Contrasting thrust generation mechanics and energetics of flapping foil locomotory states characterized by a unified - scaling

Self-propelled flapping foils with distinct locomotion-enabling kinematic restraints exhibit a remarkably similar Strouhal number ( $St$ )-Reynolds number ( $Re$ ) dependence. This similarity has been hypothesized to pervade diverse forms of oscillatory self-propulsion and undulatory biolocomotion; however, its genesis and implications on the energetic cost of locomotion remain elusive. Here, using high-resolution simulations of translationally free and restrained foils that self-propel as they are pitched, we demonstrate that a generality in the $St$ - $Re$ relationship can emerge despite significant disparities in thrust generation mechanics and locomotory performance. Specifically, owing to a recoil reaction induced passive heave, the fluid's inertial response to the prescribed rotational pitch, the principal source of thrust in unidirectionally free and towed configurations, ceases to produce thrust in a bidirectionally free configuration. Rather, the thrust generated from the leading edge suction mechanics self-propels a bidirectionally free pitching foil. Owing to the foregoing distinction in the thrust generation mechanics, the $St$ - $Re$ relationships for the bidirectionally and unidirectionally free/towed foils are dissimilar and pitching amplitude dependent, but specifically for large reduced frequencies, converge to a previously reported unified power law. Importantly, to propel at a given mean forward speed, the bidirectionally free foil must counteract the out-of-phase passive heave through a more intense rotational pitch, resulting in an appreciably higher power consumption over the range $10 \leq Re \leq 10^3$ . We highlight the critical role of thrust in introducing an offset in the $St$ - $Re$ relation, and through its amplification, being ultimately responsible for the considerable disparity in the locomotory performance of differentially constrained foils.

Influence of an L-type SALMFamide neuropeptide on locomotory performance and muscle physiology in the sea cucumber Apostichopus japonicus

ABSTRACT Neuropeptides in the SALMFamide family serve as muscle relaxants in echinoderms and may affect locomotion, as the motor behavior in sea cucumbers involves alternating contraction and extension of the body wall, which is under the control of longitudinal muscle. We evaluated the effect of an L-type SALMFamide neuropeptide (LSA) on locomotory performance of Apostichopus japonicus. We also investigated the metabolites of longitudinal muscle tissue using ultra performance liquid chromatography and quadrupole time-of-flight mass spectrometry (UPLC-Q-TOF-MS) to assess the potential physiological mechanisms underlying the effect of LSA. The hourly distance, cumulative duration and number of steps moved significantly increased in sea cucumbers in the fourth hour after injection with LSA. Also, the treatment enhanced the mean and maximum velocity by 9.8% and 17.8%, respectively, and increased the average stride by 12.4%. Levels of 27 metabolites in longitudinal muscle changed after LSA administration, and the increased concentration of pantothenic acid, arachidonic acid and lysophosphatidylethanolamine, and the altered phosphatidylethanolamine/phosphatidylcholine ratio are potential physiological mechanisms that could explain the observed effect of LSA on locomotor behavior in A. japonicus.

Work that body: fin and body movements determine herbivore feeding performance within the natural reef environment

Herbivorous fishes form a keystone component of reef ecosystems, yet the functional mechanisms underlying their feeding performance are poorly understood. In water, gravity is counter-balanced by buoyancy, hence fish are recoiled backwards after every bite they take from the substrate. To overcome this recoil and maintain contact with the algae covered substrate, fish need to generate thrust while feeding. However, the locomotory performance of reef herbivores in the context of feeding has hitherto been ignored. We used a three-dimensional high-speed video system to track mouth and body kinematics during in situ feeding strikes of fishes in the genus Zebrasoma , while synchronously recording the forces exerted on the substrate. These herbivores committed stereotypic and coordinated body and fin movements when feeding off the substrate and these movements determined algal biomass removed. Specifically, the speed of rapidly backing away from the substrate was associated with the magnitude of the pull force and the biomass of algae removed from the substrate per feeding bout. Our new framework for measuring biting performance in situ demonstrates that coordinated movements of the body and fins play a crucial role in herbivore foraging performance and may explain major axes of body and fin shape diversification across reef herbivore guilds.

Size-dependent locomotory performance creates a behaviorally mediated prey size refuge in the marine snail Olivella semistriata: a study in the natural habitat

The effects of the variability of individual prey locomotory performance on the vulnerability to predation are poorly understood, partly because individual performance is difficult to determine in natural habitats. To gain insights into the role(s) of individual variation in predatory relationships, we study a convenient model system, the neotropical sandy beach gastropod Olivella semistriata and its main predator, the carnivorous snail Agaronia propatula. The largest size class of O. semistriata is known to be missing from A. propatula’s spectrum of subdued prey, although the predator regularly captures much larger individuals of other taxa. To resolve this conundrum, we analyzed predation attempts in the wild. While A. propatula attacked O. semistriata of all sizes, large prey specimens usually escaped by ‘sculling’, an accelerated, stepping mode of locomotion. Olivella semistriata performed sculling locomotion regardless of size, but sculling velocities determined in the natural environment increased strongly with size. Thus, growth in size as such does not establish a prey size refuge in which O. semistriata is safe from predation. Rather, a behaviorally mediated size refuge is created through the size-dependence of sculling performance. Taken together, this work presents a rare quantitative characterization in the natural habitat of the causal sequence from the size-dependence of individual performance, to the prey size-dependent outcome of predation attempts, to the size bias in the predator’s prey spectrum.

Direct measurement of swimming and diving kinematics of giant Atlantic bluefin tuna ( Thunnus thynnus )

Tunas possess a range of physiological and mechanical adaptations geared towards high-performance swimming that are of considerable interest to physiologists, ecologists and engineers. Advances in biologging have provided significant improvements in understanding tuna migrations and vertical movement patterns, yet our understanding of the locomotion and swimming mechanics of these fish under natural conditions is limited. We equipped Atlantic bluefin tuna ( Thunnus thynnus ) with motion-sensitive tags and video cameras to quantify the gaits and kinematics used by wild fish. Our data reveal significant variety in the locomotory kinematics of Atlantic bluefin tuna, ranging from continuous locomotion to two types of intermittent locomotion. The tuna sustained swimming speeds in excess of 1.5 m s −1 (0.6 body lengths s −1 ), while beating their tail at a frequency of approximately 1 Hz. While diving, some descents were entirely composed of passive glides, with slower descent rates featuring more gliding, while ascents were primarily composed of active swimming. The observed swimming behaviour of Atlantic bluefin tuna is consistent with theoretical models predicting such intermittent locomotion to result in mechanical and physiological advantages. Our results confirm that Atlantic bluefin tuna possess behavioural specializations to increase their locomotory performance, which together with their unique physiology improve their capacity to use pelagic and mesopelagic habitats.

Distribution, composition and functions of gelatinous tissues in deep-sea fishes

Many deep-sea fishes have a gelatinous layer, or subdermal extracellular matrix, below the skin or around the spine. We document the distribution of gelatinous tissues across fish families (approx. 200 species in ten orders), then review and investigate their composition and function. Gelatinous tissues from nine species were analysed for water content (96.53 ± 1.78% s.d.), ionic composition, osmolality, protein (0.39 ± 0.23%), lipid (0.69 ± 0.56%) and carbohydrate (0.61 ± 0.28%). Results suggest that gelatinous tissues are mostly extracellular fluid, which may allow animals to grow inexpensively. Further, almost all gelatinous tissues floated in cold seawater, thus their lower density than seawater may contribute to buoyancy in some species. We also propose a new hypothesis: gelatinous tissues, which are inexpensive to grow, may sometimes be a method to increase swimming efficiency by fairing the transition from trunk to tail. Such a layer is particularly prominent in hadal snailfishes (Liparidae); therefore, a robotic snailfish model was designed and constructed to analyse the influence of gelatinous tissues on locomotory performance. The model swam faster with a watery layer, representing gelatinous tissue, around the tail than without. Results suggest that the tissues may, in addition to providing buoyancy and low-cost growth, aid deep-sea fish locomotion.

Optimal shape and motion of undulatory swimming organisms

Undulatory swimming animals exhibit diverse ranges of body shapes and motion patterns and are often considered as having superior locomotory performance. The extent to which morphological traits of swimming animals have evolved owing to primarily locomotion considerations is, however, not clear. To shed some light on that question, we present here the optimal shape and motion of undulatory swimming organisms obtained by optimizing locomotive performance measures within the framework of a combined hydrodynamical, structural and novel muscular model. We develop a muscular model for periodic muscle contraction which provides relevant kinematic and energetic quantities required to describe swimming. Using an evolutionary algorithm, we performed a multi-objective optimization for achieving maximum sustained swimming speed U and minimum cost of transport (COT)—two conflicting locomotive performance measures that have been conjectured as likely to increase fitness for survival. Starting from an initial population of random characteristics, our results show that, for a range of size scales, fish-like body shapes and motion indeed emerge when U and COT are optimized. Inherent boundary-layer-dependent allometric scaling between body mass and kinematic and energetic quantities of the optimal populations is observed. The trade-off between U and COT affects the geometry, kinematics and energetics of swimming organisms. Our results are corroborated by empirical data from swimming animals over nine orders of magnitude in size, supporting the notion that optimizing U and COT could be the driving force of evolution in many species.