The kinematics of voluntary steady swimming of hatchling and adult axolotls (Ambystoma mexicanum Shaw, 1789)

Axolotls swim throughout post-hatching ontogeny. This coincides with an approximately twentyfold range in total body length (L), which may imply unfavourable differences in encountered flow regime (viscous versus inertial) during ontogeny. Using high-speed video (500 fields s(-1)), we analysed the kinematics, mechanical efficiency, swimming speeds and flow regime of swimming hatchlings (approximately 0.01 m L, ``stage 1''), 2 week old animals (approximately 0.02 m L, ``stage 2'') and 20 week old animals (approximatery 0.08 m L, ``stage 3''), and compared the data with similar data horn adults (0.135-0.238 m L, ``stage 4''). All stages swim by passing waves of lateral curvature down the body. The kinematics, described by the characteristics of this wave (speed, frequency, length, amplitude) are largely comparable in all four stages : within each stage, swimming speed is increased by increasing the wave frequency only. Mechanical swimming efficiency, estimated by means of Lighthill's elongated-body theory, is about 5\% lower in hatchlings than in adults. The most striking result is that the observed, voluntary absolute swimming speeds from stage 1 to stage 4 are much more similar than would be expected given the twenty-fold L range. Possible explanations are ecological and/or hydrodynamical. Firstly predator escape success increases as the swimming speed increases. Secondly, by adopting high speeds, axolotls increase Reynolds numbers, and thus avoid having to swim in the unfavourable viscous flow regime.