Session AQ: Mini-Symposium on Biological Perspectives on Locomotion

E. coli swimming over agar in a thin aqueous film

When cells of Escherichia coli are grown in a rich medium over somewhat soft agar (0.45\%) they elongate, produce more flagella, and swarm (or flock). Their behavior is dominated by collisions: an individual cell's velocity is randomized in about 0.2 s [1]. However, cells do not swim in spirals, as they do when in a thick layer of fluid near a solid boundary [2]. This suggests that the surface of the swarm is stationary, i.e., that the cells swim in a thin film of fluid between two fixed surfaces. We showed that this is the case by following the motion of MgO smoke particles deposited at the fluid-air interface [3]. By visualizing flagella of cells in swarms, we found that cells can escape from a confined environment by swimming back through the flagellar bundle, without changing the orientation of the cell body. This maneuver involves normal-to-curly and curly-to-normal polymorphic transformations [4]. These phenomena will be illustrated.\\[4pt] [1] Darnton NC, Turner L, Rojevsky S, \& Berg HC (2010) Dynamics of bacterial swarming. Biophys. J. 98:2082-2090.\\[0pt] [2] Lauga E, DiLuzio WR, Whitesides GM, \& Stone HA (2006) Swimming in circles: motion of bacteria near solid boundaries. Biophys. J. 90:400-412.\\[0pt] [3] Zhang R, Turner L, \& Berg HC (2010) The upper surface of an Escherichia coli swarm is stationary. Proc. Natl. Acad. Sci. USA 107:288-290.\\[0pt] [4] Turner L, Zhang R, Darnton NC, \& Berg HC (2010) Visualization of flagella during bacterial swarming. J. Bacteriol. 192:3259-3267.   

Jet propulsion in animals: theoretical innovation and biological constraints

Jet propulsion is arguably the oldest and simplest form of animal locomotion, and simple hydrodynamic theory highlights the many possible ways in which animals might maximize speed and minimize metabolic cost while using jet propulsion to travel from one point to another. However, environmental and physiological reality constrains the potential for hydrodynamic innovation. We explore two heuristic examples: Antarctic scallops, in which ecological release from predation apparently constrains the evolution of improved locomotory capacity, and squids, in which the fundamental limitations of muscular contraction constrain the hydrodynamic efficiency of locomotion for all but a small range of sizes. Even simple forms of locomotion can be complex in a biological context.   

Locomotion by microscopic organisms in turbulent ambient water flow

How is the locomotion of microscopic organisms swimming or moving across the substratum affected by turbulent ambient currents and waves in marine habitats? We addressed this question using larvae of marine invertebrates. Many benthic animals produce microscopic larvae that swim and respond to environmental factors (e.g. odors). Larvae are dispersed to new sites by ambient water currents and land on surfaces in suitable habitats exposed to wavy, turbulent water flow. Using fouling communities and coral reefs as study systems, we measured water flow across them in the field and recreated it in flumes where we could quantify on the scale experienced by larvae (mm's, ms's) the instantaneous water velocities and concentrations of odors released from surfaces where larvae settle. We used these data to determine the temporal patterns of water velocities and odor concentrations encountered by larvae as they swim in the water and land on surfaces. We found that larvae have rapid on-off encounters with odors while swimming through fine filaments of odor swirling in unscented water, experience varying shear, and after landing are exposed to rapidly fluctuating hydrodynamic forces with peaks that depend on their location within the fine scale habitat topography. These data enabled us to design small-scale experiments in which larval locomotion could be analyzed in realistic, rapidly varying patterns of water movement and odor.   

Fish robotics and hydrodynamics

Studying the fluid dynamics of locomotion in freely-swimming fishes is challenging due to difficulties in controlling fish behavior. To provide better control over fish-like propulsive systems we have constructed a variety of fish-like robotic test platforms that range from highly biomimetic models of fins, to simple physical models of body movements during aquatic locomotion. First, we have constructed a series of biorobotic models of fish pectoral fins with 5 fin rays that allow detailed study of fin motion, forces, and fluid dynamics associated with fin-based locomotion. We find that by tuning fin ray stiffness and the imposed motion program we can produce thrust both on the fin outstroke and instroke. Second, we are using a robotic flapping foil system to study the self-propulsion of flexible plastic foils of varying stiffness, length, and trailing edge shape as a means of investigating the fluid dynamic effect of simple changes in the properties of undulating bodies moving through water. We find unexpected non-linear stiffness-dependent effects of changing foil length on self-propelled speed, and as well as significant effects of trailing edge shape on foil swimming speed.   

Small-scale biological, physical and chemical signals in the sea

Plankton operate at low to intermediate Reynolds numbers, generating watery signals that can be attenuated by viscosity and confused with small-scale turbulence. Yet messages are created, transmitted, perceived and recognized. These messages guide essential survival tasks of aquatic micro crustaceans. Cues created include those of escaping prey, lunging predators, attractive mates, and appropriate hosts. In this presentation, I describe some unusual and some typical examples of small-scale biological-physical-chemical signals in the sea that help to maintain the integrity of our aquatic ecosystems.