MECHANICS OF ESCAPE RESPONSES IN CRAYFISH (ORCONECTES VIRILIS)

Measurements of acceleration performance of crayfish (mean mass 0.018 kg) were made during lateral giant mediated tail flips (LG tail flips) and truncated tail flips at 15°C. The LG tail flip power stroke was composed of a lift-off phase, when crayfish accelerated vertically from the substrate, and a free swimming phase. The total duration of the power stroke was 44 ms, followed by a recovery stroke lasting 173 ms. Truncated tail flips were used in acceleration and swimming by crayfish free of the substrate. Power strokes had a mean duration of 36 ms, and recovery strokes 92 ms. Net velocities, acceleration rates, and distances travelled by the centre of mass were similar for both types of tail flips. Thrust was generated almost entirely by the uropods and telson. Velocities and angles of orientation to the horizontal of abdominal segments were similar for both types of tail flip. Angles of attack were large, varying from 30° to 90°. Pressure (drag) forces were considered negligible compared to inertial forces associated with the acceleration of added water mass. Thrust forces, energy and power were determined for exemplary tail flips. Thrust was 0.92 and 0.42 N for LG tail flip lift-off and swimming phases respectively, and 0.29 N for the swimming truncated tail flip. Rates of working were 0.39, 0.19, and 0.18 W respectively. The efficiency of converting muscle power to backward motion was estimated to be 0.5 for power strokes and 0.68 for complete swimming cycles. Comparisons with fish performance suggested fish would be less efficient (0.1-0.2). The low efficiency is attributed to energy lost in lateral recoil movements.