Stability versus Maneuvering: Challenges for Stability during Swimming by Fishes

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.

The environment to the rescue: can physics help predict predator-prey interactions?

Understanding the factors that determine the occurrence and strength of ecological interactions under specific abiotic and biotic conditions is fundamental since many aspects of ecological community stability and ecosystem functioning depend on patterns of interactions among species. Current approaches to mapping food webs are mostly based on traits, expert knowledge, experiments, and/or statistical inference. However, they do not offer clear mechanisms explaining how trophic interactions are affected by the interplay between organism characteristics and aspects of the physical environment, such as temperature, light intensity or viscosity. Hence, they cannot yet predict accurately how local food webs will respond to anthropogenic pressures, notably to climate change and species invasions. Herein, we propose a framework that synthesises recent developments in food-web theory, integrating body size and metabolism with the physical properties of ecosystems. We advocate for combination of the movement paradigm with a modular definition of the predation sequence, because movement is central to predator-prey interactions, and a generic, modular model is needed to describe all the possible variation in predator-prey interactions. Pending sufficient empirical and theoretical knowledge, our framework will help predict the food-web impacts of well-studied physical factors, such as temperature and oxygen availability, as well as less commonly considered variables such as wind, turbidity or electrical conductivity. An improved predictive capability will facilitate a better understanding of ecosystem responses to a changing world.

Multisensory navigational strategies of hatchling fish for dispersal

Animals influence how they disperse in the environment by sensing local cues and adapting how they move. However, controlling dispersal can present a particular challenge early in life when animals tend to be more limited in their capacities to sense and move. To what extent and by what mechanisms can newly hatched fish control how they disperse? Here, we reveal hatchling sensorimotor mechanisms for controlling dispersal by combining swim tracking and precise sensory manipulations of a model species, zebrafish. In controlled laboratory experiments, if we physically constrained hatchlings or blocked sensations of motion through vision and the lateral line, hatchlings responded by elevating their buoyancy and passively moving with faster surface currents. Complementarily, in stagnant water, hatchlings covered more ground using hyperstable swimming, strongly orienting based on graviception. Using experimentally calibrated hydrodynamic simulations, we show that these hatchling behaviors nearly tripled diffusivity and made dispersal robust to local conditions, suggesting this multisensory strategy may provide important advantages for early life in a variable environment.

How Free Swimming Fosters the Locomotion of a Purely Oscillating Fish-like Body

The recoil motions in free swimming, given by lateral and angular rigid motions due to the interaction with the surrounding water, are of great importance for a correct evaluation of both the forward locomotion speed and efficiency of a fish-like body. Their contribution is essential for calculating the actual movements of the body rear end whose prominent influence on the generation of the proper body deformation was established a long time ago. In particular, the recoil motions are found here to promote a dramatic improvement of the performance when damaged fishes, namely for a partial functionality of the tail or even for its complete loss, are considered. In fact, the body deformation, which turns out to become oscillating and symmetric in the extreme case, is shown to recover in the water frame a kind of undulation leading to a certain locomotion speed though at the expense of a large energy consumption. There has been a deep interest in the subject since the infancy of swimming studies, and a revival has recently arisen for biomimetic applications to robotic fish-like bodies. We intend here to apply a theoretical impulse model to the oscillating fish in free swimming as a suitable test case to strengthen our belief in the beneficial effects of the recoil motions. At the same time, we intend to exploit the linearity of the model to detect from the numerical simulations the intrinsic physical reasons related to added mass and vorticity release behind the experimental observations.

Locomotion control during curb descent: Bilateral ground reaction variables covary consistently during the double support phase regardless of future foot placement constraints

During community ambulation, anticipatory adaptations in gait are key for navigating built, populated and natural environments. It has been argued that some instability in gait can be functionally beneficial in situations demanding high maneuverability, and while the mechanisms utilized to maintain locomotor balance are well understood, relatively less is known about how the control of gait stability changes to facilitate upcoming maneuvers in challenging environments. The double support phase may be important in this regard; since both feet can push off the ground simultaneously, there is greater control authority over the body's movement during this phase. Our goal was to identify how this control authority is exploited to prepare for upcoming maneuvers in challenging environments. We used synergy indices to quantify the degree of coordination between the ground reaction forces and moments under the two feet for stabilizing the resultant force and moment on the body during the double support phase of curb descent. In contrast to our expectations, we observed that the kinetic synergy indices during curb descent were minimally influenced by expected foot targeting maneuvers for the subsequent step. Only the resultant moment in the frontal plane showed reduced stability when targeting was required, but the synergy index was still high, indicating that the resultant moment was stable. Furthermore, the synergy indices indicated that the main function of the ground reaction variables is to maintain stability of whole-body rotations during double support, and this prerogative was minimally influenced by the subsequent foot targeting tasks, likely because the cost of losing balance while descending a curb would be higher than the cost of mis-stepping on a visual target. Our work demonstrates the salience of stabilizing body rotations during curb negotiation and improves our understanding of locomotor control in challenging environments.

Keels of boxfish carapaces strongly improve stabilization against roll

Boxfish (Ostraciidae) have peculiar body shapes, with conspicuous keels formed by their bony carapaces. Previous studies have proposed various hydrodynamic roles for these keels, including reducing drag during swimming, contributing to passive stabilization of the swimming course, or providing resistance against roll rotations. Here, we tested these hypotheses using computational fluid dynamics simulations of five species of Ostraciidae with a range of carapace shapes. The hydrodynamic performance of the original carapace surface models, obtained from laser scanning of museum specimens, was compared with models where the keels had been digitally reduced. The original carapaces showed no reduced drag or increased passive stability against pitch and yaw compared to the reduced-keel carapaces. However, consistently for all studied species, a strong increase in roll drag and roll-added mass was observed for the original carapaces compared to the reduced-keel carapaces, despite the relatively small differences in keel height. In particular, the damping of roll movement by resistive drag torques increased considerably by the presence of keels. Our results suggest that the shape of the boxfish carapace is important in enabling the observed roll-free forward swimming of boxfish and may facilitate the control of manoeuvres.

How much biology is in the product? Role and relevance of biological evolution and function for bio-inspired design

Bio-inspired design (BID) means the concept of transferring functional principles from biology to technology. The core idea driving BID-related work is that evolution has shaped functional attributes, which are termed “adaptations” in biology, to a high functional performance by relentless selective pressure. For current methods and tools, such as data bases, it is implicitly supposed that the considered biological models are adaptations and their functions already clarified. Often, however, the identification of adaptations and their functional features is a difficult task which is not yet accomplished for numerous biological structures, as happens to be the case also for various organismic features from which successful BID developments were derived. This appears to question the relevance of the much stressed importance of evolution for BID. While it is obviously possible to derive an attractive technical principle from an observed biological effect without knowing its original functionality, this kind of BID (“analog BID”) has no further ties to biology. In contrast, a BID based on an adaptation and its function (“homolog BID”) is deeply embedded in biology. It is suggested that a serious and honest clarification of the functional background of a biological structure is an essential first step in devising a BID project, to recognize possible problems and pitfalls as well as to evaluate the need for further biological analysis.

Changes in rays' swimming stability due to the phase difference between left and right pectoral fin movements

Swimming motions of rays that swim using undulation locomotion are not always symmetrical; there may be a phase difference between the left and right pectoral fins. However, few studies on the swimming of rays have mentioned left and right pectoral fin movements. Moreover, the effects of movements of the left and right pectoral fins on swimming have not been clarified. This paper describes a computational study of phase differences of pectoral fin movements in the swimming of rays with the validity of fluid analysis methods. The movement and shape of the ray were made based on previous biological research and pictures. An overset grid was used to reproduce the ray’s complex motions. The analysis was performed under four phase difference conditions: 0 \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$T$$\end{document}T (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$T$$\end{document}T is the period), 0.25 \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$T$$\end{document}T, 0.5 \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$T$$\end{document}T, and 0.75 \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$T$$\end{document}T. The results show that a phase difference between the left and right pectoral fin movements affects swimming stability and maneuverability but not propulsive efficiency. We suggest that the phase difference in pectoral fin movements is essential for the swimming of rays, and rays adjust the phase difference between the movement of the left and right pectoral fins to suit their purpose.

Streamwise and lateral maneuvers of a fish-inspired hydrofoil

Fish are highly maneuverable compared to human-made underwater vehicles. Maneuvers are inherently transient, so they are often studied via observations of fish and fish-like robots, where their dynamics cannot be recorded directly. To study maneuvers in isolation, we designed a new kind of wireless carriage whose air bushings allow a hydrofoil to maneuver semi-autonomously in a water channel. We show that modulating the hydrofoil's frequency, amplitude, pitch bias, and stroke speed ratio (pitching speed of left vs right stroke) produces streamwise and lateral maneuvers with mixed effectiveness. Modulating pitch bias, for example, produces quasi-steady lateral maneuvers with classic reverse von Kármán wakes, whereas modulating the stroke speed ratio produces sudden yaw torques and vortex pairs like those observed behind turning zebrafish. Our findings provide a new framework for considering in-plane maneuvers and streamwise/lateral trajectory corrections in fish and fish-inspired robots.

The Effect of Locomotion Mode on Body Shape Evolution in Teleost Fishes

Teleost fishes vary in their reliance on median and paired fins (MPF) or undulation of the body (BCF) to generate thrust during straight-line, steady swimming. Previous work indicates that swimming mode is associated with different body shapes, though this has never been empirically demonstrated across the diversity of fishes. As the body does not play as active a mechanical role in steady swimming by MPF swimmers, this may relax constraints and spur higher rates of body shape diversification. We test these predictions by measuring the impact of the dominant steady swimming mode on the evolution of body shape across 2295 marine teleost fishes. Aligning with historical expectations, BCF swimmers exhibit a more elongate, slender body shape, while MPF propulsion is associated with deeper and wider body shapes. However, in contrast to expectations, we find that BCF propulsion is associated with higher morphological diversity and greater variance around trait optima. This surprising result is consistent with the interpretation that stronger functional trade-offs stimulate phenotypic evolution, rather than constrain it.

The ZE-Tunnel: An Affordable, Easy-to-Assemble, and User-Friendly Benchtop Zebrafish Swim Tunnel

The popularity of zebrafish in both basic biological and biomedical research has led to an increased need for understanding their behavior. Locomotor behavior is an important outcome of different factors, such as specific genotypes or external stimuli that influence the nervous and musculoskeletal system. Locomotion can be studied by forced swimming in a swim tunnel, a device capable of generating a laminar water flow at different speeds in a chamber where zebrafish can be placed. However, commercially available swim tunnels are relatively expensive and in-house built systems are mostly presented without clear building instructions or proper validation procedures. In this study, we developed an alternative, cheap (<250 euro), and user-friendly, but customizable benchtop swim tunnel, called the "Zebrafish exercise-tunnel" (ZE-Tunnel). Detailed step-by-step instructions on how to construct the tunnel components, including the frame, mechanical, and electric components are given. The ZE-Tunnel was reliably used to exercise fish for prolonged periods and its performance was successfully validated by replicating previously published experiments on critical speed testing in zebrafish. Finally, implementation of behavioral video analysis using freely available motion-tracking software showed differences in swimming dynamics in the Chihuahua skeletal zebrafish mutant.

Hydrodynamic Analysis for the Morphing Median Fins of Tuna during Yaw Motions

Tuna can change the area and shape of the median fins, including the first dorsal, second dorsal, and anal fins. The morphing median fins have the ability of adjusting the hydrodynamic forces, thereby affecting the yaw mobility of tuna to a certain extent. In this paper, the hydrodynamic analysis of the median fins under different morphing states is carried out by the numerical method, so as to clarify the influence of the erected median fins on the yaw maneuvers. By comparing the two morphing states of erected and depressed, it can be concluded that the erected median fins can increase their own hydrodynamic forces during the yaw movement. However, the second dorsal and anal fins have limited influence on the yaw maneuverability, and they tend to maintain the stability of tuna. The first dorsal fin has more lift increment in the erection state, which can obviously affect the hydrodynamic performance of tuna. Moreover, as the median fins are erected, the hydrodynamic forces of the tuna's body increase synchronously due to the interaction between the body and the median fins, which is also very beneficial to the yaw motion. This study indicates that tuna can use the morphing median fins to adjust its mobility and stability, which provides a new idea for the design of robotic fish.

Aracaniform Swimming: A Proposed New Category of Swimming Mode in Bony Fishes (Teleostei: Tetraodontiformes: Aracanidae)

The deepwater boxfishes of the family Aracanidae are the phylogenetic sister group of the shallow-water, generally more tropical boxfishes of the family Ostraciidae. Both families are among the most derived groups of teleosts. All members of both families have armored bodies, the forward 70% of which are enclosed in rigid bony boxes (carapaces). There is substantial intragroup variation in both groups in body shapes, sizes, and ornamentation of the carapaces. Swimming-related morphology, swimming mode, biomechanics, kinematics, and hydrodynamics have been studied in detail in multiple species of the ostraciids. Ostraciids are all relatively high-performance median and paired fin swimmers. They are highly maneuverable. They swim rectilinearly with substantial dynamic stability and efficiency. Aracanids have not been previously studied in these respects. This article describes swimming-related aspects of morphology, swimming modes, biomechanics, and kinematics in two south Australian species (striped cowfish and ornate cowfish) that are possibly representative of the entire group. These species differ morphologically in many respects, both from each other and from ostraciids. There are differences in numbers, sizes, and placements of keels on carapaces. The most important differences from ostraciids are openings in the posterior edges of the carapaces behind the dorsal and anal fins. The bases of those fins in ostraciids are enclosed in bone. The openings in aracanids free the fins and tail to move. As a result, aracanids are body and caudal fin swimmers. Their overall swimming performances are less stable, efficient, and effective. We propose establishing a new category of swimming mode for bony fishes called "aracaniform swimming."

Biomimetic design of dorsal fins for AUVs to enhance maneuverability

We demonstrate that shape-changing or morphing fins provide a new paradigm for improving the ability of vehicles to maneuver and move rapidly underwater. An ingenuous solution is employed by fish to accommodate both the need for stability of locomotion and the ability to perform tight maneuvers: Retractable fins can alter the stability properties of a vehicle to suit their particular goals. Tunas, for example, are large fish that are fast swimmers and yet they need rapid turning agility to track the smaller fish they pursue; they have perfected the use of their dorsal and ventral fins to ensure stability when retracted and rapid turning when erected. Although fish employ unsteady propulsors rather than propellers, we show that engineering rigid-hull underwater vehicles can also exploit similar solutions. We explore the basic flow mechanisms and design considerations of employing morphing fins to alter the stability and maneuvering qualities of vehicles and apply unsteady forces and moments under active control. We also show results from maneuvering simulations and experiments on a model of an underwater vehicle.

Hierarchical control of locomotion by distinct types of spinal V2a interneurons in zebrafish

In all vertebrates, excitatory spinal interneurons execute dynamic adjustments in the timing and amplitude of locomotor movements. Currently, it is unclear whether interneurons responsible for timing control are distinct from those involved in amplitude control. Here, we show that in larval zebrafish, molecularly, morphologically and electrophysiologically distinct types of V2a neurons exhibit complementary patterns of connectivity. Stronger higher-order connections from type I neurons to other excitatory V2a and inhibitory V0d interneurons provide timing control, while stronger last-order connections from type II neurons to motor neurons provide amplitude control. Thus, timing and amplitude are coordinated by distinct interneurons distinguished not by their occupation of hierarchically-arranged anatomical layers, but rather by differences in the reliability and probability of higher-order and last-order connections that ultimately form a single anatomical layer. These findings contribute to our understanding of the origins of timing and amplitude control in the spinal cord.

V2a excitatory interneurons in the spinal cord are important for coordinating locomotion. Here the authors describe two types of V2a neuron with differences in higher order and lower order connectivity in larval zebrafish.

Comparing Models of Lateral Station-Keeping for Pitching Hydrofoils

Fish must maneuver laterally to maintain their position in schools or near solid boundaries. Unsteady hydrodynamic models, such as the Theodorsen and Garrick models, predict forces on tethered oscillating hydrofoils aligned with the incoming flow. How well these models predict forces when bio-inspired hydrofoils are free to move laterally or when angled relative to the incoming flow is unclear. We tested the ability of five linear models to predict a small lateral adjustment made by a hydrofoil undergoing biased pitch oscillations. We compared the models to water channel tests in which air bushings gave a rigid pitching hydrofoil lateral freedom. What we found is that even with no fitted coefficients, linear models predict some features of the lateral response, particularly high frequency features like the amplitude and phase of passive heave oscillations. To predict low frequency features of the response, such as overshoot and settling time, we needed a semiempirical model based on tethered force measurements. Our results suggest that fish and fish-inspired vehicles could use linear models for some aspects of lateral station-keeping, but would need nonlinear or semiempirical wake models for more advanced maneuvers.

Body and Pectoral Fin Kinematics During Routine Yaw Turning in Bonnethead Sharks (Sphyrna tiburo)

Maneuvering is a crucial locomotor strategy among aquatic vertebrates, common in routine swimming, feeding, and escape responses. Combinations of whole body and fin movements generate an imbalance of forces resulting in deviation from an initial path. Sharks have elongate bodies that bend substantially and, in combination with pectoral fin rotation, play a role in yaw (horizontal) turning, but previous studies focus primarily on maximal turning performance rather than routine maneuvers. Routine maneuvering is largely understudied in fish swimming, despite observations that moderate maneuvering is much more common than the extreme behaviors commonly described in the literature. In this study, we target routine maneuvering in the bonnethead shark, Sphyrna tiburo. We use video reconstruction of moving morphology to describe three-dimensional pectoral fin rotation about three axes to compare to those previously described on yaw turning by the Pacific spiny dogfish. We quantify kinematic variables to understand the impacts of body and fin movements on routine turning performance. We also describe the anatomy of bonnethead pectoral fins and use muscle stimulation to confirm functional hypotheses about their role in actuating the fin. The turning performance metrics we describe for bonnethead sharks are comparable to other routine maneuvers described for the Pacific spiny dogfish and manta rays. These turns were substantially less agile and maneuverable than previously documented for other sharks, which we hypothesize results from the comparison of routine turning to maneuvering under stimulated conditions. We suggest that these results highlight the importance of considering routine maneuvering in future studies. Cinemática del Cuerpo y de las Aletas Pectorales Durante el giro en el eje Vertical en la Cabeza del Tiburón Pala (Sphyrna tiburo) (Body and Pectoral Fin Kinematics During Routine Yaw Turning in Bonnethead Sharks [Sphyrna tiburo]).

Differences in kinematic plasticity between freshwater turtle species underlie differences in swimming performance in response to varying flow conditions

The distribution and performance of aquatic vertebrates can be linked strongly to their ability to perform in variable conditions of flowing water. Performance in these variable conditions can be affected by both morphology and behaviour, and animals that experience more variable environments often show greater behavioural plasticity that improves performance in those environments. One common metric of performance is swimming stability, which can constitute a majority of the daily energy budget of swimming animals. We compared the body oscillations arising from recoil forces of the limbs of two species of freshwater turtles as they swam in different flow conditions: the lentic specialist Emydura subglobosa and the habitat generalist Chrysemys picta. We found that E. subglobosa experienced more limited oscillations in still water than C. picta, but that C. picta had a greater kinematic response to increased flow speed that might contribute to their improved performance in flowing water. These results provide insight into how secondarily aquatic tetrapods respond to the functional demands of variation in flow, helping to build understanding of the relationship between energetics, kinematics and performance of such lineages in different environments.

Morphology, Ecology, and Biogeography of Independent Origins of Cleaning Behavior Around the World

Members of an ecological guild may be anticipated to show morphological convergence, as similar functional demands exert similar selective pressures on phenotypes. Nature is rife with examples, however, where such taxa instead exhibit ‘incomplete’ convergence or even divergence. Incorporating factors such as character displacement by other guild members or variation in ecological specialization itself may therefore be necessary to gain a more complete understanding of what constrains or promotes diversity. Cleaning, a behavior in which species remove and consume ectoparasites from “clientele,” has been shown to exhibit variation in specialization and has evolved in a variety of marine habitats around the globe. To determine the extent to which specialization in this tropic strategy has affected phenotypic evolution, we examined the evolution of cleaning behavior in five marine fish families: Labridae, Gobiidae, Pomacanthidae, Pomacentridae, and Embiotocidae. We used a comparative framework to determine patterns of convergence and divergence in body shape and size across non-cleaning and cleaning members within these five clades. Highly specialized obligate cleaning, found in the Indo-Pacific and the Caribbean, evolved in the Labridae and Gobiidae at strikingly similar times. In these two regions, obligate cleaning evolves early, shows convergence on an elongate body shape, and is restricted to species of small body size. Facultative cleaning, shown either throughout ontogeny or predominately in the juvenile phase, exhibits a much more varied phenotype, especially in geographic regions where obligate cleaning occurs. Collectively, our results are consistent with varying extents of an ecological specialization constraining or spurring morphological evolution in recurrent ways across regions.

Energetics and behavior of coral reef fishes during oscillatory swimming in a simulated wave surge

Oxygen consumption rates were measured for coral reef fishes during swimming in a bidirectional, oscillatory pattern to simulate station-holding in wave-induced, shallow-water flows. For all species examined, increases in wave intensity, as simulated by increases in frequency and amplitude of oscillation, yielded increased metabolic rates and net costs of swimming (NCOS; swimming metabolic rate minus standard metabolic rate). Comparing species with different swimming modes, the caudal fin swimming Kuhlia spp. (Kuhliidae) and simultaneous pectoral-caudal fin swimming Amphiprion ocellaris (Pomacentridae) turned around to face the direction of swimming most of the time, whereas the median-paired fin (MPF) swimmers, the pectoral fin swimming Ctenochaetus strigosus (Acanthuridae) and dorsal-anal fin swimming Sufflamen bursa (Balistidae), more frequently swam in reverse for one half of the oscillation to avoid turning. Contrary to expectations, the body-caudal fin (BCF) swimming Kuhlia spp. had the lowest overall NCOS in the oscillatory swimming regime compared with the MPF swimmers. However, when examining the effect of increasing frequency of oscillation at similar average velocities, Ku hlia spp. showed a 24% increase in NCOS with a 50% increase in direction changes and accelerations. The two strict MPF swimmers had lower increases on average, suggestive of reduced added costs with increasing frequency of direction changes with this swimming mode. Further studies are needed on the costs of unsteady swimming to determine whether these differences can explain the observed prevalence of fishes using the MPF pectoral fin swimming mode in reef habitats exposed to high, wave-surge-induced water flows.

Three-dimensional movements of the pectoral fin during yaw turns in the Pacific spiny dogfish, Squalus suckleyi

Fish pectoral fins move in complex ways, acting as control surfaces to affect force balance during swimming and maneuvering. Though objectively less dynamic than their actinopterygian relatives, shark pectoral fins undergo complex conformational changes and movements during maneuvering. Asynchronous pectoral fin movement is documented during yaw turning in at least two shark species but the three-dimensional (3D) rotation of the fin about the body axes is unknown. We quantify the 3D actuation of the pectoral fin base relative to the body axes. We hypothesized that Pacific spiny dogfish rotate pectoral fins with three degrees of freedom relative to the body during volitional turning. The pectoral fin on the inside of the turn is consistently protracted, supinated and depressed. Additionally, turning angular velocity increased with increasing fin rotation. Estimated drag on the fin increased and the shark decelerated during turning. Based on these findings, we propose that Pacific spiny dogfish uses drag-based turning during volitional swimming. Post-mortem muscle stimulation revealed depression, protraction and supination of the pectoral fin through stimulation of the ventral and cranial pterygoideus muscles. These data confirm functional hypotheses about pectoral fin musculature and suggest that Pacific spiny dogfish actively rotate pectoral fins to facilitate drag-based turning.This article has an associated First Person interview with the first author of the paper.

Variation in Morphology and Kinematics Underlies Variation in Swimming Stability and Turning Performance in Freshwater Turtles

Among swimming animals, stable body designs often sacrifice performance in turning, and high turning performance may entail costs in stability. However, some rigid-bodied animals appear capable of both high stability and turning performance during swimming by propelling themselves with independently controlled structures that generate mutually opposing forces. Because such species have traditionally been studied in isolation, little is known about how variation within rigid-bodied designs might influence swimming performance. Turtles are a lineage of rigid-bodied animals, in which most species use contralateral limbs and mutually opposing forces to swim. We tested the stability and turning performance of two species of turtles, the pleurodire Emydura subglobosa and the cryptodire Chrysemys picta. Emydura subglobosa exhibited both greater stability and turning performance than C. picta, potentially through the use of subequally-sized (and larger) propulsive structures, faster limb movements, and decreased limb excursions. These data show how, within a given body design, combinations of different traits can serve as mechanisms to improve aspects of performance with competing functional demands.

The impact of keels and tails on turtle swimming performance and their potential as models for biomimetic design

Stability and turning performance are two key metrics of locomotor performance in animals, and performance in both of these metrics can be improved through a variety of morphological structures. Aquatic vehicles are often designed with keels and rudders to improve their stability and turning performance, but how keels and rudders function in rigid-bodied animals is less understood. Aquatic turtles are a lineage of rigid-bodied animals that have the potential to function similarly to engineered vehicles, and also might make use of keels and rudders to improve their stability and turning performance. To test these possibilities, we trained turtles to follow a mechanically controlled prey stimulus under three sets of conditions: with no structural modifications, with different sized and shaped keels, and with restricted tail use. We predicted that keels in turtles would function similarly to those in aquatic vehicles to reduce oscillations, and that turtles would use the tail like a rudder to reduce oscillations and improve turning performance. We found that the keel designs we tested did not reduce oscillations in turtles, but that the tail was used similarly to a rudder, with benefits to both the magnitude of oscillations they experienced and turning performance. These data show how variation in the accessory structures of rigid-bodied animals can impact swimming performance, and suggest that such variation among turtles could serve as a biomimetic model in designing aquatic vehicles that are stable as well as maneuverable and agile.

Development of vestibular behaviors in zebrafish

Most animals orient their bodies with respect to gravity to facilitate locomotion and perception. The neural circuits responsible for these orienting movements have long served as a model to address fundamental questions in systems neuroscience. Though postural control is vital, we know little about development of either balance reflexes or the neural circuitry that produces them. Recent work in a genetically and optically accessible vertebrate, the larval zebrafish, has begun to reveal the mechanisms by which such vestibular behaviors and circuits come to function. Here we highlight recent work that leverages the particular advantages of the larval zebrafish to illuminate mechanisms of postural development, the role of sensation for balance circuit development, and the organization of developing vestibular circuits. Further, we frame open questions regarding the developmental mechanisms for functional circuit assembly and maturation where studying the zebrafish vestibular system is likely to open new frontiers.

Healthy individuals are more maneuverable when walking slower while navigating a virtual obstacle course

INTRODUCTION

Maintaining stability, especially in the mediolateral direction, is important for successful walking. Navigating in the community, however, may require people to reduce stability to make quick lateral transitions, creating a tradeoff between stability and maneuverability. Walking slower can improve stability during steady state walking, but there remains a need to better understand how walking speed influences maneuverability. This study investigated how walking at different speeds influenced how individuals modulate both stability and maneuverability in a virtual obstacle course.

METHODS

Fifteen healthy adults walked on a treadmill in a virtual environment for 6 trials each at typical and slower speed. Participants made repeated transitions between virtual sets of arches displayed in any of 4 lanes. Participants were instructed to walk under the arches and hit as few arches as possible. To quantify stability, mean step width and mean lateral margin of stability (Mean MOS) were calculated and averaged for ipsilateral and contralateral steps. To quantify maneuverability, the number of arches hit when entering or exiting each arch set was calculated and averaged for each condition.

RESULTS

Participants exhibited high levels of variability in their stepping patterns. Mean MOS and mean step width were significantly greater for the typical speed than slower speed for the ipsilateral steps (p < 0.001). Participants hit more arches during the typical speed than during the slow speed (p = 0.039).

CONCLUSION

When walking at the slower speed, healthy individuals exhibited decreased stability of ipsilateral steps, but increased maneuverability and better transition performance.

Hydraulic control of tuna fins: A role for the lymphatic system in vertebrate locomotion

The lymphatic system in teleost fish has genetic and developmental origins similar to those of the mammalian lymphatic system, which is involved in immune response and fluid homeostasis. Here, we show that the lymphatic system of tunas functions in swimming hydrodynamics. Specifically, a musculo-vascular complex, consisting of fin muscles, bones, and lymphatic vessels, is involved in the hydraulic control of median fins. This specialization of the lymphatic system is associated with fish in the family Scombridae and may have evolved in response to the demand for swimming and maneuvering control in these high-performance species.

A pressure-based force and torque prediction technique for the study of fish-like swimming

Many outstanding questions about the evolution and function of fish morphology are linked to swimming dynamics, and a detailed knowledge of time-varying forces and torques along the animal’s body is a key component in answering many of these questions. Yet, quantifying these forces and torques experimentally represents a major challenge that to date prevents a full understanding of fish-like swimming. Here, we develop a method for obtaining these force and torque data non-invasively using standard 2D digital particle image velocimetry in conjunction with a pressure field algorithm. We use a mechanical flapping foil apparatus to model fish-like swimming and measure forces and torques directly with a load cell, and compare these measured values to those estimated simultaneously using our pressure-based approach. We demonstrate that, when out-of-plane flows are relatively small compared to the planar flow, and when pressure effects sufficiently dominate shear effects, this technique is able to accurately reproduce the shape, magnitude, and timing of locomotor forces and torques experienced by a fish-like swimmer. We conclude by exploring of the limits of this approach and its feasibility in the study of freely-swimming fishes.

Control of Movement Initiation Underlies the Development of Balance

Balance arises from the interplay of external forces acting on the body and internally generated movements. Many animal bodies are inherently unstable, necessitating corrective locomotion to maintain stability. Understanding how developing animals come to balance remains a challenge. Here we study the interplay among environment, sensation, and action as balance develops in larval zebrafish. We first model the physical forces that challenge underwater balance and experimentally confirm that larvae are subject to constant destabilization. Larvae propel in swim bouts that, we find, tend to stabilize the body. We confirm the relationship between locomotion and balance by changing larval body composition, exacerbating instability and eliciting more frequent swimming. Intriguingly, developing zebrafish come to control the initiation of locomotion, swimming preferentially when unstable, thus restoring preferred postures. To test the sufficiency of locomotor-driven stabilization and the developing control of movement timing, we incorporate both into a generative model of swimming. Simulated larvae recapitulate observed postures and movement timing across early development, but only when locomotor-driven stabilization and control of movement initiation are both utilized. We conclude the ability to move when unstable is the key developmental improvement to balance in larval zebrafish. Our work informs how emerging sensorimotor ability comes to impact how and why animals move when they do.