyzed the magnitude and frequency components of COM motion in three dimensions (x: surge, y: sway, z: heave) in three fish species (eel, bluegill sunfish, and clown knifefish) swimming with four locomotor modes at three speeds using high-speed video, and used an image cross-correlation technique to estimate COM motion, thus enabling untethered and unrestrained locomotion. Anguilliform swimming by eels shows reduced COM surge oscillation magnitude relative to carangiform swimming, but not compared to knifefish using a gymnotiform locomotor style. Labriform swimming (bluegill at 0.5 body lengths/s) displays reduced COM sway oscillation relative to swimming in a carangiform style at higher speeds. Oscillation frequency of the COM in the surge direction occurs at twice the tail beat frequency for carangiform and anguilliform swimming, but at the same frequency as the tail beat for gymnotiform locomotion in clown knifefish. Scaling analysis of COM heave oscillation for terrestrial locomotion suggests that COM heave motion scales with positive allometry, and that fish have relatively low COM oscillations for their body size.

Measurement of hydrodynamic force generation by swimming dolphins using bubble DPIV

Attempts to measure the propulsive forces produced by swimming dolphins have been limited. Previous uses of computational hydrodynamic models and gliding experiments have provided estimates of thrust production by dolphins, but these were indirect tests that relied on various assumptions. The thrust produced by two actively swimming bottlenose dolphins (Tursiops truncatus) was directly measured using digital particle image velocimetry (DPIV). For dolphins swimming in a large outdoor pool, the DPIV method used illuminated microbubbles that were generated in a narrow sheet from a finely porous hose and a compressed air source. The movement of the bubbles was tracked with a high-speed video camera. Dolphins swam at speeds of 0.7 to 3.4 m s−1 within the bubble sheet oriented along the midsagittal plane of the animal. The wake of the dolphin was visualized as the microbubbles were displaced because of the action of the propulsive flukes and jet flow. The oscillations of the dolphin flukes were shown to generate strong vortices in the wake. Thrust production was measured from the vortex strength through the Kutta–Joukowski theorem of aerodynamics. The dolphins generated up to 700 N during small amplitude swimming and up to 1468 N during large amplitude starts. The results of this study demonstrated that bubble DPIV can be used effectively to measure the thrust produced by large-bodied dolphins.

Kinematics of ribbon-fin locomotion in the bowfin, Amia calva

An elongated dorsal and/or anal ribbon-fin to produce forward and backward propulsion has independently evolved in several groups of fishes. In these fishes, fin ray movements along the fin generate a series of waves that drive propulsion. There are no published data on the use of the dorsal ribbon-fin in the basal freshwater bowfin, Amia calva. In this study, frequency, amplitude, wavelength, and wave speed along the fin were measured in Amia swimming at different speeds (up to 1.0 body length/sec) to understand how the ribbon-fin generates propulsion. These wave properties were analyzed to (1) determine whether regional specialization occurs along the ribbon-fin, and (2) to reveal how the undulatory waves are used to control swimming speed. Wave properties were also compared between swimming with sole use of the ribbon-fin, and swimming with simultaneous use of the ribbon and pectoral fins. Statistical analysis of ribbon-fin kinematics revealed no differences in kinematic patterns along the ribbon-fin, and that forward propulsive speed in Amia is controlled by the frequency of the wave in the ribbon-fin, irrespective of the contribution of the pectoral fin. This study is the first kinematic analysis of the ribbon-fin in a basal fish and the model species for Amiiform locomotion, providing a basis for understanding ribbon-fin locomotion among a broad range of teleosts.

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.

Contemporary and historical evolutionary processes interact to shape patterns of within-lake phenotypic divergences in polyphenic pumpkinseed sunfish, Lepomis gibbosus

Historical and contemporary evolutionary processes can both contribute to patterns of phenotypic variation among populations of a species. Recent studies are revealing how interactions between historical and contemporary processes better explain observed patterns of phenotypic divergence than either process alone. Here, we investigate the roles of evolutionary history and adaptation to current environmental conditions in structuring phenotypic variation among polyphenic populations of sunfish inhabiting 12 postglacial lakes in eastern North America. The pumpkinseed sunfish polyphenism includes sympatric ecomorphs specialized for littoral or pelagic lake habitats. First, we use population genetic methods to test the evolutionary independence of within-lake phenotypic divergences of ecomorphs and to describe patterns of genetic structure among lake populations that clustered into three geographical groupings. We then used multivariate analysis of covariance (MANCOVA) to partition body shape variation (quantified with geometric morphometrics) among the effects of evolutionary history (reflecting phenotypic variation among genetic clusters), the shared phenotypic response of all populations to alternate habitats within lakes (reflecting adaptation to contemporary conditions), and unique phenotypic responses to habitats within lakes nested within genetic clusters. All effects had a significant influence on body form, but the effects of history and the interaction between history and contemporary habitat were larger than contemporary processes in structuring phenotypic variation. This highlights how divergence can be better understood against a known backdrop of evolutionary history.

Fin-tail coordination during escape and predatory behavior in larval zebrafish

Larval zebrafish innately perform a suite of behaviors that are tightly linked to their evolutionary past, notably escape from threatening stimuli and pursuit and capture of prey. These behaviors have been carefully examined in the past, but mostly with regard to the movements of the trunk and tail of the larvae. Here, we employ kinematics analyses to describe the movements of the pectoral fins during escape and predatory behavior. In accord with previous studies, we find roles for the pectoral fins in slow swimming and immediately after striking prey. We find novel roles for the pectoral fins in long-latency, but not in short-latency C-bends. We also observe fin movements that occur during orienting J-turns and S-starts that drive high-velocity predatory strikes. Finally, we find that the use of pectoral fins following a predatory strike is scaled to the velocity of the strike, supporting a role for the fins in braking. The implications of these results for central control of coordinated movements are discussed, and we hope that these results will provide baselines for future analyses of cross-body coordination using mutants, morphants, and transgenic approaches.

Movement and function of the pectoral fins of the larval zebrafish (Danio rerio) during slow swimming

Pectoral fins are known to play important roles in swimming for many adult fish; however, their functions in fish larvae are unclear. We examined routine pectoral fin movement during rhythmic forward swimming and used genetic ablation to test hypotheses of fin function in larval zebrafish. Fins were active throughout bouts of slow swimming. Initiation was characterized by asymmetric fin abduction that transitioned to alternating rhythmic movement with first fin adduction. During subsequent swimming, fin beat amplitude decreased while tail beat amplitude increased over swimming speeds ranging from 1.47 to 4.56 body lengths per second. There was no change in fin or tail beat frequency with speed (means ± s.d.: 28.2±3.5 and 29.6±1.9 Hz, respectively). To examine potential roles of the pectoral fins in swimming, we compared the kinematics of finless larvae generated with a morpholino knockdown of the gene fgf24 to those of normal fish. Pectoral fins were not required for initiation nor did they significantly impact forward rhythmic swimming. We investigated an alternative hypothesis that the fins function in respiration. Dye visualization demonstrated that pectoral fin beats bring distant fluid toward the body and move it caudally behind the fins, disrupting the boundary layer along the body's surface, a major site of oxygen absorption in larvae. Larval zebrafish also demonstrated more fin beating in low oxygen conditions. Our data reject the hypothesis that the pectoral fins of larval zebrafish have a locomotor function during slow, forward locomotion, but are consistent with the hypothesis that the fins have a respiratory function.

Flexibility in the patterning and control of axial locomotor networks in lamprey

In lower vertebrates, locomotor burst generators for axial muscles generally produce unitary bursts that alternate between the two sides of the body. In lamprey, a lower vertebrate, locomotor activity in the axial ventral roots of the isolated spinal cord can exhibit flexibility in the timings of bursts to dorsally-located myotomal muscle fibers versus ventrally-located myotomal muscle fibers. These episodes of decreased synchrony can occur spontaneously, especially in the rostral spinal cord where the propagating body waves of swimming originate. Application of serotonin, an endogenous spinal neurotransmitter known to presynaptically inhibit excitatory synapses in lamprey, can promote decreased synchrony of dorsal-ventral bursting. These observations suggest the possible existence of dorsal and ventral locomotor networks with modifiable coupling strength between them. Intracellular recordings of motoneurons during locomotor activity provide some support for this model. Pairs of motoneurons innervating myotomal muscle fibers of similar ipsilateral dorsoventral location tend to have higher correlations of fast synaptic activity during fictive locomotion than do pairs of motoneurons innervating myotomes of different ipsilateral dorsoventral locations, suggesting their control by different populations of premotor interneurons. Further, these different motoneuron pools receive different patterns of excitatory and inhibitory inputs from individual reticulospinal neurons, conveyed in part by different sets of premotor interneurons. Perhaps, then, the locomotor network of the lamprey is not simply a unitary burst generator on each side of the spinal cord that activates all ipsilateral body muscles simultaneously. Instead, the burst generator on each side may comprise at least two coupled burst generators, one controlling motoneurons innervating dorsal body muscles and one controlling motoneurons innervating ventral body muscles. The coupling strength between these two ipsilateral burst generators may be modifiable and weakening when greater swimming maneuverability is required. Variable coupling of intrasegmental burst generators in the lamprey may be a precursor to the variable coupling of burst generators observed in the control of locomotion in the joints of limbed vertebrates.

Volumetric imaging of fish locomotion

Fishes use multiple flexible fins in order to move and maintain stability in a complex fluid environment. We used a new approach, a volumetric velocimetry imaging system, to provide the first instantaneous three-dimensional views of wake structures as they are produced by freely swimming fishes. This new technology allowed us to demonstrate conclusively the linked ring vortex wake pattern that is produced by the symmetrical (homocercal) tail of fishes, and to visualize for the first time the three-dimensional vortex wake interaction between the dorsal and anal fins and the tail. We found that the dorsal and anal fin wakes were rapidly (within one tail beat) assimilated into the caudal fin vortex wake. These results show that volumetric imaging of biologically generated flow patterns can reveal new features of locomotor dynamics, and provides an avenue for future investigations of the diversity of fish swimming patterns and their hydrodynamic consequences.

The effects of turbulent eddies on the stability and critical swimming speed of creek chub (Semotilus atromaculatus)

The effect of turbulent eddy diameter, vorticity and orientation on the 2 min critical swimming speed and stability of creek chub (Semotilus atromaculatus) is reported. Turbulent eddies were visualized and their properties were quantified using particle image velocimetry (PIV). Flow fields with an increasing range in eddy diameter were created by inserting cylinder arrays upstream from the swimming test section. Eddy vorticity increased with increasing velocity. Two orientations of eddies, eddies spinning about a vertical axis and eddies spinning about a horizontal (wall-to-wall) axis, were investigated. Stability challenges were not observed until the largest (95th percentile) eddy diameters reached 76% of the fish body total length. Under these conditions fish were observed to spin in an orientation consistent with the rotational axis of the large eddies and translate downstream. These losses in postural control were termed 'spills'. Spills were 230% more frequent and lasted 24% longer in turbulent flow fields dominated by horizontal eddies than by vertical eddies of the same diameter. The onset of spills coincided with a 10% and 22% reduction in critical swimming speed in turbulent flows dominated by large vertical and horizontal eddies, respectively. These observations confirm predictions by Pavlov et al., Cada and Odeh, Lupandin, and Liao that the eddy diameter, vorticity and orientation play an important role in the swimming capacity of fishes.

Hydrodynamic fin function of brief squid, Lolliguncula brevis

Although the pulsed jet is often considered the foundation of a squid's locomotive system, the lateral fins also probably play an important role in swimming, potentially providing thrust, lift and dynamic stability as needed. Fin morphology and movement vary greatly among squid species, but the locomotive role of the fins is not well understood. To begin to elucidate the locomotive role of the fins in squids, fin hydrodynamics were studied in the brief squid Lolliguncula brevis, a species that exhibits a wide range of fin movements depending on swimming speed. Individual squid were trained to swim in both the arms-first and tail-first orientations against currents in a water tunnel seeded with light-reflective particles. Particle-laden water around the fins was illuminated with lasers and videotaped so that flow dynamics around the fins could be analyzed using digital particle image velocimetry (DPIV). Time-averaged forces generated by the fin were quantified from vorticity fields of the fin wake. During the low swimming speeds considered in this study [<2.5 dorsal mantle lengths (DML) per second], L. brevis exhibited four unique fin wake patterns, each with distinctive vortical structures: (1) fin mode I, in which one vortex is shed with each downstroke, generally occurring at low speeds; (2) fin mode II, an undulatory mode in which a continuous linked chain of vortices is produced; (3) fin mode III, in which one vortex is shed with each downstroke and upstroke, and; (4) fin mode IV, in which a discontinuous chain of linked double vortex structures is produced. All modes were detected during tail-first swimming but only fin modes II and III were observed during arms-first swimming. The fins produced horizontal and vertical forces of varying degrees depending on stroke phase, swimming speed, and swimming orientation. During tail-first swimming, the fins functioned primarily as stabilizers at low speeds before shifting to propulsors as speed increased, all while generating net lift. During arms-first swimming, the fins primarily provided lift with thrust production playing a reduced role. These results demonstrate the lateral fins are an integral component of the complex locomotive system of L. brevis, producing lift and thrust forces through different locomotive modes.

Maneuvers during legged locomotion

Maneuverability is essential for locomotion. For animals in the environment, maneuverability is directly related to survival. For humans, maneuvers such as turning are associated with increased risk for injury, either directly through tissue loading or indirectly through destabilization. Consequently, understanding the mechanics and motor control of maneuverability is a critical part of locomotion research. We briefly review the literature on maneuvering during locomotion with a focus on turning in bipeds. Walking turns can use one of several different strategies. Anticipation can be important to adjust kinematics and dynamics for smooth and stable maneuvers. During running, turns may be substantially constrained by the requirement for body orientation to match movement direction at the end of a turn. A simple mathematical model based on the requirement for rotation to match direction can describe leg forces used by bipeds (humans and ostriches). During running turns, both humans and ostriches control body rotation by generating fore-aft forces. However, whereas humans must generate large braking forces to prevent body over-rotation, ostriches do not. For ostriches, generating the lateral forces necessary to change movement direction results in appropriate body rotation. Although ostriches required smaller braking forces due in part to increased rotational inertia relative to body mass, other movement parameters also played a role. Turning performance resulted from the coordinated behavior of an integrated biomechanical system. Results from preliminary experiments on horizontal-plane stabilization support the hypothesis that controlling body rotation is an important aspect of stable maneuvers. In humans, body orientation relative to movement direction is rapidly stabilized during running turns within the minimum of two steps theoretically required to complete analogous maneuvers. During straight running and cutting turns, humans exhibit spring-mass behavior in the horizontal plane. Changes in the horizontal projection of leg length were linearly related to changes in horizontal-plane leg forces. Consequently, the passive dynamic stabilization associated with spring-mass behavior may contribute to stability during maneuvers in bipeds. Understanding the mechanics of maneuverability will be important for understanding the motor control of maneuvers and also potentially be useful for understanding stability.

Pitching stabilization via caudal fin-wave propagation in a forward-sinking parrot cichlid (Cichlasoma citrinellum x Cichlasoma synspilum)

Caudal fin-wave propagation (CFP) is a commonly observed behavior in a fish but has been little investigated. Our objective is to understand the function of a CFP for a forward-sinking parrot cichlid that adopts a tilted-down swimming posture. We utilized stereoscopic digital particle-image velocimetry to measure the velocity fields in the wake of both the caudal fin and the pectoral fins and to evaluate the corresponding hydrodynamic forces. The tilted-down posture of this fish is inherently unstable because of the presence of the head-down pitching moment induced from the buoyant force of the body. The down-stroke of the pectoral fins results also in a head-down pitching moment that destabilizes the fish. Our results indicate that a CFP facilitates the pitching stabilization of a fish. In a forward-sinking parrot cichlid, a CFP produces periodic jets (CFP jets) that are oriented laterally and posterodorsally, which result in both thrust and negative lift that induce a head-up pitching moment. The CFP jets are initially trapped by the ventral part of the caudal fin, strengthened and reoriented by the dorsally propagating fin wave, and expelled near the dorsal part of the caudal fin.

The effect of temperature and thermal acclimation on the sustainable performance of swimming scup

There is a significant reduction in overall maximum power output of muscle at low temperatures due to reduced steady-state (i.e. maximum activation) power-generating capabilities of muscle. However, during cyclical locomotion, a further reduction in power is due to the interplay between non-steady-state contractile properties of muscle (i.e. rates of activation and relaxation) and the stimulation and the length-change pattern muscle undergoes in vivo. In particular, even though the relaxation rate of scup red muscle is slowed greatly at cold temperatures (10 degrees C), warm-acclimated scup swim with the same stimulus duty cycles at cold as they do at warm temperature, not affording slow-relaxing muscle any additional time to relax. Hence, at 10 degrees C, red muscle generates extremely low or negative work in most parts of the body, at all but the slowest swimming speeds. Do scup shorten their stimulation duration and increase muscle relaxation rate during cold acclimation? At 10 degrees C, electromyography (EMG) duty cycles were 18% shorter in cold-acclimated scup than in warm-acclimated scup. But contrary to the expectations, the red muscle did not have a faster relaxation rate, rather, cold-acclimated muscle had an approximately 50% faster activation rate. By driving cold- and warm-acclimated muscle through cold- and warm-acclimated conditions, we found a very large increase in red muscle power during swimming at 10 degrees C. As expected, reducing stimulation duration markedly increased power output. However, the increased rate of activation alone produced an even greater effect. Hence, to fully understand thermal acclimation, it is necessary to examine the whole system under realistic physiological conditions.

Advances in comparative physiology from high-speed imaging of animal and fluid motion

Since the time of Muybridge and Marey in the last half of the nineteenth century, studies of animal movement have relied on some form of high-speed or stop-action imaging to permit analysis of appendage and body motion. In the past ten years, the advent of megapixel-resolution high-speed digital imaging with maximal framing rates of 250 to 100,000 images per second has allowed new views of musculoskeletal function in comparative physiology that now extend to imaging flow around moving animals and the calculation of fluid forces produced by animals moving in fluids. In particular, the technique of digital particle image velocimetry (DPIV) has revolutionized our ability to understand how moving animals generate fluid forces and propel themselves through air and water. DPIV algorithms generate a matrix of velocity vectors through the use of image cross-correlation, which can then be used to calculate the force exerted on the fluid as well as locomotor work and power. DPIV algorithms can also be applied to images of moving animals to calculate the velocity of different regions of the moving animal, providing a much more detailed picture of animal motion than can traditional digitizing methods. Although three-dimensional measurement of animal motion is now routine, in the near future model-based kinematic reconstructions and volumetric analyses of animal-generated fluid flow patterns will provide the next step in imaging animal biomechanics and physiology.

An overview of a Lagrangian method for analysis of animal wake dynamics

The fluid dynamic analysis of animal wakes is becoming increasingly popular in studies of animal swimming and flying, due in part to the development of quantitative flow visualization techniques such as digital particle imaging velocimetry (DPIV). In most studies, quasi-steady flow is assumed and the flow analysis is based on velocity and/or vorticity fields measured at a single time instant during the stroke cycle. The assumption of quasi-steady flow leads to neglect of unsteady (time-dependent) wake vortex added-mass effects,which can contribute significantly to the instantaneous locomotive forces. In this paper we review a Lagrangian approach recently introduced to determine unsteady wake vortex structure by tracking the trajectories of individual fluid particles in the flow, rather than by analyzing the velocity/vorticity fields at fixed locations and single instants in time as in the Eulerian perspective. Once the momentum of the wake vortex and its added mass are determined, the corresponding unsteady locomotive forces can be quantified. Unlike previous studies that estimated the time-averaged forces over the stroke cycle, this approach enables study of how instantaneous locomotive forces evolve over time. The utility of this method for analyses of DPIV velocity measurements is explored, with the goal of demonstrating its applicability to data that are typically available to investigators studying animal swimming and flying. The methods are equally applicable to computational fluid dynamics studies where velocity field calculations are available.

Routine turning maneuvers of koi carpCyprinus carpio koi:effects of turning rate on kinematics and hydrodynamics

Spontaneous swimming behaviors of koi carp Cyprinus carpio koiwere recorded using a video tracking system. Routine single-beat turns were selected from the recorded image sequences for kinematic and hydrodynamic analysis. As with C-starts, the turns can be divided into two stages (stage 1 and stage 2), based on kinematics. Stage 1 involves a bend to one side forming a C-shaped curve in the body, while stage 2 corresponds to the return flip of the body and tail. The turning angle in stage 1 accounts for the greatest portion of the total turning angle and the mean turning rate in stage 1 reflects the intensity of turn. The effects of the turning rate in stage 1 on both kinematics and hydrodynamics were examined. The duration of stage 1 remained relatively stable over a nearly tenfold change in turning rate. Consequently, the turning angle is dominated by the turning rate in stage 1. The turning radius is not related to the swimming speed. Moreover, except in very fast turns, the turning radius is also not affected by the turning rate. The angle between the side jet and the carp's initial orientation of a turn does not change substantially with the turning rate, and it is always close to 90° (94.2±3.1°, N=41), so the orientation of the side jet in the forthcoming turn can be predicted. The angle between the jet and the line joining the center of mass of the carp and the trailing edge of the tail (mean value in stage 1) is also always close to 90°(95.3±1.3°, N=41). It is helpful for the carp to maximize the torque so as to improve the turning efficiency. In stage 1, the impulsive moment obtained from the beat of the body and tail and the mean angular momentum of the carp show an agreement in magnitude. Two types of flow patterns in the wake of routine single-beat turns are revealed. The difference between the two types of wakes is in whether or not a vortex ring and a thrust jet are generated in stage 2. The recoil speed of the tail, the bending amplitude of the turn, and the angle of attack of the tail are three probable factors influencing the flow patterns in stage 2.

Non-invasive measurement of instantaneous forces during aquatic locomotion: a case study of the bluegill sunfish pectoral fin

Swimming and flying animals generate unsteady locomotive forces by delivering net momentum into the fluid wake. Hence, swimming and flying forces can be quantified by measuring the momentum of animal wakes. A recently developed model provides an approach to empirically deduce swimming and flying forces based on the measurement of velocity and vortex added-mass in the animal wake. The model is contingent on the identification of the vortex boundary in the wake. This paper demonstrates the application of that method to a case study quantifying the instantaneous locomotive forces generated by the pectoral fins of the bluegill sunfish (Lepomis macrochirusRafinesque), measured using digital particle image velocimetry (DPIV). The finite-time Lyapunov exponent (FTLE) field calculated from the DPIV data was used to determine the wake vortex boundary, according to recently developed fluid dynamics theory. Momentum of the vortex wake and its added-mass were determined and the corresponding instantaneous locomotive forces were quantified at discrete time points during the fin stroke. The instantaneous forces estimated in this study agree in magnitude with the time-averaged forces quantified for the pectoral fin of the same species swimming in similar conditions and are consistent with the observed global motion of the animals. A key result of this study is its suggestion that the dynamical effect of the vortex wake on locomotion is to replace the real animal fin with an `effective appendage', whose geometry is dictated by the FTLE field and whose interaction with the surrounding fluid is wholly dictated by inviscid concepts from potential flow theory. Benefits and limitations of this new framework for non-invasive instantaneous force measurement are discussed, and its application to comparative biomechanics and engineering studies is suggested.

Hydrodynamic function of dorsal and anal fins in brook trout(Salvelinus fontinalis)

Recent kinematic and hydrodynamic studies on fish median fins have shown that dorsal fins actively produce jets with large lateral forces. Because of the location of dorsal fins above the fish's rolling axis, these lateral forces, if unchecked, would cause fish to roll. In this paper we examine the hydrodynamics of trout anal fin function and hypothesize that anal fins,located below the fish's rolling axis, produce similar jets to the dorsal fin and help balance rolling torques during swimming. We simultaneously quantify the wake generated by dorsal and anal fins in brook trout by swimming fish in two horizontal light sheets filmed by two synchronized high speed cameras during steady swimming and manoeuvring. Six major conclusions emerge from these experiments.

First, anal fins produce lateral jets to the same side as dorsal fins,confirming the hypothesis that anal fins produce fluid jets that balance those produced by dorsal fins. Second, in contrast to previous work on sunfish,neither dorsal nor anal fins produce significant thrust during steady swimming; flow leaves the dorsal and anal fins in the form of a shear layer that rolls up into vortices similar to those seen in steady swimming of eels. Third, dorsal and anal fin lateral jets are more coincident in time than would be predicted from simple kinematic expectations; shape, heave and pitch differences between fins, and incident flow conditions may account for the differences in timing of jet shedding. Fourth, relative force and torque magnitudes of the anal fin are larger than those of the dorsal fin; force differences may be due primarily to a larger span and a more squarely shaped trailing edge of the anal fin compared to the dorsal fin; torque differences are also strongly influenced by the location of each fin relative to the fish's centre of mass. Fifth, flow is actively modified by dorsal and anal fins resulting in complex flow patterns surrounding the caudal fin. The caudal fin does not encounter free-stream flow, but rather moves through incident flow greatly altered by the action of dorsal and anal fins. Sixth, trout anal fin function differs from dorsal fin function; although dorsal and anal fins appear to cooperate functionally, there are complex interactions between other fins and free stream perturbations that require independent dorsal and anal fin motion and torque production to maintain control of body position.

Feeding, fins and braking maneuvers: locomotion during prey capture in centrarchid fishes

Locomotion is an integral aspect of the prey capture strategy of almost every predatory animal. For fishes that employ suction to draw prey into their mouths, locomotor movements are vital for the correct positioning of the mouth relative to the prey item. Despite this, little is known regarding the relationships between locomotor movements and prey capture. To gain insights into how fishes move during prey capture and the mechanisms underlying deceleration during prey capture, I measured the fin and body movements of largemouth bass, Micropterus salmoides, and bluegill sunfish, Lepomis macrochirus. Using a high-speed video camera (500 frames s-1), I captured locomotor and feeding movements in lateral and ventral (via a mirror) view. Largemouth bass swam considerably faster than bluegill during the approach to the prey item, and both species decelerated substantially following prey capture. The mean magnitude of deceleration was significantly higher in largemouth bass (-1089 cm s-2) than bluegill (-235 cm s-2), and the timing of maximum deceleration was much later for largemouth bass (30.3 ms after maximum gape) than bluegill (6.7 ms after maximum gape). Both species employed their pectoral, anal and caudal fins in order to decelerate during prey capture. However, largemouth bass protracted their pectoral fins more and faster,likely contributing to the greater magnitude of deceleration in the species. The primary mechanism for increased deceleration was an increase in approach speed. The drag forces experienced by the fins and body are proportional to the velocity of the flow squared. Thus, the braking forces exerted by fins,without any change in kinematics, will increase exponentially with small increases in swimming speed, perhaps allowing these fishes to achieve higher braking forces at higher swimming speeds without altering body or fin kinematics. This result can likely be extended to other maneuvers such as turning.

Locomotion with flexible propulsors: I. Experimental analysis of pectoral fin swimming in sunfish

A full understanding of the mechanics of locomotion can be achieved by incorporating descriptions of (1) three-dimensional kinematics of propulsor movement, (2) material properties of the propulsor, (3) power input and control and (4) the fluid dynamics effects of propulsor motion into (5) a three-dimensional computational framework that models the complexity of propulsors that deform and change area. In addition, robotic models would allow for further experimental investigation of changes to propulsor design and for testing of hypothesized relationships between movement and force production. Such a comprehensive suite of data is not yet available for any flexible propulsor. In this paper, we summarize our research program with the goal of producing a comprehensive data set for each of the five components noted above through a study of pectoral fin locomotion in one species of fish: the bluegill sunfish Lepomis macrochirus. Many fish use pectoral fins exclusively for locomotion, and pectoral fins in most fish are integral to generating force during maneuvering. Pectoral fins are complex structures composed of jointed bony supports that are under active control via pectoral fin musculature. During propulsion in sunfish, the fin deforms considerably, has two leading edges, and sunfish can rotate the whole fin or just control individual sections to vector thrust. Fin material properties vary along the length of fin rays and among rays. Experimental fluid dynamic analysis of sunfish pectoral fin locomotion reveals that the fin generates thrust throughout the fin beat cycle, and that the upper and lower edges each produce distinct simultaneous leading edge vortices. The following companion paper provides data on the computational approach taken to understand locomotion using flexible pectoral fins.

Locomotion with flexible propulsors: II. Computational modeling of pectoral fin swimming in sunfish

This paper describes a computational fluid dynamics (CFD) based investigation of the pectoral fin hydrodynamics of a bluegill sunfish. The pectoral fin of this fish undergoes significant shape-change during its abduction-adduction cycle and the effect of this deformation on the thrust performance remains far from understood. The current study is part of a combined experimental-numerical approach wherein the numerical simulations are being used to examine features and issues that are not easily amenable to the experiments. These numerical simulations are highly challenging and we briefly describe the computational methodology that has been developed to handle such flows. Finally, we describe some of the key computational results including wake vortex topologies and hydrodynamics forces.

Locomotor function of the dorsal fin in rainbow trout: kinematic patterns and hydrodynamic forces

In this study, we examine the kinematics and hydrodynamics of the soft dorsal fin in a representative basal teleost, the rainbow trout (Oncorhynchus mykiss), during steady rectilinear locomotion at 0.5-2.0 body lengths (L) s(-1) and during maneuvering. During steady swimming, dorsal fin height and sweep amplitude decrease with increasing speed. The dorsal fin wake, as viewed within a horizontal plane, consists of paired vortices on each side of the body (0.5 L s(-1)) or nearly linearly arrayed vortex centers above the body (1.0 L s(-1)) with central jet flows directed predominately laterally (lateral:thrust force ratio = 5-6). At 2.0 L s(-1), the dorsal fin is no longer recruited to add momentum to the wake. This pattern of decreasing involvement of the trout dorsal fin in thrust production with increasing speed contrasts with the results of our previous study of the soft dorsal fin of sunfish (Lepomis), which is hydrodynamically inactive at low speed and sheds a propulsive vortex wake at higher speed. Yawing maneuvers by trout involve unilateral production of a single vortex ring by the dorsal fin with a strong jet flow oriented almost directly laterally. During steady swimming, interception by the tail of the dorsal fin's vortical wake and the adipose fin's non-vortical (drag) wake is hypothesized as a mechanism for enhancing tail thrust. This study provides the first experimental evidence that the plesiomorphic soft dorsal fin of ray-finned fishes acts as an ancillary force generator during axial locomotion. We suggest that the distinction often made between median and paired fin (MPF) propulsion and body and caudal fin (BCF) propulsion in fishes obscures the important role of multiple propulsors acting in a coordinated fashion. Using a combination of anterior median fin oscillation and axial undulation, without continuous paired fin excursions, trout employ an ;M-BCF' gait during steady swimming. The primarily lateral orientation of dorsal fin force in trout induces corresponding roll and yaw moments, which must be countered by forces from the caudal, anal and paired fins. Locomotion in trout therefore involves the simultaneous active use of multiple fins, presumably to maintain body stability in the face of environmental perturbations.

Dorsal and anal fin function in bluegill sunfishLepomis macrochirus: three-dimensional kinematics during propulsion and maneuvering

Dorsal and anal fins are median fins located above and below the centre of mass of fishes, each having a moment arm relative to the longitudinal axis. Understanding the kinematics of dorsal and anal fins may elucidate how these fins are used in concert to maintain and change fish body position and yet little is known about the functions of these fins. Using three synchronized high-speed cameras (500 frames s–1) we studied the three-dimensional kinematics of dorsal and anal fins during steady swimming(0.5–2.5 TL s–1, where TL=total length) and during slow speed maneuvers (0.5 TLs–1). By digitizing points along every other fin ray in the soft-rayed portion of the fins we were able to determine not only the movement of the fin surface but also the curvature of individual fin rays and the resulting fin surface shape. We found that dorsal and anal fins begin oscillating, in phase, at steady swimming speeds above 1.0 TLs–1 and that maximum lateral displacement of the trailing edge of the fins as well as fin area increase with increasing steady swimming speed. Differences in area, lateral displacement and moment arm between the dorsal and anal fin suggest that dorsal and anal fins produce balancing torques during steady swimming. During maneuvers, fin area is maximized and mean lateral excursion of both fins is greater than during steady swimming,with large variation among maneuvers. Fin surface shape changes dramatically during maneuvers. At any given point in time the spanwise (base to tip)curvature along fin rays can differ between adjacent rays, suggesting that fish have a high level of control over fin surface shape. Also, during maneuvers the whole surface of both dorsal and anal fins can be bent without individual fin rays exhibiting significant curvature.

The morphology, behavior, and biomechanics of swimming in ascidian larvae

Ascidian larvae use a simple set of locomotor behaviors during dispersal and settlement. The swimming performance and the ability of an ascidian larva to orient within its environment depend on the biomechanics of its rudimentary locomotor morphology. The undulatory motion of the tail generates fluid forces that propel the body with a rate and direction determined by body mass and its spatial distribution. Differences in morphology and behavior among species influence these dynamics and create differences in swimming performance. Furthermore, the ability of a larva to orient within its environment depends on its ability to coordinate the motion of its body with respect to perceived sensory cues. Research on the swimming of ascidian larvae demonstrates the biomechanical basis of tactic orientation and interspecific differences in performance in a large and diverse group of animals.

'Optimal' vortex rings and aquatic propulsion mechanisms

Fishes swim by flapping their tail and other fins. Other sea creatures, such as squid and salps, eject fluid intermittently as a jet. We discuss the fluid mechanics behind these propulsion mechanisms and show that these animals produce optimal vortex rings, which give the maximum thrust for a given energy input. We show that fishes optimize both their steady swimming efficiency and their ability to accelerate and turn by producing an individual optimal ring with each flap of the tail or fin. Salps produce vortex rings directly by ejecting a volume of fluid through a rear orifice, and these are also optimal. An important implication of this paper is that the repetition of vortex production is not necessary for an individual vortex to have the 'optimal' characteristics.