The influence of unsteady (time varying) motion on the energetics of swimming was investigated with measurements and theoretical estimates of the specific cost of locomotion for two species of hydrozoan medusae: Gonionemus vertens L. Agassiz and Stomotoca atra L. Agassiz. These species, both about 1 g, provide a broad range of swimming speeds for which locomotor energetics can be explored. The cost of locomotion (a dimensionless ratio defined as the rate of energy consumption divided by the product of an animal's weight and speed) was estimated from the oxygen consumption rate of medusae tethered to a force platform. Swimming beat frequency, as monitored by the force platform, was correlated with velocity obtained from ciné-films of freely swimming medusae.

The specific cost of locomotion was 6.3 (dimensionless), nearly one order of magnitude greater than the extrapolated cost of locomotion for a vertebrate swimmer of equivalent body mass. The great magnitude of this cost is attributed to two aspects of the periodic pulsatile pattern of locomotion by these cnidarians: (1) the energy expenditure for periodic accelerations of the animal's mass and some mass of fluid about it and (2) the energy dissipated in bell deformations and recovery strokes. Nearly 25% of the augmented cost is attributed to the energy required to overcome an unsteady flow force, that is the force required to accelerate fluid about the animal. Such a high cost of locomotion is apparently a general consequence of swimming with a discontinuous production of thrust.

The mechanics of discontinuous swimming are explored by measuring the hydrodynamic coefficients associated with unsteady flows (added-mass coefficients) for models of medusae. The results suggest that the effects of vortex formation and shedding may significantly increase the magnitude of the forces produced by or resisting unsteady animal locomotion.

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