Hydrodynamics of linear acceleration in bluegill sunfish, Lepomis macrochirus

Integration of Metabolomics and Transcriptomics to Reveal the Metabolic Characteristics of Exercise-Improved Bone Mass

(1) Background: Exercise is effective in promoting and maintaining bone mass. The aim of this study was to detect the exercise-induced metabolic changes in bone tissue of zebrafish. (2) Methods: Thirty-eight zebrafish (Danio rerio, six months old) were analyzed. The exercise group (n = 19) received 8 weeks of counter-current swimming training. The control group (n = 19) was not subjected to exercise. Mineralization was quantified, and alkaline phosphatase (Alp) and anti-tartrate acid phosphatase (Trap) activities were estimated (n = 12). The metabolomics (n = 12) and transcriptomics (n = 14) data of bone tissue were used for the integration analyses. (3) Results: The results showed that the exercise training improved the bone mineralization of zebrafish, e.g., the exercise group (5.74 × 104 ± 7.63 × 103) had a higher mean optical density than the control group (5.26 × 104 ± 8.56 × 103, p = 0.046) for the caudal vertebrae. The amount of mineralized matrix in scales of the exercised zebrafish was also higher (0.156 ± 0.012 vs. 0.102 ± 0.003, p = 0.005). Both histological staining and biochemical analysis revealed increased Alp activity (0.81 ± 0.26 vs. 0.76 ± 0.01, p = 0.002) and decreased Trap activity (1.34 ± 0.01 vs. 1.36 ± 0.01, p = 0.005) in the exercise group. A total of 103 different metabolites (DMs, VIP ≥ 1, fold change (FC) ≥ 1.20 or ≤0.83, p < 0.050) were identified. Alanine, aspartate and glutamate metabolism, β-alanine metabolism, pyrimidine metabolism, and pantothenate and CoA biosynthesis were the significantly enriched metabolic pathways (p < 0.050). A total of 35 genes (q ≤ 0.050 (BH), |Log2FC| ≥ 0.5) were coenriched with the 103 DMs in the four identified pathways. Protein–protein interaction network analysis of the 35 genes showed that entpd3, entpd1, and cmpk2 were the core genes. (4) Conclusions: The results of this study suggest that alanine, aspartate and glutamate metabolism, β-alanine metabolism, pyrimidine metabolism, and pantothenate and CoA biosynthesis contributed to exercise-induced improvements in bone mass.

The Kinematics and Dynamics of Schizopygopsis malacanthus Swimming during Ucrit Testing

The swimming kinematics (how fish move) and dynamics (how forces effect movement) of Schizopygopsis malacanthus were investigated during the determination of Ucrit by stepped velocity testing. A video tracking program was used to record and analyze the motion of five test fish in a Brett-type flume during each velocity step. The findings fell into three groups: (1) Even when flow was uniform, fish did not swim steadily, with speeds fluctuating by 2.2% to 8.4% during steady swimming. The proportion of unsteady swimming time increased with water velocity, and defining steady and unsteady swimming statistically, in terms of the definition of standard deviation of instantaneous displacements, may have higher accuracy. (2) In steady swimming, the forward velocity and acceleration of fish were correlated with body length (p < 0.05), but in unsteady swimming the correlations were not significant. The maximum swimming speed (1.504 m/s) and acceleration (16.54 m/s2) occurred during unsteady swimming, but these measurements may not be definitive because of tank space constraints on fish movement and the passive behavior of the test fish with respect to acceleration. (3) Burst-coast swimming in still water, investigated by previous scholars as an energy conserving behavior, is not the same as the gait transition from steady to unsteady swimming in flowing water. In this study, the axial force of fish swimming in the unsteady mode was significantly higher (×1.2~1.6) than in the steady mode, as was the energy consumed (×1.27~3.33). Thus, gait transition increases, rather than decreases, energy consumption. Our characterization of the kinematics and dynamics of fish swimming provides important new information to consider when indices of swimming ability from controlled tank testing are applied to fish passage design.

Streamwise and lateral maneuvers of a fish-inspired hydrofoil

Fish are highly maneuverable compared to human-made underwater vehicles. Maneuvers are inherently transient, so they are often studied via observations of fish and fish-like robots, where their dynamics cannot be recorded directly. To study maneuvers in isolation, we designed a new kind of wireless carriage whose air bushings allow a hydrofoil to maneuver semi-autonomously in a water channel. We show that modulating the hydrofoil's frequency, amplitude, pitch bias, and stroke speed ratio (pitching speed of left vs right stroke) produces streamwise and lateral maneuvers with mixed effectiveness. Modulating pitch bias, for example, produces quasi-steady lateral maneuvers with classic reverse von Kármán wakes, whereas modulating the stroke speed ratio produces sudden yaw torques and vortex pairs like those observed behind turning zebrafish. Our findings provide a new framework for considering in-plane maneuvers and streamwise/lateral trajectory corrections in fish and fish-inspired robots.

Tuna robotics: hydrodynamics of rapid linear accelerations

Fish routinely accelerate during locomotor manoeuvres, yet little is known about the dynamics of acceleration performance. Thunniform fish use their lunate caudal fin to generate lift-based thrust during steady swimming, but the lift is limited during acceleration from rest because required oncoming flows are slow. To investigate what other thrust-generating mechanisms occur during this behaviour, we used the robotic system termed Tunabot Flex, which is a research platform featuring yellowfin tuna-inspired body and tail profiles. We generated linear accelerations from rest of various magnitudes (maximum acceleration of [Formula: see text] at [Formula: see text] tail beat frequency) and recorded instantaneous electrical power consumption. Using particle image velocimetry data, we quantified body kinematics and flow patterns to then compute surface pressures, thrust forces and mechanical power output along the body through time. We found that the head generates net drag and that the posterior body generates significant thrust, which reveals an additional propulsion mechanism to the lift-based caudal fin in this thunniform swimmer during linear accelerations from rest. Studying fish acceleration performance with an experimental platform capable of simultaneously measuring electrical power consumption, kinematics, fluid flow and mechanical power output provides a new opportunity to understand unsteady locomotor behaviours in both fishes and bioinspired aquatic robotic systems.

Airfoil-like mechanics generate thrust on the anterior body of swimming fishes

The anterior body of many fishes is shaped like an airfoil turned on its side. With an oscillating angle to the swimming direction, such an airfoil experiences negative pressure due to both its shape and pitching movements. This negative pressure acts as thrust forces on the anterior body. Here, we apply a high-resolution, pressure-based approach to describe how two fishes, bluegill sunfish (Lepomis macrochirus Rafinesque) and brook trout (Salvelinus fontinalis Mitchill), swimming in the carangiform mode, the most common fish swimming mode, generate thrust on their anterior bodies using leading-edge suction mechanics, much like an airfoil. These mechanics contrast with those previously reported in lampreys-anguilliform swimmers-which produce thrust with negative pressure but do so through undulatory mechanics. The thrust produced on the anterior bodies of these carangiform swimmers through negative pressure comprises 28% of the total thrust produced over the body and caudal fin, substantially decreasing the net drag on the anterior body. On the posterior region, subtle differences in body shape and kinematics allow trout to produce more thrust than bluegill, suggesting that they may swim more effectively. Despite the large phylogenetic distance between these species, and differences near the tail, the pressure profiles around the anterior body are similar. We suggest that such airfoil-like mechanics are highly efficient, because they require very little movement and therefore relatively little active muscular energy, and may be used by a wide range of fishes since many species have appropriately shaped bodies.

Surprising simplicities and syntheses in limbless self-propulsion in sand

Animals moving on and in fluids and solids move their bodies in diverse ways to generate propulsion and lift forces. In fluids, animals can wiggle, stroke, paddle or slap, whereas on hard frictional terrain, animals largely engage their appendages with the substrate to avoid slip. Granular substrates, such as desert sand, can display complex responses to animal interactions. This complexity has led to locomotor strategies that make use of fluid-like or solid-like features of this substrate, or combinations of the two. Here, we use examples from our work to demonstrate the diverse array of methods used and insights gained in the study of both surface and subsurface limbless locomotion in these habitats. Counterintuitively, these seemingly complex granular environments offer certain experimental, theoretical, robotic and computational advantages for studying terrestrial movement, with the potential for providing broad insights into morphology and locomotor control in fluids and solids, including neuromechanical control templates and morphological and behavioral evolution. In particular, granular media provide an excellent testbed for a locomotion framework called geometric mechanics, which was introduced by particle physicists and control engineers in the last century, and which allows quantitative analysis of alternative locomotor patterns and morphology to test for control templates, optimality and evolutionary alternatives. Thus, we posit that insights gained from movement in granular environments can be translated into principles that have broader applications across taxa, habitats and movement patterns, including those at microscopic scales.

Red muscle activity in bluegill sunfish Lepomis macrochirus during forward accelerations

Fishes generate force to swim by activating muscles on either side of their flexible bodies. To accelerate, they must produce higher muscle forces, which leads to higher reaction forces back on their bodies from the environment. If their bodies are too flexible, the forces during acceleration could not be transmitted effectively to the environment, but fish can potentially use their muscles to increase the effective stiffness of their body. Here, we quantified red muscle activity during acceleration and steady swimming, looking for patterns that would be consistent with the hypothesis of body stiffening. We used high-speed video, electromyographic recordings, and a new digital inertial measurement unit to quantify body kinematics, red muscle activity, and 3D orientation and centre of mass acceleration during forward accelerations and steady swimming over several speeds. During acceleration, fish co-activated anterior muscle on the left and right side, and activated all muscle sooner and kept it active for a larger fraction of the tail beat cycle. These activity patterns are both known to increase effective stiffness for muscle tissue in vitro, which is consistent with our hypothesis that fish use their red muscle to stiffen their bodies during acceleration. We suggest that during impulsive movements, flexible organisms like fishes can use their muscles not only to generate propulsive power but to tune the effective mechanical properties of their bodies, increasing performance during rapid movements and maintaining flexibility for slow, steady movements.