Function of the Caudal Fin During Locomotion in Fishes: Kinematics, Flow Visualization, and Evolutionary Patterns1
01 Feb 2000-Integrative and Comparative Biology (Society for Integrative and Comparative Biology)-Vol. 40, Iss: 1, pp 101-122
TL;DR: In this article, three-dimensional kinematic analysis, and quantitative flow measurements in the wake of freely-swimming fishes using digital particle image velocimetry (DPIV) were applied to the function of the caudal fin during steady swimming in fishes.
Abstract: One of the most prominent characteristics of early vertebrates is the elongate caudal fin bearing fin rays. The caudal fin represents a fundamental design feature of vertebrates that predates the origin of jaws and is found in both agnathans and gnathostomes. The caudal fin also represents the most posterior region of the vertebrate axis and is the location where fluid, accelerated by movement of the body anteriorly, is shed into the surrounding medium. Despite the extensive fossil record of the caudal fin, the use of caudal characters for systematic studies, and the importance of tail function for understanding locomotor dynamics in fishes, few experimental studies have been undertaken of caudal fin function. In this paper I review two experimental approaches which promise to provide new insights into the function and evolution of the caudal fin: three-dimensional kinematic analysis, and quantitative flow measurements in the wake of freely-swimming fishes using digital particle image velocimetry (DPIV). These methods are then applied to the function of the caudal fin during steady swimming in fishes with heterocercal and homocercal morphologies: chondrichthyians (leopard sharks) and ray-fined fishes (sturgeon and bluegill sunfish). The caudal fin of leopard sharks functions in a manner consistent with the classical model of heterocercal tail function in which the caudal surface moves at an acute angle to the horizontal plane, and hence is expected to generate lift forces and torques which must be counteracted anteriorly by the body and pectoral fins. An alternative model in which the shark tail produces a reactive force that acts through the center of mass is not supported. The sturgeon heterocercal tail is extremely flexible and the upper tail lobe trails the lower during the fin beat cycle. The sturgeon tail does not function according to the classical model of the heterocercal tail, and is hypothesized to generate reactive forces oriented near the center of mass of the body which is tilted at an angle to the flow during steady locomotion. Functional analysis of the homocercal tail of bluegill shows that the dorsal and ventral lobes do not function symmetrically as expected. Rather, the dorsal lobe undergoes greater lateral excursions and moves at higher velocities than the ventral lobe. The surface of the dorsal lobe also achieves a significantly acute angle to the horizontal plane suggesting that the homocercal tail of bluegill generates lift during steady swimming. These movements are actively generated by the hypochordal longitudinalis muscle within the tail. This result, combined with DPIV flow visualization data, suggest a new hypothesis for the function of the homocercal tail: the homocercal tail generates tilted and linked vortex rings with a central jet inclined posteroventrally, producing an anterodorsal reactive force on the body which generates lift and torque in the manner expected of a heterocercal tail. These results show that the application of new techniques to the study of caudal fin function in fishes reveals a previously unknown diversity of homocercal and heterocercal tail function, and that morphological characterizations of caudal fins do not accurately reflect in vivo function.
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TL;DR: The vortex wake shed by the tail differs between eel-like fishes and fishes with a discrete narrowing of the body in front of the tail, and three-dimensional effects may play a major role in determining wake structure in most fishes.
Abstract: What mechanisms of flow control do animals use to enhance hydrodynamic performance? Animals are capable of manipulating flow around the body and appendages both passively and actively. Passive mechanisms rely on structural and morphological components of the body (i.e., humpback whale tubercles, riblets). Active flow control mechanisms use appendage or body musculature to directly generate wake flow structures or stiffen fins against external hydrodynamic loads. Fish can actively control fin curvature, displacement, and area. The vortex wake shed by the tail differs between eel-like fishes and fishes with a discrete narrowing of the body in front of the tail, and three-dimensional effects may play a major role in determining wake structure in most fishes.
684 citations
TL;DR: In this article, a review of the basic mechanisms of force production and flow manipulation in oscillating foils for underwater use is presented, focusing primarily on experimental studies on some of the, at least partially understood, mechanisms, which include the formation of streets of vortices around and behind two-and three-dimensional propulsive oscillating flapping foils.
Abstract: Significant progress has been made in understanding some of the basic mechanisms of force production and flow manipulation in oscillating foils for underwater use. Biomimetic observations, however, show that there is a lot more to be learned, since many of the functions and details of fish fins remain unexplored. This review focuses primarily on experimental studies on some of the, at least partially understood, mechanisms, which include 1) the formation of streets of vortices around and behind two- and three-dimensional propulsive oscillating foils; 2) the formation of vortical structures around and behind two- and three-dimensional foils used for maneuvering, hovering, or fast-starting; 3) the formation of leading-edge vortices in flapping foils, under steady flapping or transient conditions; 4) the interaction of foils with oncoming, externally generated vorticity; multiple foils, or foils operating near a body or wall.
442 citations
TL;DR: The hydrodynamics of American eels swimming steadily at 1.4 L s-1 are examined and it is inferred that the lack of downstream flow results from a spatial and temporal balance of momentum removal and thrust generated along the body, due to the relatively uniform shape of eels.
Abstract: Eels undulate a larger portion of their bodies while swimming than many other fishes, but the hydrodynamic consequences of this swimming mode are poorly understood. In this study, we examine in detail the hydrodynamics of American eels (Anguilla rostrata) swimming steadily at 1.4 L s(-1) and compare them with previous results from other fishes. We performed high-resolution particle image velocimetry (PIV) to quantify the wake structure, measure the swimming efficiency, and force and power output. The wake consists of jets of fluid that point almost directly laterally, separated by an unstable shear layer that rolls up into two or more vortices over time. Previously, the wake of swimming eels was hypothesized to consist of unlinked vortex rings, resulting from a phase offset between vorticity distributed along the body and vorticity shed at the tail. Our high-resolution flow data suggest that the body anterior to the tail tip produces relatively low vorticity, and instead the wake structure results from the instability of the shear layers separating the lateral jets, reflecting pulses of high vorticity shed at the tail tip. We compare the wake structure to large-amplitude elongated body theory and to a previous computational fluid dynamic model and note several discrepancies between the models and the measured values. The wake of steadily swimming eels differs substantially in structure from the wake of previously studied carangiform fishes in that it lacks any significant downstream flow, previously interpreted as signifying thrust. We infer that the lack of downstream flow results from a spatial and temporal balance of momentum removal (drag) and thrust generated along the body, due to the relatively uniform shape of eels. Carangiform swimmers typically have a narrow caudal peduncle, which probably allows them to separate thrust from drag both spatially and temporally. Eels seem to lack this separation, which may explain why they produce a wake with little downstream momentum while carangiform swimmers produce a wake with a clear thrust signature.
389 citations
TL;DR: In this paper, the role of fin conformation in propulsion and maneuvering is investigated in a variety of fishes, including sharks, sturgeon, trout, sunfish, and surfperch.
Abstract: Over the past 520 million years, the process of evolution has produced a diversity of nearly 25000 species of fish. This diversity includes thousands of different fin designs which are largely the product of natural selection for locomotor performance. Fish fins can be grouped into two major categories: median and paired fins. Fins are typically supported at their base by a series of segmentally arranged bony or cartilaginous elements, and fish have extensive muscular control over fin conformation. Recent experimental hydrodynamic investigation of fish fin function in a diversity of freely swimming fish (including sharks, sturgeon, trout, sunfish, and surfperch) has demonstrated the role of fins in propulsion and maneuvering. Fish pectoral fins generate either separate or linked vortex rings during propulsion, and the lateral forces generated by pectoral fins are of similar magnitudes to thrust force during slow swimming. Yawing maneuvers involve differentiation of hydrodynamic function between left and right fins via vortex ring reorientation. Low-aspect ratio pectoral fins in sharks function to alter body pitch and induce vertical maneuvers through conformational changes of the fin trailing edge. The dorsal fin of fish displays a diversity of hydrodynamic function, from a discrete thrust-generating propulsor acting independently from the body, to a stabilizer generating only side forces. Dorsal fins play an active role in generating off-axis forces during maneuvering. Locomotor efficiency may be enhanced when the caudal fin intercepts the dorsal fin wake. The caudal fin of fish moves in a complex three-dimensional manner and evidence for thrust vectoring of caudal fin forces is presented for sturgeon which appear to have active control of the angle of vortices shed from the tail. Fish are designed to be unstable and are constantly using their control surfaces to generate opposing and balancing forces in addition to thrust. Lessons from fish for autonomous underwater vehicle (AUV) design include: 1) location of multiple control surfaces distributed widely about the center of mass, 2) design of control surfaces that have a high degree of three-dimensional motion through a flexible articulation with the body, 3) the ability to modulate fin surface conformation, and 4) the simultaneous use of numerous control surfaces including locating some fin elements in the downstream wake generated by other fins. The ability to manufacture an AUV that takes advantage of these design features is currently limited by the nature of available materials and mechanical drive trains. But future developments in polymer artificial muscle technology will provide a new approach to propulsor design that will permit construction of biomimetic propulsors with conformational and articulational flexibility similar to that of fish fins.
295 citations
TL;DR: Empirical evidence is presented that vortex structures generated by the soft dorsal fin upstream can constructively interact with those produced by the caudal fin downstream, and Reinforcement of circulation around the tail through interception of the dorsal fin's vortices is proposed as a mechanism for augmenting wake energy and enhancing thrust.
Abstract: A key evolutionary transformation of the locomotor system of ray-finned fishes is the morphological elaboration of the dorsal fin. Within Teleostei, the dorsal fin primitively is a single midline structure supported by soft, flexible fin rays. In its derived condition, the fin is made up of two anatomically distinct portions: an anterior section supported by spines, and a posterior section that is soft-rayed. We have a very limited understanding of the functional significance of this evolutionary variation in dorsal fin design. To initiate empirical hydrodynamic study of dorsal fin function in teleost fishes, we analyzed the wake created by the soft dorsal fin of bluegill sunfish (Lepomis macrochirus) during both steady swimming and unsteady turning maneuvers. Digital particle image velocimetry was used to visualize wake structures and to calculate in vivo locomotor forces. Study of the vortices generated simultaneously by the soft dorsal and caudal fins during locomotion allowed experimental characterization of median-fin wake interactions. During high-speed swimming (i.e. above the gait transition from pectoral- to median-fin locomotion), the soft dorsal fin undergoes regular oscillatory motion which, in comparison with analogous movement by the tail, is phase-advanced (by 30% of the cycle period) and of lower sweep amplitude (by 1.0 cm). Undulations of the soft dorsal fin during steady swimming at 1.1 bodylength s(-1) generate a reverse von Karman vortex street wake that contributes 12% of total thrust. During low-speed turns, the soft dorsal fin produces discrete pairs of counterrotating vortices with a central region of high-velocity jet flow. This vortex wake, generated in the latter stage of the turn and posterior to the center of mass of the body, counteracts torque generated earlier in the turn by the anteriorly positioned pectoral fins and thereby corrects the heading of the fish as it begins to translate forward away from the turning stimulus. One-third of the laterally directed fluid force measured during turning is developed by the soft dorsal fin. For steady swimming, we present empirical evidence that vortex structures generated by the soft dorsal fin upstream can constructively interact with those produced by the caudal fin downstream. Reinforcement of circulation around the tail through interception of the dorsal fin's vortices is proposed as a mechanism for augmenting wake energy and enhancing thrust. Swimming in fishes involves the partitioning of locomotor force among several independent fin systems. Coordinated use of the pectoral fins, caudal fin and soft dorsal fin to increase wake momentum, as documented for L. macrochirus, highlights the ability of teleost fishes to employ multiple propulsors simultaneously for controlling complex swimming behaviors.
250 citations
References
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Book•
11 Jun 2002TL;DR: In this paper, the authors present a practical guide for the planning, performance and understanding of experiments employing the PIV technique, which is primarily intended for engineers, scientists and students, who already have some basic knowledge of fluid mechanics and nonintrusive optical measurement techniques.
Abstract: This practical guide intends to provide comprehensive information on the PIV technique that in the past decade has gained significant popularity
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an internationally recognized expert. This book is primarily intended for engineers, scientists and students, who already have some basic
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Abstract: Preface Part I: Introduction The science of ichthyology Systematic procedures Part II: Form, Function and Ontogeny Skeleton and skin Soft anatomy Oxygen, metabolism and energetics The sensory systems Homeostasis Functional morphology of locomotion and feeding Early life history Juveniles, adults, age and growth Part III: Taxonomy, Phylogeny and Evolution "A history of fishes" Chondrichthyes: sharks, skates, rays and chimaeras Living representatives of primitive fishes Teleosts at last I: bonytongues through angler fishes Teleosts at last II: spiny-rayed fishes Part IV: Zoogeography, Habitats and Adaptations Zoogeography Special habitats and special adaptations Part V: Behavior and Ecology Fishes as predators Fishes as prey Fishes as social animals: reproduction Fishes as social animals: aggregation, aggression and cooperation Cycles of activity and behaviour Individuals, populations and assemblages Communities, ecosystems and the functional role of fishes Part VI: The Future of Fishes Conservation References Index
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