Effects of Shear Flows on Nonlinear Waves in Excitable Media

If an excitable medium is moving with relative shear, the waves of excitation may be broken by the motion. We consider such breaks for the case of a constant linear shear flow. The mechanisms and conditions for the breaking of solitary waves and wavetrains are essentially different: the solitary waves require the velocity gradient to exceed a certain threshold, whilst the breaking of repetitive wavetrains happens for arbitrarily small velocity gradients. Since broken waves evolve into new spiral wave sources, this leads to spatio-temporal irregularity.     

Temporal Changes in Swimming Direction during the Phototactic Orientation of Individual Cells in Cryptomonas sp.

The response of individual Cryptomonas cells to continuous lightwas recorded using infrared video-micrography. Swimming directionsand temporal shifts in swimming direction of each cell weremeasured. White light of 0.1–1 W m^–2 elicited apositive phototactic orientation, but did not induce any photophobicresponse. Light of 100 W m^–2 induced a photophobic responseat the onset of actinic irradiation, but did not induce positivephototactic orientation. No correlation between positive phototacticorientation and photophobic response was found in this species.The direction toward the light source was defined as 0°,and the direction away from the source as 180°. Within 2s after the onset of lateral monochromatic light of 570 nm at0.1 W m^–2, cells which were swimming in a direction ofless than 120° predominantly shifted their course towardthe light source. Cells swimming in directions of larger than120° shifted their course as randomly as those in the dark.Thus, for phototactic orientation, the cells must perceive thelight from their anterior side. (Received July 29, 1985; Accepted November 4, 1985)     

Swimming in mantids

The initiation, form of leg strokes and minimal requirements for swimming in Sphodromantis lineola are described.     

Springs in Swimming Animals

Animals can lower the metabolic cost of swimming by using appropriatelytuned, elastic springs. Jet-powered invertebrates use springsthat lie in functional parallel to their swimming muscles topower half the locomotor cycle. The parallel geometry constrainsthe spring to be non-linearly elastic; muscle power is divertedto load the spring only when swimming muscles are not capableof producing maximal hydrodynamic thrust. The springs of jellyfishand scallops are forced at or near their resonant frequency,producing large energy savings. Measuring the contribution ofelastic energy storage to jet-powered locomotion has been facilitatedby the relatively simple geometries of invertebrate locomotorsystems. In contrast, complex musculoskeletal systems and kinematicshave complicated the study of springs in swimming vertebrates.Skins, tendons and axial skeletons of some vertebrate swimmershave appropriate mechanical properties to act as springs. Todate, though, there exist just a handful of studies that haveinvestigated the mechanical behaviors of these locomotor structuresin swimming vertebrates, and these data have yet to be integratedwith measures of swimming power. Integrating mechanical, kinematic,hydrodynamic and metabolic data are required to understand morefully the role of elastic springs in vertebrate swimming energetics.     

The Kinematics of Swimming and Relocation Jumps in Copepod Nauplii

Copepod nauplii move in a world dominated by viscosity. Their swimming-by-jumping propulsion mode, with alternating power and recovery strokes of three pairs of cephalic appendages, is fundamentally different from the way other microplankters move. Protozoans move using cilia or flagella, and copepodites are equipped with highly specialized swimming legs. In some species the nauplius may also propel itself more slowly through the water by beating and rotating the appendages in a different, more complex pattern. We use high-speed video to describe jumping and swimming in nauplii of three species of pelagic copepods: Temora longicornis, Oithona davisae and Acartia tonsa. The kinematics of jumping is similar between the three species. Jumps result in a very erratic translation with no phase of passive coasting and the nauplii move backwards during recovery strokes. This is due to poorly synchronized recovery strokes and a low beat frequency relative to the coasting time scale. For the same reason, the propulsion efficiency of the nauplii is low. Given the universality of the nauplius body plan, it is surprising that they seem to be inefficient when jumping, which is different from the very efficient larger copepodites. A slow-swimming mode is only displayed by T. longicornis. In this mode, beating of the appendages results in the creation of a strong feeding current that is about 10 times faster than the average translation speed of the nauplius. The nauplius is thus essentially hovering when feeding, which results in a higher feeding efficiency than that of a nauplius cruising through the water.     

The Long-Time Dynamics of Two Hydrodynamically-Coupled Swimming Cells

Swimming microorganisms such as bacteria or spermatozoa are typically found in dense suspensions, and exhibit collective modes of locomotion qualitatively different from that displayed by isolated cells. In the dilute limit where fluid-mediated interactions can be treated rigorously, the long-time hydrodynamics of a collection of cells result from interactions with many other cells, and as such typically eludes an analytical approach. Here, we consider the only case where such problem can be treated rigorously analytically, namely when the cells have spatially confined trajectories, such as the spermatozoa of some marine invertebrates. We consider two spherical cells swimming, when isolated, with arbitrary circular trajectories, and derive the long-time kinematics of their relative locomotion. We show that in the dilute limit where the cells are much further away than their size, and the size of their circular motion, a separation of time scale occurs between a fast (intrinsic) swimming time, and a slow time where hydrodynamic interactions lead to change in the relative position and orientation of the swimmers. We perform a multiple-scale analysis and derive the effective dynamical system—of dimension two—describing the long-time behavior of the pair of cells. We show that the system displays one type of equilibrium, and two types of rotational equilibrium, all of which are found to be unstable. A detailed mathematical analysis of the dynamical systems further allows us to show that only two cell-cell behaviors are possible in the limit of t→∞, either the cells are attracted to each other (possibly monotonically), or they are repelled (possibly monotonically as well), which we confirm with numerical computations. Our analysis shows therefore that, even in the dilute limit, hydrodynamic interactions lead to new modes of cell-cell locomotion.     

Foot Infections in Swimming Baths

A 10% random sample of all bathers at a public swimming bath were examined for tinea pedis and verruca.The overall incidence of tinea pedis was 8·5% and of verruca 4·8%. The incidence of tinea pedis in 205 male adults was 21·5%, in 288 boys 6·3%, in 60 adult females 3·3%, and in 220 girls 0·9%. The incidence of verruca in juveniles ranged from 4·2% in boys to 10·5% in girls.It was clear that both infections spread within the baths, and since a relatively small proportion of users admitted to taking precautions to avoid contracting or developing infections it seems advisable that more publicity about recommendations on foot care should be provided.     

The Tiny Zebrafish Keep Swimming Fast in the Developmental Biology Pond

In 1996,the journal Development published a special issue on zebrafish solely focusing on characterization of dozens of phenotypic mutants chosen from hundreds of mutants identified through chemical(ENU)mutagenesis by two zebrafish groups in Tubingen and Boston.This milestone formally catapulted zebrafish to a league of genetically tractable model     

Neural Control of Swimming in the Leech

Leeches swim by undulating; they alternately form crests thentroughs at their anterior end and move Them backward, therebyproducing forward thrust. These movements are accomplished byalternating contractions of dorsal and ventral longitudinalmuscles in each of the 21 body segments. These contractionsare caused by bursts of impulses in groups of excitatory andinhibitory motor neurons. Connections among motor neurons helpto coordinate these bursts: synergistic muscle excitors areelectrotonically coupled, which aids in keeping their burstsnearly synchronous; muscle inhibitors also inhibit the excitorsto the same muscles, and it is this inhibition which keeps theexcitors from being tonically active during swimming. Neuronssensitive to either dorsal or ventral body wall stretch producereciprocal stretch reflexes to the muscle excitors, probablyvia the inhibitors. That these stretch reflexes may be involvedin generating the periodic bursts is supported by the resultsof both behavioral and electrophysiological experiments.     

Switching of Swimming Modes in Magnetospirillium gryphiswaldense

The microaerophilic magnetotactic bacterium Magnetospirillum gryphiswaldense swims along magnetic field lines using a single flagellum at each cell pole. It is believed that this magnetotactic behavior enables cells to seek optimal oxygen concentration with maximal efficiency. We analyze the trajectories of swimming M. gryphiswaldense cells in external magnetic fields larger than the earth’s field, and show that each cell can switch very rapidly (in <0.2 s) between a fast and a slow swimming mode. Close to a glass surface, a variety of trajectories were observed, from straight swimming that systematically deviates from field lines to various helices. A model in which fast (slow) swimming is solely due to the rotation of the trailing (leading) flagellum can account for these observations. We determined the magnetic moment of this bacterium using a to our knowledge new method, and obtained a value of (2.0 ± 0.6) × 10^?16 A · m². This value is found to be consistent with parameters emerging from quantitative fitting of trajectories to our model.     

Sudden Failure of Swimming in Cold Water

To investigate the effect of cold water on swimming four men who declared themselves good swimmers were immersed fully clothed on separate days in water at 23·7° and 4·7° C. The time that they were able to swim in the cold water was much shorter than in the warm. The two shortest swims ended after 1·5 and 7·6 minutes, before rectal temperature fell, when the men suddenly floundered after developing respiratory distress with breathing rates of 56–60/min. The other cold swims, by the two fattest men, ended less abruptly with signs of general and peripheral hypothermia.It is concluded that swimming in cold water was stopped partly by respiratory reflexes in the thin men and hypothermia in the fat, and partly by the cold water''s high viscosity. The longer swimming times of the fat men are attributed largely to their greater buoyancy enabling them to keep their heads above water during the early hyperventilation.The findings explain some reports of sudden death in cold water. It is clearly highly dangerous to attempt to swim short distances to shore without a life-jacket in water near 0° C.     

The Interaction between Water Currents and Salmon Swimming Behaviour in Sea Cages

Positioning of sea cages at sites with high water current velocities expose the fish to a largely unknown environmental challenge. In this study we observed the swimming behaviour of Atlantic salmon (Salmo salar L.) at a commercial farm with tidal currents altering between low, moderate and high velocities. At high current velocities the salmon switched from the traditional circular polarized group structure, seen at low and moderate current velocities, to a group structure where all fish kept stations at fixed positions swimming against the current. This type of group behaviour has not been described in sea cages previously. The structural changes could be explained by a preferred swimming speed of salmon spatially restricted in a cage in combination with a behavioural plasticity of the fish.     

The Effect of Swimming with Dolphins on Human Well-Being and Anxiety

ABSTRACT

The present investigation aimed to explore the psychological effects for humans of swimming with dolphins as opposed to swimming in the ocean without dolphins. It was hypothesized that people swimming with dolphins would experience significantly greater levels of well-being and reduced levels of anxiety than those who swam without dolphins. Participants were sampled from Perth's UnderWater World marine park and at the Bunbury Dolphin Discovery Centre, Australia. Participants completed well-being and anxiety measures before and after their swim. Well-being was greater in participants who swam with dolphins than in those who did not, both before and after their swim. However, well-being increased to the same extent in both groups. In contrast, anxiety decreased for participants swimming with dolphins but not in those who swam without dolphins. The findings suggest that anticipation of a new and exciting experience, and swimming, itself increase well-being. In addition, swimming specifically with dolphins may lower anxiety. Whether these effects are responsible for the therapeutic benefits associated with human–dolphin interactions requires further investigation.     

Muscle Dynamics in Fish During Steady Swimming

SYNOPSIS. Recent research in fish locomotion has been dominatedby an interest in the dynamic mechanical properties of the swimmingmusculature. Prior observations have indicated that waves ofmuscle activation travel along the body of an undulating fishfaster than the resulting waves of muscular contraction, suggestingthat the phase relation between the muscle strain cycle andits activation must vary along the body. Since this phase relationis critical in determining how the muscle performs in cycliccontractions, the possibility has emerged that dynamic musclefunction may change with axial position in swimming fish. Quantificationof muscle contractile properties in cyclic contractions relieson in vitro experiments using strain and activation data collectedin vivo. In this paper we discuss the relation between theseparameters and body kinematics. Using videoradiographic datafrom swimming mackerel we demonstrate that red muscle straincan be accurately predicted from midline curvature but not fromlateral displacement. Electromyographic recordings show neuronalactivation patterns that are consistent with red muscle performingnet positive work at all axial positions. The relatively constantcross-section of red muscle along much of the body suggeststhat positive power for swimming is generated fairly uniformlyalong the length of the fish.     

The Discrete Multi-Hybrid System for the Simulation of Solid-Liquid Flows

This study proposes a model based on the combination of Smoothed Particle Hydrodynamics, Coarse Grained Molecular Dynamics and the Discrete Element Method for the simulation of dispersed solid-liquid flows. The model can deal with a large variety of particle types (non-spherical, elastic, breakable, melting, solidifying, swelling), flow conditions (confined, free-surface, microscopic), and scales (from microns to meters). Various examples, ranging from biological fluids to lava flows, are simulated and discussed. In all cases, the model captures the most important features of the flow.     

Mechanics of Undulatory Swimming in a Frictional Fluid

The sandfish lizard (Scincus scincus) swims within granular media (sand) using axial body undulations to propel itself without the use of limbs. In previous work we predicted average swimming speed by developing a numerical simulation that incorporated experimentally measured biological kinematics into a multibody sandfish model. The model was coupled to an experimentally validated soft sphere discrete element method simulation of the granular medium. In this paper, we use the simulation to study the detailed mechanics of undulatory swimming in a “granular frictional fluid” and compare the predictions to our previously developed resistive force theory (RFT) which models sand-swimming using empirically determined granular drag laws. The simulation reveals that the forward speed of the center of mass (CoM) oscillates about its average speed in antiphase with head drag. The coupling between overall body motion and body deformation results in a non-trivial pattern in the magnitude of lateral displacement of the segments along the body. The actuator torque and segment power are maximal near the center of the body and decrease to zero toward the head and the tail. Approximately 30% of the net swimming power is dissipated in head drag. The power consumption is proportional to the frequency in the biologically relevant range, which confirms that frictional forces dominate during sand-swimming by the sandfish. Comparison of the segmental forces measured in simulation with the force on a laterally oscillating rod reveals that a granular hysteresis effect causes the overestimation of the body thrust forces in the RFT. Our models provide detailed testable predictions for biological locomotion in a granular environment.