Jet-propelled swimming in scallops: swimming mechanics and ontogenic scaling

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Abstract
Scallop locomotion was investigated on the basis of an analysis of fluid forces acting on the body and the balance of the forces during swimming. A hydrodynamic model for unsteady jet propulsion was developed in which propulsion performance is characterized by three nondimensional parameters: the storage/discharge volume ratio, reduced clapping frequency, and reduced discharge frequency. Pulsed jet propulsion is designed to achieve high thrust, although not necessarily with low hydrodynamic propulsive efficiency, as was previously widely considered. Swimming in scallops is realized by orientating the body at a certain angle of attack and maintaining a minimum swimming speed to prevent sinking. The working frequency of the locomotor system is determined and adjusted by the swimming strategy (angle of attack, swimming speed, and trajectory angle). For Placopecten magellanicus, the optimum angle of attack is about 6° – 12°, at which swimming requires the lowest energy input (lowest frequency) and hydrodynamic behaviour is ideal (without severe separation and stall). To maintain level swimming, P. magellanicus, during almost all their life, must swim at 5 – 7 body lengths per second if postured at a 6° – 12° angle of attack. The estimated Froude efficiency decreases during growth from about 0.5 to 0.3 for level swimming and from about 0.4 to 0.2 for climbing at an angle of 25°. It is suggested that the heavy body and inferior hydrodynamic characteristics (low aspect ratio and imperfect planform shape) have prevented scallops from becoming good swimmers. These problems are enhanced as the animals grow.

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