Aeration rates and tank geometry result in different larvae survival rates [1]-[6]. This study reports new results about the flow field in a rearing tank. Specifically, the effects of the aspect ratio AR (the ratio of water depth to tank radius) on overall flow fields were clarified from flow visualization and PIV (Particle Image Velocimetry) experiments. Two different types of flow patterns, a single-pair vortex system and two-pair vortex system, were confirmed as the value of tank aspect ratio changed from 1.0 to 2.0.

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In a computational model of hydrodynamics based on the Multiple Collision Dynamics modelling technique, we investigate hydrodynamic advantages of fish swimming in an infinite school. We show that the average Froude efficiency of individuals swimming in a school is higher than of individuals swimming solitarily for each of four spatial configurations (line, phalanx, rectangular and diamond-shaped). These benefits may emerge from several kinds of interactions among wakes and individuals.

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Mammalian sperm swim in fluids containing proteins and cellular debris that may increase resistance to flow. We explore how swimming speeds and trajectories of sperm with symmetric and asymmetric waveforms are affected by this enhanced resistance using a Brinkman model. The effective flow is determined using regularized fundamental solutions and we represent the flagellum as an elastic rod with forces from a preferred curvature model. For certain ranges of stiffness parameters, we can see an increase in swimming speed for the finite length swimmer with a symmetric waveform. The path curvature decreases for an asymmetric waveform as resistance increases.

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A biased one-dimensional random walk model is proposed. This model adopts biased rules that include the features of bacterial chemo-taxis. In this model, a model cell moves along a discretized number line sensing whether it has approached or receded from the origin where a chemical attractant exists. A steady probability distribution function of the cell's existence is analytically obtained and it is confirmed by numerical simulations. The biased rules introduce advection effect into random diffusive motion: the probability distribution indicates model cells' accumulation around the origin, which corresponds to the spatial migration of bacterial cells around a chemical attractant.

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Underwater propulsion that leaves little traceable wake structure while producing high levels of thrust is a highly desired goal. A potential biological model is the California sea lion, a highly maneuverable aquatic mammal that produces thrust primarily with its foreflippers without a characteristic flapping frequency. In an effort to mimic the sea lion flipper, a flipper from a sea lion carcass has been scanned using several non invasive methods with the goal of manufacturing a robotic flipper with identical geometry. The scanning methods are hereby compared for overall quality, accuracy, and level of detail captured.

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To elucidate the characteristics of flow in the collective motion of bacteria, we performed microscopic observation of the cells cultured on a semi-fluid medium. The collective motion occurs when the average cell-cell distance decreases to 5 μm, almost the same as the size of the cell. The standard deviation of the angle between velocity vectors represents the degree of collective motion well. The characteristic size of the swarming group is deduced by the analysis of the correlation with respect to the direction parallel and perpendicular to the swimming direction; 30 μm in length and 10 μm in width.

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Some semiterrestrial beach fleas are capable of swimming movement using their pleopods in water. Movements of swimming organ in beach flea were examined by high-speed video camera system. The structural properties of terrestrial beach flea pleopods were studied through the measurements of some morphological parameters. The swimming legs of the beach flea were examined with a color 3D laser scanning microscope. The relation between morphology of pleopod and motility of the beach flea was considered.

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Wing of bird is a sophisticated organ for flying in various conditions. One typical example is a primary feather that is supposed to attenuate the wing tip vortex and thus contribute to reduce the induced drag. Although its great availability for the flight, the elasticity of feather causes the problem of flutter due to the coupling of the aerodynamic and the elastic forces. Interestingly, flutters are hardly observed in the primary feathers of flying birds. The reflection in the primary feather is supposed to be the key factor of flutter inhibition. The wind tunnel tests with feather models composed of various types of reflection confirmed that the reflection effectively inhibited the flutter generation.

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The objective of this study was to investigate the swimming movement of an octopus-inspired propulsion mechanism. First, a simulation model for the mechanism was constructed. After a simulation based on observation data, the optimization of the arm stroke was carried out. In the simulation based on the observation data, successful propulsion was confirmed. If large stroke amplitudes were allowed in the optimization, the arm tips bent forward in the late recovery stroke. If large joint torques were allowed in the optimization, the arms were opened widely in the recovery stroke.

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Many kinds of flapping small insects are able to fly stably and hover. Many researchers have studied the mechanism of complex flapping flight ability. In this paper, an experimental device with a flapping mechanism, which was allowed to generate both roll and yaw moments simultaneously, was developed and evaluated. The results indicated the possibilities of controlling the roll moment, yaw moment, and lift. Therefore, we assumed that the flapping mechanism with the proposed mechanism had the possibility of achieving postural control with the fewest actuators.

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Bovine sperm motion in environments of various viscosities is studied. We used the semen of Japanese cattle and investigated the following parameters: the sperm velocity, the straight-line velocity, and the amplitude from the observed sperm trajectory. As the viscosity increased, the motility of the sperm decreased. On the other hand, the power expended by the sperm flagellum was of the same order of magnitude over the viscosity range of 0.0007-0.0226 Pa·s. Additionally, the increase in the viscosity brought about a change in the flagellum shape and an increase in the percentage of sperm with a nonrotating head. The existence of rotation caused a change in the sperm velocity, amplitude, and frequency of the flagellum. These results suggest that bovine sperm has evolved to swim effectively in the oviduct, which is a high-viscosity environment.

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The present paper describes new flexible ciliary actuators made of thin urethane rubber, to imitate the hydraulic skeleton found in soft creatures, such as earthworms. Several trial actuators are manufactured while varying the number of fluid chambers from 1 to 3. These actuators are driven by fluid pressure and different methods of posture restoration. A new multiple two-way ciliary actuator sheet with an embedded multi-layered fluid circuit is proposed. A pipe inspection robot of 27 mm in height with ciliary actuator arrays moves at 5.4 mm/s. A new ciliary actuator with a z-shaped body is also discussed.

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A constrained swimming test showed that at fastest flow speeds, the tail-beat frequencies (TBF) of followers in a school of Japanese chub mackerels were significantly lower than those of the leading fish. The gross cost of transport (GCOT) of schooling fish was 4.8-6.6 J·kg^-1·m^-1, whereas that of solitary swimming fish was significantly larger (9.2-11.5 J·kg^-1·m^-1). To examine the association between the configuration of individuals in the school and energy-saving, the three-dimensional positions of individuals in a school were measured in a large tank. Although the school was not arranged in the diamond pattern known to be most effective for energy conservation, the TBF of the followers remarkably decreased when within the range of 1.4 BL (Body Length) from the leading fish in the school. The energy gain of schooling fish was greater than that derived from the potential-theory basis, implying a potential hydrodynamic synergy effect.

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