Locomotory behaviour of the intertidal marble crab (Pachygrapsus marmoratus) supports the underwater spring loaded inverted pendulum as fundamental model for punting in animals.

In aquatic pedestrial locomotion, the dynamics of terrestrial and aquatic environments are coupled. Here we study terrestrial running and aquatic punting locomotion of the marine-living crabPachygrapsus marmoratus. We detected both active and passive phases of running and punting through the observation of crab locomotory behavior in standardized settings and by 3D kinematic analysis of its dynamic gaits using high speed video cameras. Variations in different stride parameters were studied and compared. The comparison was done based on the dimensionless parameter Froude number (Fr) to account for the effect of buoyancy and size variability among the crabs. The Underwater Spring Loaded Inverted Pendulum (USLIP) model better fitted the dynamics of aquatic punting. This model comprehends the damping effect of the aquatic environment, variable not considered by the Spring Loaded Inverted Pendulum (SLIP) in reduced gravity. The results of the present study highlight the underlying principles of aquatic-terrestrial locomotion by comparing itself with terrestrial locomotion. Comparing punting to running, we show evidence that: stride period increased; the duty cycle decreased; and, the orientation of carapace was more inclined with the horizontal plane, indicating the significance of fluid forces on the dynamics attributed by the aquatic environment. Moreover, we discovered periodicity in punting locomotion of crabs and two different gaits, namely, long flight punting and short flight punting, distinguished by both footfall patterns and kinematic parameters. The generic fundamental model which belongs to all animal performing both terrestrial and aquatic legged locomotion has implications both on control strategies, evolution, and translation on robotic artifacts.