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 app roximately twentyfold range in total body length (L), which may imply unfav ourable differences in encountered flow regime (viscous versus inertial) du ring ontogeny. Using high-speed video (500 fields s(-1)), we analysed the k inematics, mechanical efficiency, swimming speeds and flow regime of swimmi ng hatchlings (approximately 0.01 m L, "stage 1"), 2 week old animals (appr oximately 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 ki nematics, 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. M echanical 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 Reyno lds numbers, and thus avoid having to swim in the unfavourable viscous flow regime.