Social learning in juvenile lemon sharks, Negaprion brevirostris.

Social learning is taxonomically widespread and can provide distinct behavioural advantages, such as in finding food or avoiding predators more efficiently. Although extensively studied in bony fishes, no such empirical evidence exists for cartilaginous fishes. Our aim in this study was to experimentally investigate the social learning capabilities of juvenile lemon sharks, Negaprion brevirostris. We designed a novel food task, where sharks were required to enter a start zone and subsequently make physical contact with a target in order to receive a food reward. Naive sharks were then able to interact with and observe (a) pre-trained sharks, that is, 'demonstrators', or (b) sharks with no previous experience, that is, 'sham demonstrators'. On completion, observer sharks were then isolated and tested individually in a similar task. During the exposure phase observers paired with 'demonstrator' sharks performed a greater number of task-related behaviours and made significantly more transitions from the start zone to the target, than observers paired with 'sham demonstrators'. When tested in isolation, observers previously paired with 'demonstrator' sharks completed a greater number of trials and made contact with the target significantly more often than observers previously paired with 'sham demonstrators'. Such experience also tended to result in faster overall task performance. These results indicate that juvenile lemon sharks, like numerous other animals, are capable of using socially derived information to learn about novel features in their environment. The results likely have important implications for behavioural processes, ecotourism and fisheries.

Bio-Model Selection, Processing and Results for Bio-Inspired Truck Streamlining.

We introduce a method for the selection and processing of a biological model to derive an outline that provides morphometric information for a novel aerodynamic truck design. Because of the dynamic similarities, our new truck design will be inspired by biological shapes with a known high level of streamlining and low drag for operation near the seabed, i.e., the head of a trout, but other model organisms will also be used later. Demersal fish are chosen because they live near the bottom of rivers or the sea. Complementary to many biomimetic studies so far, we plan to concentrate on reshaping the outline of the fish's head and extend it to a 3D design for the tractor that, at the same time, fits within EU regulations and maintains the truck's normal use and stability. We intend to explore this biological model selection and formulization involving the following elements: (i) the reason for selecting fish as a biological model for a streamlined truck design; (ii) The choice of a fish model via a functional similarity method; (iii) biological shape formulization based on the morphometric information of models in (ii) outline pick-up, a reshaping step and a subsequent design process; (iv) modify the biomimetic designs and test utilizing CFD; (v) further discussion, outputs and results from the bio-inspired design process.

Biomimetic Design of Turbine Blades for Ocean Current Power Generation.

The enhancement of energy technology and innovation play a crucial role in order to meet the challenges related to global warming in the coming decades. Inspired by bird wings, the performance of a bio-inspired blade assembled to a marine turbine model, is examined. Following a biomimetic pathway, the aerodynamic performance of the bird wings of the species Common Guillemot (Uria aalge) was tested in a wind tunnel laboratory. Based on our results, we derived a bio-inspired blade model by following a laser scanning method. Lastly, the bio-inspired blades were assembled to a marine turbine model and tested in a large flow tank facility. We found efficiencies (Cp) up to 0.3 which is around 53% of the maximum power that can be expected from the turbine model according to the Betz approach. Our findings are analyzed in the discussion section as well as considerations for future research.

Design optimization and wind tunnel investigation of a flapping system based on the flapping wing trajectories of a beetle's hindwings.

To design a flapping-wing micro air vehicle (FWMAV), the hovering flight action of a beetle species (Protaetia brevitarsis) was captured, and various parameters, such as the hindwing flapping frequency, flapping amplitude, angle of attack, rotation angle, and stroke plane angle, were obtained. The wing tip trajectories of the hindwings were recorded and analyzed, and the flapping kinematics were assessed. Based on the wing tip trajectory functions, bioinspired wings and a linkage mechanism flapping system were designed. The critical parameters for the aerodynamic characteristics were investigated and optimized by means of wind tunnel tests, and the artificial flapping system with the best wing parameters was compared with the natural beetle. This work provides insight into how natural flyers execute flight by experimentally duplicating beetle hindwing kinematics and paves the way for the future development of beetle-mimicking FWMAVs.

Modulating yaw with an unstable rigid body and a course-stabilizing or steering caudal fin in the yellow boxfish (Ostracion cubicus).

Despite that boxfishes have a rigid carapace that restricts body undulation, they are highly manoeuvrable and manage to swim with remarkably dynamic stability. Recent research has indicated that the rigid body shape of boxfishes shows an inherently unstable response in its rotations caused by course-disturbing flows. Hence, any net stabilizing effect should come from the fishes' fins. The aim of the current study was to determine the effect of the surface area and orientation of the caudal fin on the yaw torque exerted on the yellow boxfish, Ostracion cubicus, a square cross-sectional shaped species of boxfish. Yaw torques quantified in a flow tank using a physical model with an attachable closed or open caudal fin at different body and tail angles and at different water flow speeds showed that the caudal fin is crucial for controlling yaw. These flow tank results were confirmed by computational fluid dynamics simulations. The caudal fin acts as both a course-stabilizer and rudder for the naturally unstable rigid body with regard to yaw. Boxfishes seem to use the interaction of the unstable body and active changes in the shape and orientation of the caudal fin to modulate manoeuvrability and stability.

Coasting in live-bearing fish: the drag penalty of being pregnant.

Swimming performance of pregnant live-bearing fish is presumably constrained by the additional drag associated with the reproductive burden. Yet, it is still unclear how and to what extent the reproductive investment affects body drag of the females. We examined the effect of different levels of reproductive investment on body drag. The biggest measured increase in body volume due to pregnancy was about 43%, linked to a wetted area increase of about 16% and 69% for the frontal area. We printed three-dimensional models of live-bearing fish in a straight body posture representing different reproductive allocation (RA) levels. We measured the drag and visualized the flow around these models in a flow tunnel at different speeds. Drag grew in a power fashion with speed and exponentially with the increase of RA, thus drag penalty for becoming thicker was relatively low for low speeds compared to high ones. We show that the drag increase with increasing RA was most probably due to bigger regions of flow separation behind the enlarged belly. We suggest that the rising drag penalty with an increasing RA, possibly together with pregnancy-related negative effects on muscle- and abdominal bending performance, will reduce the maximum swimming speed.

The effects of wing twist in slow-speed flapping flight of birds: trading brute force against efficiency.

In aircraft propellers that are used to propel aircraft forward at some speed, propeller blade twist is important to make the individual propeller 'wings' operate at a relatively constant effective angle of attack over the full span. Wing twist is sometimes also assumed to be essential in flapping flight, especially in bird flight. For small insects, it has however been shown that wing twist has little effect on the forces generated by a flapping wing. The unimportance of twist was attributed to the prominent role of unsteady aerodynamic mechanisms. These were recently also shown to be important in bird flight. It has therefore become necessary to verify the role of wing twist in the flapping flight of birds. The aim of the study is to compare the efficiency and the aerodynamic forces of twisted and non-twisted wings that mimic the slow-speed flapping flight of birds. The analyses were performed by using physical models with different amounts of spanwise twist (0°, 10°, 40°). The flow was mapped in three-dimensions using digital particle image velocimetry. The spanwise circulation, the induced drag, the lift-to-drag ratio and the span efficiency were determined. Twist and Strouhal number (St) both determine the local effective angles of attack of the flapping wing. Wings with low average effective angles of attack (resulting from high twist and/or low St) are more efficient, but generate significantly lower aerodynamic forces. High average effective angles of attack result in lower efficiency and high aerodynamic forces. Efficiency and the magnitude of aerodynamic forces are competing parameters. Wing twist is beneficial only in the cases where efficiency is most important-e.g. in cruising flight. Take-off, landing and maneuvering, however, require large and robust aerodynamic forces to be generated. The additional force comes at the cost of efficiency, but it enables birds to perform extreme manoeuvres, increasing their overall fitness.

Feeding current characteristics of three morphologically different bivalve suspension feeders, Crassostrea gigas, Mytilus edulis and Cerastoderma edule, in relation to food competition.

Introduced Pacific oysters (Crassostrea gigas) have shown rapid expansion in the Oosterschelde estuary, while stocks of native bivalves declined slightly or remained stable. This indicates that they might have an advantage over native bivalve filter feeders. Hence, at the scale of individual bivalves, we studied whether this advantage occurs in optimizing food intake over native bivalves. We investigated feeding current characteristics, in which potential differences may ultimately lead to a differential food intake. We compared feeding currents of the invasive epibenthic non-siphonate Pacific oyster to those of two native bivalve suspension feeders: the epibenthic siphonate blue mussel Mytilus edulis and the endobenthic siphonate common cockle Cerastoderma edule. Inhalant flow fields were studied empirically using digital particle image velocimetry and particle tracking velocimetry. Exhalant jet speeds were modelled for a range of exhalant-aperture cross-sectional areas as determined in the laboratory and a range of filtration rates derived from literature. Significant differences were found in inhalant and exhalant current velocities and properties of the inhalant flow field (acceleration and distance of influence). At comparable body weight, inhalant current velocities were lower in C. gigas than in the other species. Modelled exhalant jets were higher in C. gigas, but oriented horizontally instead of vertically as in the other species. Despite these significant differences and apparent morphological differences between the three species, absolute differences in feeding current characteristics were small and are not expected to lead to significant differences in feeding efficiency.

Propulsive force calculations in swimming frogs. II. Application of a vortex ring model to DPIV data.

Frogs propel themselves by kicking water backwards using a synchronised extension of their hind limbs and webbed feet. To understand this propulsion process, we quantified the water movements and displacements resulting from swimming in the green frog Rana esculenta, applying digital particle image velocimetry (DPIV) to the frog's wake. The wake showed two vortex rings left behind by the two feet. The rings appeared to be elliptic in planform, urging for correction of the observed ring radii. The rings' long and short axes (average ratio 1.75:1) were about the same size as the length and width of the propelling frog foot and the ellipsoid mass of water accelerated with it. Average thrust forces were derived from the vortex rings, assuming all propulsive energy to be compiled in the rings. The calculated average forces (F(av)=0.10+/-0.04 N) were in close agreement with our parallel study applying a momentum-impulse approach to water displacements during the leg extension phase. We did not find any support for previously assumed propulsion enhancement mechanisms. The feet do not clap together at the end of the power stroke and no "wedge-action" jetting is observed. Each foot accelerates its own water mantle, ending up in a separate vortex ring without interference by the other leg.

Propulsive force calculations in swimming frogs. I. A momentum-impulse approach.

Frogs are animals that are capable of locomotion in two physically different media, aquatic and terrestrial. A comparison of the kinematics of swimming frogs in a previous study revealed a difference in propulsive impulse between jumping and swimming. To explore this difference further, we determined the instantaneous forces during propulsion in swimming using an impulse-momentum approach based on DPIV flow data. The force profile obtained was compared with force profiles obtained from drag-thrust equilibrium of the centre of mass and with the force profiles generated during jumping. The new approach to quantifying the instantaneous forces during swimming was tested and proved to be a valid method for determining the external forces on the feet of swimming frogs. On the kinematic profiles of swimming, leg extension precedes propulsion. This means that it is not only the acceleration of water backwards that provides thrust, but also that the deceleration of water flowing towards the frog as a result of recovery accelerates the centre of mass prior to leg extension. The force profile obtained from the impulse-momentum approach exposed an overestimation of drag by 30% in the drag-thrust calculations. This means that the difference in impulse between jumping and swimming in frogs is even larger than previously stated. The difference between the force profiles, apart from a slightly higher peak force during jumping, lies mainly in a difference in shape. During swimming, maximal force is reached early in the extension phase, 20% into it, while during jumping, peak force is attained at 80% of the extension phase. This difference is caused by a difference in inter-limb coordination.

Riding the waves: the role of the body wave in undulatory fish swimming.

A continuously swimming mullet modulates its thrust production by changing slip-the ratio between its swimming speed U and the speed V with which the body wave travels down its body. This variation in thrust is reflected in the wake of the fish. We obtained 2-dimensional impressions of the wake behind a mullet swimming at a slip of 0.7 equivalent to active swimming, at a slip of 0.9 close to free-wheeling, and at a slip of 1.1 when the fish is braking. Independent of the slip, vortices are shed at the tail when the tail tip reaches its maximum lateral excursion. The manner in which the wake changes as it decays depends on the degree of slip: At a slip well below unity, the wake decays without any qualitative changes in shape, the medio-frontal cross section of the mature wake consists of a double row of alternating vortices separated by an undulating jet, and the angle between the jet flow and the mean path of motion is close to 45°; at a slip above unity, the vortices stretch out laterally and the mature wake resembles a single row of oval vortices with two vortex cores, and the jet between the vortices is almost perpendicular to the mean path of motion; the wake at slip of 0.9 exhibits a pattern intermediate between the wakes at slips 0.7 and 0.9 with slightly elongate vortices and a jet angle of 61°.

The scaling and structure of aquatic animal wakes.

Animal generated water movements are visualized and quantified using two-dimensional particle image velocimetry (PIV). The resulting vector flow fields allow for the study of the distribution of velocity, vorticity and vortices. Structural and temporal aspects of animal-induced flows covering a range of Reynolds (Re) numbers between less than 1 to more than 10(4) are presented.Maps of flow induced by continuous foraging and intermittent escape responses of tethered nauplius and copepodid stages of the marine copepod Temora longicornis offer insight in viscosity-dominated flow regimes. Fast escape responses of the equally sized largest nauplius stage and the smallest copepodid stage are compared. The nauplius moves by generating a viscous flow pattern with high velocities and vorticity; the copepodid moves by using inertial effects to produce a vortex ring with a rearward jet through the center.Larvae and small adult fish (zebra danio) use a burst-and-coast-swimming mode at Re numbers up to 6,000, shedding a vortex ring with the associated jet at the tail during the burst phase. Flow patterns during the coasting phase differ between the small larvae and larger adults due to the changes in importance of viscosity.A 12 cm long mullet swimming in a continuous mode generates a chain of vortex rings with a backward undulating jet through the centers of the rings at Re numbers of 4 × 10(4) in inertia-dominated regimes.Our empirical results provide realistic insight in the scale effects determining the morphology of the interactions between animals and water.

Copepod feeding currents: flow patterns, filtration rates and energetics.

Particle image velocimetry was used to construct a quasi 3-dimensional image of the flow generated by the feeding appendages of the calanoid copepod Temora longicornis. By scanning layers of flow, detailed information was obtained on flow velocity and velocity gradients. The flow around feeding T. longicornis was laminar, and was symmetrical viewed dorsally, but highly asymmetrical viewed laterally, with high levels of vorticity on the ventral side. The flow rate through the feeding appendages varied between 77 and 220 ml day(-1) per individual. The morphology of the flow field ensured that water was entrained over the full length of the first antennae. These were kept out of areas with high velocity gradients that could interfere with distant mechano- or chemoreception. The volume of influence, i.e. the volume of water around the foraging copepod, where shear rates were significantly higher than background levels, was calculated. Implications for encounter probability and mechanoreception are discussed. The average rate of energy dissipation within the copepod's volume of influence is several times higher than the levels of turbulent energy dissipation these animals are likely to encounter in their environment. Even in highly turbulent environments, adult T. longicornis will not experience very significant effects of turbulence. Within the volume of influence of the copepods the energy dissipation due to viscous friction varied between 6.6 x 10(-11) and 2.3 x 10(-10)W. Taking mechanical efficiency and muscle efficiency into account, this results in a total energetic cost of the feeding current of 1.6 x 10(-9)W per copepod. This value represents only a small percentage of the total energy budget of small calanoid copepods.