Historically, aquatic locomotion has intrigued scientists across a wide range of disciplines, from hydrodynamics to ecology. Many diverse forms of life representing all five kingdoms reside in water. Of these organisms, the largest and most motile are the vertebrates, which must contend with the challenges of moving through a highly viscous and dense medium. Yet in spite of these challenges over half of all known vertebrate species, the fishes, have flourished in aquatic environments.

One of the key design features of fishes responsible for their success in the aquatic medium is the elaboration of the body into a variety of blade-like control surfaces: the fins. In particular, the function of the tail (or caudal) fin has been the focus of interest as biologists attempt to understand the dynamics of aquatic locomotion. The evolution of the caudal fin has traditionally been understood from the perspective of swimming efficiency, with the idea that tail shapes have evolved to perform more efficiently. The symmetrical homocercal tail characteristic of advanced fishes (Fig. 1A ) is considered to be highly efficient because it produces a horizontal force component (thrust) without a vertical force component (lift). Energy is therefore conserved by producing force only in the direction of motion. In contrast, the asymmetrical heterocercal tail characteristic of primitive fishes (Fig. 1B, C) produces lift in addition to thrust. The generation of lift represents wasted energy, for in order to stop rising in the water column a swimming fish must produce a counteracting downward force (e.g. with pectoral fins or negative buoyancy) to achieve propulsion in the horizontal plane (Ferry and Lauder, 1996). Although there have been significant advances in experimental kinematic and theoretical analyses of tail fin locomotion, the lack of sophisticated technology has until now prevented essential progress towards the quantitative analysis of flow structure behind tail fins. The inability to quantify the flow around a moving tail precludes resolution of force production questions and thus swimming efficiency.

Fluid mechanics engineers have recently developed a technique called Digital Particle Image Velocimetry (DPIV) which allows the visualization and quantification of the three-dimensional flow field around immersed objects. Images of suspended particles in the flow illuminated by a powerful laser are recorded by a high-speed video system. Pairs of consecutive images are digitized and processed with fluid dynamics software, which performs a spatial cross-correlation analysis to produce a vector plot of flow velocity. Three-dimensional flow patterns are then reconstructed from data collected by orienting the laser sheet in three orthogonal planes corresponding to transverse, parasagittal, and frontal sections through the wake of a fish.

DPIV represents a powerful tool with which we can ask previously unanswerable questions. Question # (Fig. 1): How do the flow patterns around a homocercal and heterocercal tail differ? Question #: What differences, if any, are seen when comparing flow between tails of similar morphology?


To begin addressing these questions, over the past summer I have acquired preliminary data using the technology available in the laboratory of my advisor, Dr. George Lauder. These data show that the tail of the white sturgeon (Acipenser transmontanus) produces jets of water directed posteriorly from the body centered within vortex rings, a shape originally inferred from the wake of birds (Kokshaysky, 1979). I intend to determine whether or not the tail produces lift by measuring the angle with which the rings are shed relative to the long axis of the animal. Can we assume functional similarity between the homologous tails of sturgeon and sharks based solely on morphology? That is, does the heterocercal tail of a sturgeon provide lift like the heterocercal tail of a shark (Ferry and Lauder, 1996)? If so, DPIV analysis would reveal that the rings are oriented postero-ventrally to the long axis, generating a vertical force component. Alternatively, if the sturgeon tail produced vortex rings in line with its long axis, a strong argument would be made against the classical lift-based model of the heterocercal shark tail. Can a heterocercal tail of a primitive fish have the function reserved for the traditional role of a homocercal tail characteristic of teleosts? What would enable two similar morphologies (Fig. 1B, C) to possess different functions? Could it be differences in the stiffness of the tail, or differences in muscular or neural output control?

Over the course of my preliminary experiments I have analyzed sequential images tracking the spatial evolution of flow structures behind the sturgeon tail that suggest the formation of discrete, unlinked rings. This result does not conform with the predominant view of homocercal tail locomotion in the literature or with previous data collected on the homocercal tail of bluegill sunfish (Lepomis macrochirus), in which continuous linked rings are formed. Does the shape of the tail dictate whether the rings are linked or distinct, or is it oscillation frequency? If validated, the production of unlinked rings would represent a previously undescribed hydrodynamical method of caudal fin locomotion.

Visualization and quantification of flow structure with DPIV represents the next logical step in the field of fish locomotion. In conjunction with the wealth of kinematic data, DPIV has the potential to allow re-evaluation of the relationship between caudal fin function and morphology. For the first time, forces produced by the tail can be directly quantified by calculating the change in the momentum of the ring with respect to time. Consequently, the results of the proposed experiments can be coupled with force data gathered from other fins to create full-body, force-balance diagrams that would determine the relationship of lift and thrust to weight and drag. Other applications may include determining force production in fish turning and the functional effects of differential fin placement on the body.

Both the Ecology and Evolution department and the Engineering department at UC Irvine possess high-caliber faculty with diverse interests, with a particularly strong group in organismal physiology. The education I will receive from working with Dr. Lauder and the interactions I will have with other members of the department will prepare me well for my pursuit of a tenure-track professorship. The multi-disciplinary nature of my project, requiring collaborations with experts in other fields and literacy with constantly evolving computer programs, will provide me with a sound basis for the creative utilization of current technology in answering biological questions.

 

 

Literature Cited

 

Ferry, L.A. and Lauder, G.V. (1996). Heterocercal tail function in leopard sharks: a three-

dimensional kinematic analysis of two models. J. Exp. Biol. 199, 2253-2268.

 

Kokshaysky, N.V. (1979) Tracing the wake of a flying bird. Nature. 279, 146-148.

 

Jimmys CV