UNDERWATER PUNTING BY AN INTERTIDAL CRAB - A NOVEL GAIT REVEALED BY THE KINEMATICS OF PEDESTRIAN LOCOMOTION IN AIR VERSUS WATER

As an animal moves from air to water, its effective weight is substant ially reduced by buoyancy while the fluid-dynamic forces (e,g. lift an d drag) are increased 800-fold. The changes in the magnitude of these forces are likely to have substantial consequences for locomotion as w ell as for resistance to being overturned. We began our investigation of aquatic pedestrian locomotion by quantifying the kinematics of crab s at slow speeds where buoyant forces are more important relative to f luid-dynamic forces. At these slow speeds, we used reduced-gravity mod els of terrestrial locomotion to predict trends in the kinematics of a quatic pedestrian locomotion. Using these models, we expected animals in water to use running gaits even at slow speeds. We hypothesized tha t aquatic pedestrians would (1) use lower duty factors and longer peri ods with no ground contact, (2) demonstrate more variable kinematics a nd (3) adopt wider stances for increased horizontal stability against fluid-dynamic forces than animals moving at the same speed on land. We tested these predictions by measuring the three-dimensional kinematic s of intertidal rock crabs (Grapsus tenuicrustatus) locomoting through water and air at the same velocity (9 cm s(-1)) over a flat substratu m. As predicted from reduced-gravity models of running, crabs moving u nder water showed decreased leg contact times and duty factors relativ e to locomotion on land, In water, the legs cycled intermittently, few er legs were in contact with the substratum and leg kinematics were mu ch more variable than on land, The width of the crab's stance was 19 % greater in water than in air, thereby increasing stability against ov erturning by hydrodynamic forces, Rather than an alternating tetrapod or metachronal wave gait, crabs in water used a novel gait we termed ' underwater punting', characterized by alternating phases of generating thrust against the substratum and gliding through the water.