Fish swimming mode and body morphology affect the energetics of swimming in a wave-surge water flow

Fish swimming modes and the shape of both the fins and body are expected to affect their swimming ability under different flow conditions. These swimming strategies and body morphologies often correspond to distributional patterns of distinct functional groups exposed to natural and variable water flows. In this study, we used a swimming-respirometer to measure energetic costs during prolonged, steady swimming and while station holding in a range of simulated oscillatory wave-surge water flows, within the natural range of flow speeds and wave frequencies on coral reefs. We quantified the net cost of swimming (NCOS, metabolic costs above resting) for four reef fish species with differences in swimming mode and morphologies of the fin and body: a body and caudal fin (BCF) swimmer, the Hawaiian flagtail, Kuhlia xenura, and three pectoral fin swimmers, the kole tang, Ctenochaetus strigosus, the saddle wrasse, Thalassoma duperrey, and the Indo-Pacific sergeant major, Abudefduf vaigiensis. We found that the BCF swimmer had the highest rates of increase in NCOS with increasing wave frequency (i.e. increased turning frequency) compared with the pectoral fin swimmers. The wrasse, with a more streamlined, higher body fineness, had lower rates of increase in NCOS with increasing swimming speeds than the low body fineness species, but overall had the highest swimming NCOS, which may be a result of a higher aerobic swimming capacity. The deep-bodied (low fineness) pectoral fin swimmers (A. vaigiensis and C. strigosus) were the most efficient at station holding in oscillating, wave-surge water flows.

Sticky, stickier and stickiest - a comparison of adhesive performance in clingfish, lumpsuckers and snailfish

The coastal waters of the North Pacific are home to the northern clingfish (Gobiesox maeandricus), Pacific spiny lumpsucker (Eumicrotremus orbis) and marbled snailfish (Liparis dennyi) - three fishes that have evolved ventral adhesive discs. Clingfish adhesive performance has been studied extensively, but relatively little is known about the performance of other sticky fishes. Here, we compared the peak adhesive forces and work to detachment of clingfish, lumpsuckers and snailfish on surfaces of varying roughness and over ontogeny. We also investigated the morphology of their adhesive discs through micro-computed tomography scanning and scanning electron microscopy. We found evidence that adhesive performance is tied to the intensity and variability of flow regimes in the fishes' habitats. The northern clingfish generates the highest adhesive forces and lives in the rocky intertidal zone where it must resist exposure to crashing waves. Lumpsuckers and snailfish both generate only a fraction of the clingfish's adhesive force, but live more subtidal where currents are slower and less variable. However, lumpsuckers generate more adhesive force relative to their body weight than snailfish, which we attribute to their higher-drag body shape and frequent bouts into the intertidal zone. Even so, the performance and morphology data suggest that snailfish adhesive discs are stiffer and built more efficiently than lumpsucker discs. Future studies should focus on sampling additional diversity and designing more ecologically relevant experiments when investigating differences in adhesive performance.

Asymmetrical Oscillating Morphology Hydrodynamic Performance of a Novel Bionic Pectoral Fin

This research proposes a novel bionic pectoral fin and experimentally studied the effects of the oscillation parameters on the hydrodynamic performance of a bionic experimental prototype. Inspired by manta rays, the bionic pectoral fin was simplified and modeled based on the natural pectoral fin skeleton structure and oscillation morphology of this underwater creature. A dual-degree-of-freedom bionic pectoral fin was designed. The active spatial motion was realized by the space six-link mechanism driven by two motors, and the passive deformation was achieved by carbon fiber. The motion analysis of the bionic pectoral fin proves that the pectoral fin can realize an “8”-shaped spatial trajectory. An experimental prototype was developed accordingly. The experimental prototype could flap between 0.1 Hz and 0.6 Hz and produce a maximum thrust of 20 N. The hydrodynamic performance under different oscillation parameters was studied experimentally in a water pool. The experimental results indicate that the hydrodynamic performance of the pectoral fin oscillation is closely related to the motion equation parameters including the amplitude, frequency, phase difference, and initial bias. In addition to considering the impact of parameters on thrust and lift, the influences of asymmetrical oscillation on the position of the equivalent point were also studied. The results show that the pectoral fin proposed in this research exhibited the expected spatial deformation and outstanding hydrodynamic performance. The obtained results shed light on the updated design and control of a bionic robot fish.

Weak Relationships Between Swimming Morphology and Water Depth in Wrasses and Parrotfish Belie Multiple Selective Demands on Form-Function Evolution

Mechanical tradeoffs in performance are predicted to sculpt macroevolutionary patterns of morphological diversity across environmental gradients. Water depth shapes the amount of wave energy organisms' experience, which should result in evolutionary tradeoffs between speed and maneuverability in fish swimming morphology. Here, we tested whether morphological evolution would reflect functional tradeoffs in swimming performance in 131 species of wrasses and parrotfish (Family: Labridae) across a water depth gradient. We found that maximum water depth predicts variation in pectoral fin aspect ratio (AR) in wrasses, but not in parrotfish. Shallow-water wrasses exhibit wing-like pectoral fins that help with "flapping," which allows more efficient swimming at faster speeds. Deeper water species, in contrast, exhibit more paddle-like pectoral fins associated with enhanced maneuverability at slower speeds. Functional morphology responds to a number of different, potentially contrasting selective pressures. Furthermore, many-to-one mapping may release some traits from selection on performance at the expense of others. As such, deciphering the signatures of mechanical tradeoffs on phenotypic evolution will require integrating multiple aspects of ecological and morphological variation. As the field of evolutionary biomechanics moves into the era of big data, we will be uniquely poised to disentangle the intrinsic and extrinsic predictors of functional diversity.

Theoretical and numerical studies on a five-ray flexible pectoral fin during labriform swimming

Natural fish have evolved with an excellent swimming performance after millions of years. Based on the flexible features of the pectoral fin, this paper focuses on the kinematics and hydrodynamics of the fin when fish are swimming stably in still water in labriform mode. The locomotion mechanism based on the morphology of the pectoral fin is applied to establish a kinematic model composed of five rays and membranes, which is adopted to control the pectoral fin to reach deformation in approximately the same way as the labriform mode. A semi-empirical theoretical model based on the kinematics is proposed to calculate the hydrodynamic force. In order to study the flow field, the numerical simulation of fluid-structure interaction is carried out and the results are validated by the present semi-empirical model, which also verifies the feasibility of the semi-empirical theoretical model for describing the dynamics of the pectoral fin under a complex water environment. In addition, the relationship between propulsion performance and locomotion parameters (e.g. frequency of motion, amplitude of flapping and rowing angle, and phase lag between flapping and rowing) of the multi-degree of freedom flexible pectoral fin is also revealed. It is found that the frequency and amplitude of the flapping angle have a significant influence on the hydrodynamic thrust, while the rowing angle and phase lag have little effect. The established models and the results provide effective tools and significant reference for the design of bionic pectoral fins.

Scythes, sickles and other blades: defining the diversity of pectoral fin morphotypes in Pachycormiformes

The traditional terminology of ‘scythe’ or ‘sickle’ shaped is observed to be flawed as an effective descriptor for pectoral fin shape in pachycormids. The diversity of pachycormid pectoral fin shapes is assessed across the 14 recognised genera that preserve complete pectoral fins, and improved terms are defined to more effectively describe their form, supported by anatomical observation and aspect ratio analysis of individual fins, and corroborated by landmark analysis. Three clear and distinct pectoral fin structural morphotypes emerge (falceform, gladiform, falcataform), reflecting a diversity of pachycormid lifestyles throughout the Mesozoic, from agile pursuit predator to slow-cruising suspension feeder.

Fins as Mechanosensors for Movement and Touch-Related Behaviors

Mechanosensation is a universal feature of animals that is essential for behavior, allowing detection of animals' own body movement and position as well as physical characteristics of the environment. The extraordinary morphological and behavioral diversity that exists across fish species provide rich opportunities for comparative mechanosensory studies in fins. The fins of fishes have been found to function as proprioceptors, by providing feedback on fin ray position and movement, and as tactile sensors, by encoding pressures applied to the fin surface. Across fish species, and among fins, the afferent response is remarkably consistent, suggesting that the ability of fin rays and membrane to sense deformation is a fundamental feature of fish fins. While fin mechanosensation has been known in select, often highly specialized, species for decades, only in the last decade have we explored mechanosensation in typical propulsive fins and considered its role in behavior, particularly locomotion. In this paper, we synthesize the current understanding of the anatomy and physiology of fin mechanosensation, looking toward key directions for research. We argue that a mechanosensory perspective informs studies of fin-based propulsion and other fin-driven behaviors and should be considered in the interpretation of fin morphology and behavior. In addition, we compare the mechanosensory system innervating the fins of fishes to the systems innervating the limbs of mammals and wings of insects in order to identify shared mechanosensory strategies and how different organisms have evolved to meet similar functional challenges. Finally, we discuss how understanding the biological organization and function of fin sensors can inform the design of control systems for engineered fins and fin-driven robotics.

Integration and modularity of teleostean pectoral fin shape and its role in the diversification of acanthomorph fishes

Phenotypic integration and modularity describe the strength and pattern of interdependencies between traits. Integration and modularity have been proposed to influence the trajectory of evolution, either acting as constraints or facilitators. Here, we examine trends in the integration and modularity of pectoral fin morphology in teleost fishes using geometric morphometrics. We compare the fin shapes of the highly diverse radiation of acanthomorph fishes to lower teleosts. Integration and modularity are measured using two-block partial least squares analysis and the covariance ratio coefficient between the radial bones and lepidotrichia of the pectoral fins. We show that the fins of acanthomorph fishes are more tightly integrated but also more morphologically diverse and faster evolving compared to nonacanthomorph fishes. The main pattern of shape covariation in nonacanthomorphs is concordant with the main trajectory of evolution between nonacanthomorphs and acanthomorphs. Our findings support a facilitating role for integration during the acanthomorph diversification. Potential functional consequences and developmental mechanisms of fin integration are discussed.