Turbulence: does vorticity affect the structure and shape of body and fin propulsors?

Over the past century, many ideas have been developed on the relationships between water flow and the structure and shape of the body and fins of fishes, largely during swimming in relatively steady flows. However, both swimming by fishes and the habitats they occupy are associated with vorticity, typically concentrated as eddies characteristic of turbulent flow. Deployment of methods to examine flow in detail suggests that vorticity impacts the lives of fishes. First, vorticity near the body and fins can increase thrust and smooth variations in thrust that are a consequence of using oscillating and undulating propulsors to swim. Second, substantial mechanical energy is dissipated in eddies in the wake and adaptations that minimize these losses would be anticipated. We suggest that such mechanisms may be found in varying the length of the propulsive wave, stiffening propulsive surfaces, and shifting to using median and paired fins when swimming at low speeds. Eddies in the flow encountered by fishes may be beneficial, but when eddy radii are of the order of 0.25 of the fish's total length, negative impacts occur due to greater difficulties in controlling stability. The archetypal streamlined "fish" shape reduces destabilizing forces for fishes swimming into eddies.     

Nonlinear muscles, passive viscoelasticity and body taper conspire to create neuromechanical phase lags in anguilliform swimmers

Locomotion provides superb examples of cooperation among neuromuscular systems, environmental reaction forces, and sensory feedback. As part of a program to understand the neuromechanics of locomotion, here we construct a model of anguilliform (eel-like) swimming in slender fishes. Building on a continuum mechanical representation of the body as an viscoelastic rod, actuated by a traveling wave of preferred curvature and subject to hydrodynamic reaction forces, we incorporate a new version of a calcium release and muscle force model, fitted to data from the lamprey Ichthyomyzon unicuspis, that interactively generates the curvature wave. We use the model to investigate the source of the difference in speeds observed between electromyographic waves of muscle activation and mechanical waves of body curvature, concluding that it is due to a combination of passive viscoelastic and geometric properties of the body and active muscle properties. Moreover, we find that nonlinear force dependence on muscle length and shortening velocity may reduce the work done by the swimming muscles in steady swimming.     

Herbivory, connectivity, and ecosystem resilience: response of a coral reef to a large-scale perturbation

Coral reefs world-wide are threatened by escalating local and global impacts, and some impacted reefs have shifted from coral dominance to a state dominated by macroalgae. Therefore, there is a growing need to understand the processes that affect the capacity of these ecosystems to return to coral dominance following disturbances, including those that prevent the establishment of persistent stands of macroalgae. Unlike many reefs in the Caribbean, over the last several decades, reefs around the Indo-Pacific island of Moorea, French Polynesia have consistently returned to coral dominance following major perturbations without shifting to a macroalgae-dominated state. Here, we present evidence of a rapid increase in populations of herbivorous fishes following the most recent perturbation, and show that grazing by these herbivores has prevented the establishment of macroalgae following near complete loss of coral on offshore reefs. Importantly, we found the positive response of herbivorous fishes to increased benthic primary productivity associated with coral loss was driven largely by parrotfishes that initially recruit to stable nursery habitat within the lagoons before moving to offshore reefs later in life. These results underscore the importance of connectivity between the lagoon and offshore reefs for preventing the establishment of macroalgae following disturbances, and indicate that protecting nearshore nursery habitat of herbivorous fishes is critical for maintaining reef resilience.     

Experimental Hydrodynamics and Evolution: Function of Median Fins in Ray-finned Fishes

The median fins of fishes consist of the dorsal, anal, and caudal fins and have long been thought to play an important role in generating locomotor force during both steady swimming and maneuvering. But the orientations and magnitudes of these forces, the mechanisms by which they are generated, and how fish modulate median fin forces have remained largely unknown until the recent advent of Digital Particle Image Velocimetry (DPIV) which allows empirical analysis of force magnitude and direction. Experimental hydrodynamic studies of median fin function in fishes are of special utility when conducted in a comparative phylogenetic context, and we have examined fin function in four ray-finned fish clades (sturgeon, trout, sunfish, and mackerel) with the goal of testing classical hypotheses of fin function and evolution. In this paper we summarize two recent technical developments in DPIV methodology, and discuss key recent findings relevant to median fin function. High-resolution DPIV using a recursive local-correlation algorithm allows quantification of small vortices, while stereo-DPIV permits simultaneous measurement of x, y, and z flow velocity components within a single planar light sheet. Analyses of median fin wakes reveal that lateral forces are high relative to thrust force, and that mechanical performance of median fins (i.e., thrust as a proportion of total force) averages 0.35, a surprisingly low value. Large lateral forces which could arise as an unavoidable consequence of thrust generation using an undulatory propulsor may also enhance stability and maneuverability. Analysis of hydrodynamic function of the soft dorsal fin in bluegill sunfish shows that a thrust wake is generated that accounts for 12% of total thrust and that the thrust generation by the caudal fin may be enhanced by interception of the dorsal fin wake. Integration of experimental studies of fin wakes, computational approaches, and mechanical models of fin function promise understanding of instantaneous forces on fish fins during the propulsive cycle as well as exploration of a broader locomotor design space and its hydrodynamic consequences.     

Hydrodynamic Characteristics of the Sailfish (Istiophorus platypterus) and Swordfish (Xiphias gladius) in Gliding Postures at Their Cruise Speeds

The sailfish and swordfish are known as the fastest sea animals, reaching their maximum speeds of around 100 km/h. In the present study, we investigate the hydrodynamic characteristics of these fishes in their cruise speeds of about 1 body length per second. We install a taxidermy specimen of each fish in a wind tunnel, and measure the drag on its body and boundary-layer velocity above its body surface at the Reynolds number corresponding to its cruising condition. The drag coefficients of the sailfish and swordfish based on the free-stream velocity and their wetted areas are measured to be 0.0075 and 0.0091, respectively, at their cruising conditions. These drag coefficients are very low and comparable to those of tuna and pike and smaller than those of dogfish and small-size trout. On the other hand, the long bill is one of the most distinguished features of these fishes from other fishes, and we study its role on the ability of drag modification. The drag on the fish without the bill or with an artificially-made shorter one is slightly smaller than that with the original bill, indicating that the bill itself does not contribute to any drag reduction at its cruise speed. From the velocity measurement near the body surface, we find that at the cruise speed flow separation does not occur over the whole body even without the bill, and the boundary layer flow is affected only at the anterior part of the body by the bill.     

The overlapping roles of the inner ear and lateral line: the active space of dipole source detection

The problems associated with the detection of sounds and other mechanical disturbances in the aquatic environment differ greatly from those associated with airborne sounds. The differences are primarily due to the incompressibility of water and the corresponding increase in importance of the acoustic near field. The near field, or hydrodynamic field, is characterized by steep spatial gradients in pressure, and detection of the accelerations associated with these gradients is performed by both the inner ear and the lateral line systems of fishes. Acceleration-sensitive otolithic organs are present in all fishes and provide these animals with a form of inertial audition. The detection of pressure gradients, by both the lateral line and inner ear, is the taxonomically most widespread mechanism of sound-source detection amongst vertebrates, and is thus the most likely primitive mode of detecting sound sources. Surprisingly, little is known about the capabilities of either the lateral line or the otolithic endorgan in the detection of vibratory dipole sources. Theoretical considerations for the overlapping roles of the inner ear and lateral line systems in midwater predict that the lateral line will operate over a shorter distance range than the inner ear, although with a much greater spatial resolution. Our empirical results of dipole detection by mottled sculpin, a benthic fish, do not agree with theoretical predictions based on midwater fishes, in that the distance ranges of the two systems appear to be approximately equal. This is almost certainly as a result of physical coupling between the fishes and the substrate. Thus, rather than having a greater active range, the inner ear appears to have a reduced distance range in benthic fishes, and the lateral line distance range may be concomitantly extended.     

Modes and scaling in aquatic locomotion

Organisms spanning a 10(7)-fold range in length of the body engage in aquatic propulsion-swimming; they do so with several kinds of propulsors and take advantage of several different fluid mechanical mechanisms. A hierarchical classification of swimming modes can impose some order on this complexity. More difficult are the issues surrounding the different kinds of propulsive devices used by different organisms. These issues can be in part exposed by an examination of how speeds and accelerations scale with changes in body length, both for different lineages of swimmers and for all swimmers collectively. Clearly, fluid mechanical factors impose general rules and constraints; just as clearly, these only roughly anticipate actual scaling. Indeed, collections of data on scaling can serve as useful correctives for assumptions about functional mechanisms. They can also reveal size-dependent constraints on biological designs.     

A treadmill-mounted force platform

Muscle, bone, and tendon forces; the movement of the center of mass, and the spring properties of the body during terrestrial locomotion can be measured using ground-mounted force platforms. These measurements have been extremely time consuming because of the difficulty in obtaining repeatable constant speed trials (particularly with animals). We have overcome this difficulty by mounting a force platform directly under the belt of a motorized treadmill. With this arrangement, vertical force can be recorded from an unlimited number of successive ground contacts in a much shorter time. With this treadmill-mounted force platform it is possible to accurately make the following measurements over the full range of steady speeds and under various perturbations of normal gait: 1) vertical ground reaction force over the course of the contact phase; 2) peak forces in bone, muscle, and tendon; 3) the vertical displacement of the center of mass; and 4) contact time for the limbs. In our treadmill-force platform design, belt forces and frictional forces cause no measurable cross-talk problem. Natural frequency (160 Hz), nonlinearity (less than 5%), and position independence (less than 2%) are all quite acceptable. Motor-caused vibrations are greater than 150 Hz and thus can be easily filtered.     

Swimming behavior in relation to buoyancy in an open swimbladder fish, the Chinese sturgeon

The swimbladder of fishes is readily compressed by hydrostatic pressure with depth, causing changes in buoyancy. While modern fishes can regulate buoyancy by secreting gases from the blood into the swimbladder, primitive fishes, such as sturgeons, lack this secretion mechanism and rely entirely on air gulped at the surface to inflate the swimbladder. Therefore, sturgeons may experience changes in buoyancy that will affect their behavior at different depths. To test this prediction, we attached data loggers to seven free-ranging Chinese sturgeons Acipenser sinensis in the Yangtze River, China, to monitor their depth utilization, tail-beating activity, swim speed and body inclination. Two distinct, individual-specific, behavioral patterns were observed. Four fish swam at shallow depths (7–31 m), at speeds of 0.5–0.6 m s⁻¹, with ascending and descending movements of 1.0–2.4 m in amplitude. They beat their tails continuously, indicating that their buoyancy was close to neutral with their inflated swimbladders. In addition, their occasional visits to the surface suggest that they gulped air to inflate their swimbladders. The other three fish spent most of their time (88–94%) on the river bottom at a depth of 106–122 m with minimum activity. They occasionally swam upwards at speeds of 0.6–0.8 m s⁻¹ with intense tailbeats before gliding back passively to the bottom, in a manner similar to fishes that lack a swimbladder. Their bladders were probably collapsed by hydrostatic pressure, resulting in negative buoyancy. We conclude that Chinese sturgeons behave according to their buoyancy, which varies with depth due to hydrostatic compression of the swimbladder.     

Motor control and learning in altered dynamic environments

Dynamic perturbations of reaching movements are an important technique for studying motor learning and adaptation. Adaptation to non-contacting, velocity-dependent inertial Coriolis forces generated by arm movements during passive body rotation is very rapid, and when complete the Coriolis forces are no longer sensed. Adaptation to velocity-dependent forces delivered by a robotic manipulandum takes longer and the perturbations continue to be perceived even when adaptation is complete. These differences reflect adaptive self-calibration of motor control versus learning the behavior of an external object or 'tool'. Velocity-dependent inertial Coriolis forces also arise in everyday behavior during voluntary turn and reach movements but because of anticipatory feedforward motor compensations do not affect movement accuracy despite being larger than the velocity-dependent forces typically used in experimental studies. Progress has been made in understanding: the common features that determine adaptive responses to velocity-dependent perturbations of jaw and limb movements; the transfer of adaptation to mechanical perturbations across different contact sites on a limb; and the parcellation and separate representation of the static and dynamic components of multiforce perturbations.     

Simple Physical Principles and Vertebrate Aquatic Locomotion

SYNOPSIS. Swimming is a common and important component of vertebratebehavior. Therefore the study of swimming form and functionis essential to understanding vertebrate biology. An interactiveprocess between biologists and hydrodynamicists has proven extremelysuccessful in providing such understanding. This process startswith a definition of forces that may act on an animal; viscousand pressure drag, acceleration reaction, lift and body inertia,and ground reaction. Dominant force components are selectedon the basis of ground contact, Reynolds number, reduced frequency,and shape. Conservation of momentum requires forces and theirmoments be in balance leading to models describing swimmingmotions and associated forces. These provide the bridge betweenthe hydrodynamics studies and biological applications. Numericalsolutions to models estimate thrust and drag, and rates of working,which are several times greater than expected for manmade non-flexingbodies. These solutions are used in prediction of optimal two-phaseswimming behaviors, alternating high resistance swimming withlow resistance behavior such as gliding. Qualitative predictionsfrom models define optimal forms for various behaviors and habitats.Optimal forms tend to be mutually exclusive, providing benchmarksfor analyses relating form and function in an ecological context.Life history patterns are also affected by locomotor capabilitiesof various morphologies. Scale effects limit the distributionof propulsors. Numerical solutions to models are probably onlygood within an order of magnitude because of the assumptionsin their formulation, but rankings of mechanisms or organismsare adequate for comparative study. Predictions must be temperedby consideration of non-locomotor function, developmental limitationsof structural options, and non-equilibrium community structures.     

Rarity and extinction risk in coral reef angelfishes on isolated islands: interrelationships among abundance, geographic range size and specialisation

Determining the species most vulnerable to increasing degradation of coral reef habitats requires identification of the ecological traits that increase extinction risk. In the terrestrial environment, endemic species often face a high risk of extinction because of an association among three traits that threaten species persistence: small geographic range size, low abundance and ecological specialisation. To test whether these traits are associated in coral reef fishes, this study compared abundance and specialisation in endemic and widespread angelfishes at the remote Christmas and Cocos Islands in the Indian Ocean. The interrelationships among traits conferring high extinction risk in terrestrial communities did not apply to these fishes. Endemic angelfishes were 50–80 times more abundant than widespread species at these islands. Furthermore, there was no relationship between abundance and ecological specialisation. Endemic species were not more specialised than widespread congeners and endemics used similar resources to many widespread species. Three widespread species exhibited low abundance and some degree of specialisation, which may expose them to a greater risk of local extinction. For endemic species, high abundance and lack of specialisation on susceptible habitats may compensate for the global extinction risk posed by having extremely small geographic ranges. However, recent extinctions of small range reef fishes confirm that endemics are not immune to the increasing severity of large-scale disturbances that can affect species throughout their geographic range.     

How do non-muscular torques contribute to the kinetics of postural recovery following a support surface translation?

The common platform translation paradigm used in balance control studies employs a disturbance event that applies non-muscular forces to the body for the duration of the disturbance. Previous research has explored the process of constructing the balance recovery by considering these perturbations to be trigger events, not events with an ongoing force application timeline. The purpose of this study was to quantify the effect of muscular and non-muscular torques on post-perturbation balance with particular interest in the role of the external perturbation in balance recovery. Five young adult males experienced backward translations of the support surface at three different speeds. Integration intervals were defined for each segment and angular impulses were calculated for a period of increasing angular momentum (destabilization), and a period of decreasing angular momentum (restabilization). Destabilization of distal segments was primarily due to impulse generated by the motion of the support surface. For the trunk, however, muscle and motion-dependent sources contributed most to increasing momentum. Restabilization of distal segments was achieved by muscle and platform impulses while trunk restabilization was achieved by muscle and motion-dependent terms in opposition to gravity. Increased platform speed resulted in increased muscular contribution only in the control of the trunk, while demand on distal musculature decreased with change in platform speed as the platform contribution to restabilization increased in these segments. Therefore, impulses from non-muscular sources, including the perturbation itself, are significant modifiers of the response to balance disturbances and must be accounted for in balance research.     

The reactions of the Norway lobster,Nephrops norvegicus (L.), to water currents

Observations were made on the reactions of the Norway lobster, Nephrops norvegicus, to water currents in a sea‐water flume tank. Blind animals were used to prevent visually‐guided behaviour. Nephrops adopted a downstream orientation, and usually walked downstream, in response to water current speeds in the range of 0.07 to 0.20 ms^?1. Patterns of water flow around the body revealed that it was most effectively streamlined when the animal adopted a downstream orientation. Direct measurements of the forces acting on the body revealed that animals with a downstream orientation experienced the least hydrodynamic drag and the greatest downforce.     

Maintenance of fish diversity on disturbed coral reefs

Habitat perturbations play a major role in shaping community structure; however, the elements of disturbance-related habitat change that affect diversity are not always apparent. This study examined the effects of habitat disturbances on species richness of coral reef fish assemblages using annual surveys of habitat and 210 fish species from 10 reefs on the Great Barrier Reef (GBR). Over a period of 11 years, major disturbances, including localised outbreaks of crown-of-thorns sea star (Acanthaster planci), severe storms or coral bleaching, resulted in coral decline of 46–96% in all the 10 reefs. Despite declines in coral cover, structural complexity of the reef framework was retained on five and species richness of coral reef fishes maintained on nine of the disturbed reefs. Extensive loss of coral resulted in localised declines of highly specialised coral-dependent species, but this loss of diversity was more than compensated for by increases in the number of species that feed on the epilithic algal matrix (EAM). A unimodal relationship between areal coral cover and species richness indicated species richness was greatest at approximately 20% coral cover declining by 3–4 species (6–8% of average richness) at higher and lower coral cover. Results revealed that declines in coral cover on reefs may have limited short-term impact on the diversity of coral reef fishes, though there may be fundamental changes in the community structure of fishes.     

Contributions of muscles to mediolateral ground reaction force over a range of walking speeds

Impaired control of mediolateral body motion during walking is an important health concern. Developing treatments to improve mediolateral control is challenging, partly because the mechanisms by which muscles modulate mediolateral ground reaction force (and thereby modulate mediolateral acceleration of the body mass center) during unimpaired walking are poorly understood. To investigate this, we examined mediolateral ground reaction forces in eight unimpaired subjects walking at four speeds and determined the contributions of muscles, gravity, and velocity-related forces to the mediolateral ground reaction force by analyzing muscle-driven simulations of these subjects. During early stance (0-6% gait cycle), peak ground reaction force on the leading foot was directed laterally and increased significantly (p<0.05) with walking speed. During early single support (14-30% gait cycle), peak ground reaction force on the stance foot was directed medially and increased significantly (p<0.01) with speed. Muscles accounted for more than 92% of the mediolateral ground reaction force over all walking speeds, whereas gravity and velocity-related forces made relatively small contributions. Muscles coordinate mediolateral acceleration via an interplay between the medial ground reaction force contributed by the abductors and the lateral ground reaction forces contributed by the knee extensors, plantarflexors, and adductors. Our findings show how muscles that contribute to forward progression and body-weight support also modulate mediolateral acceleration of the body mass center while weight is transferred from one leg to another during double support.     

Fruit preferences by fishes in a Neotropical floodplain

The diets of frugivorous animals result from the interaction between feeding preference and ecological factors such as availability of alternative resources and interactions with other frugivores. A better understanding of frugivore diets will enable predictions about the vulnerability of plant populations to anthropogenic or natural environmental changes. In addition, frugivores with greater variation in diet (generalists) are potentially more resilient to habitat changes than specialists. Here, we combined data on diets of frugivorous fishes with visual censuses on fruit availability to evaluate if fishes are selective or opportunistic in their use of fruits in floodplain habitats. We found a high diversity of fruit species (74 species) frequently consumed by four common species of frugivorous fishes during the flooding season. The identity of preferred fruits, rather than their availability, had a stronger effect on the patterns of fruit consumed by fishes in this system. Fruit preferences by fish may influence the long-term persistence and the maintenance of plant diversity in floodplain habitats. Possible disturbances of fish populations with selective consumption and the plant populations on which they depend could increase the vulnerability of both populations involved. Thus, selectivity may affect ecological and evolutionary processes associated with frugivory by fish.     

Flexibility of tropocollagen from sedimentation and viscosity

Sedimentation constant and intrinsic viscosity were measured on purified tropocollagens extracted from earthworm-cuticle and lathyritic ratskin. A cartesian diver viscometer was used to make viscosity measurements at small shear stress and to avoid the effects of surface forces. By comparing the experimental data with the hydrodynamic theories of wormlike-coil of Ullman a value of 1300 Å has been assigned for the persistence length of these tropocollagens. Other factors which may affect the estimate are discussed.