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Hydrodynamics of Balistiform swimming in the Picasso Triggerfish, Rhinecanthus aculeatus Loofbourrow, Hale

Abstract

Aquatic propulsion by means of undulatory movements of the median (dorsal and anal) fins is the primary mode of transport for the Picasso triggerfish (Rhinecanthus aculeatus). Known as balistiform locomotion, this form of propulsion is an adaptation for highly efficient movement within complex environments such as coral reefs. A principle component of balistiform locomotion has been the development of momentum enhancement, a fin-force multiplier that increases swimming efficiency. This study examines the kinematics and energetics of balistiform locomotion employing theoretical models of thrust, power, and efficiency. Thrust and power were calculated and compared with theoretical values modeled by Lighthill and Blake (1990). This model has heretofore not been thoroughly vetted and was tested for accuracy and applicability. Thrust force was estimated from resistance (drag) using a vertical dead drop to determine terminal velocity; power was calculated from oxygen consumption measurements at different speeds. The Lighthill and Blake (1990) model requires median fin kinematics (frequency, wavelength, amplitude, wave angle), which were measured from high-speed videography and followed statistically predicted trends with frequency being the dominant variable, and the others changing little or not at all with speed. Momentum enhancement was found to be 3.6, close to Lighthill and Blake’s (1990) theoretically predicted value of 2.5. Momentum enhancement is experimentally proven here for the first time. Theoretical and empirical thrust force values are closely matched; theoretical thrust is greater at lower speeds and lower at higher speeds. The ratio of theoretical thrust to drag-estimated thrust averages 1.08. Theoretical values for power are greater than those measured by a factor of about 3.6 and cannot be explained by measurement error.

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Attribution-NonCommercial-NoDerivatives 4.0 International

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