The biomechanics of fast-starts during ontogeny in the common carp cyprinus carpio

Common carp Cyprinus carpio L. were reared a constant temperature of 20 degrees C from the larval (7 mm total length) to the juvenile (80 mm) stage. Body morphology and white muscle mass distribution were measured. Fast-start escape responses were recorded using high-speed cinematography from which the velocities, accelerations and hydrodynamic power requirements were estimated. All three measures of fast-start performance increased during development. White muscle contraction regimes were calculated from changes in body shape during the fast-starts and used to predict the muscle force and power production for all longitudinal positions along the body. Scaling arguments predicted that increases in body length would constrain the fish to bend less rapidly because the cross-sectional muscle area, and hence force production, does not increase at the same rate as the inertial mass that resists bending. As predicted, the increases in body length resulted in decreases in muscle shortening velocity, and this coincided with increases in both the force and power produced by the muscles. The hydrodynamic efficiency, which relates the mechanical power produced by the muscles to the inertial power requirements in the direction of travel, showed no significant change during ontogeny. The increasing hydrodynamic power requirements were thus met by increases in the power available from the muscles. The majority of the increases in fast-start swimming performance during ontogeny can be explained by size-dependent increases in muscle power output. For all sizes, there was a decrease in muscle-mass-specific power output and an increase in muscle stress in a posterior direction along the body due to systematic variations in fibre strain. These changing strain regimes result in the central muscle bulk producing the majority of the power requirements during the fast-start, and this power is transmitted to the tail region of the fish and ultimately to the water via muscle in the caudal myotomes.