Locomotion of Hermit Crabs (coenobita Compressus) on Beach and Treadmill

Coenobita compressus (H. Milne Edwards) walk forward on six legs using an alternating tripod gait similar to that of insects. The first walking leg provides the driving force for locomotion aided secondarily by the second walking leg, while the chelipeds act largely as supports. The left appendages are longer and heavier than the right, and they extend further laterally from the midline during their stride, thus compensating for the asymmetry of the crab which has a dextrally coiled shell and an abdomen displaced to the right. The abdomen is normally carried off the ground, but it is dragged when the shell is large. Bilateral leg autotomy alters gait patterns; usually a diagonal quadrupedal gait was adopted. Walking was poor in crabs without chelae (Li and Ri) because of problems of balance. Crabs without their first walking legs (L2 and R2) were the most accomplished amputee walkers. Crabs lacking legs L3 and R3 showed the most gait diversity. Velocity of travel is a function of crab size and the substrate walked upon. Large crabs travel faster than small ones on the beach by increasing their stride length rather than stepping frequency. Studies on a miniature treadmill showed individual crabs change velocity by changing both stepping frequency and stride length. Snail shells of the genus Nerita are carried; they are the lightest shells on the beach. Shell mass for an individual may vary three-fold, but usually the masses of the shell and crab are similar. Crabs running with and without shells have the same step frequency and stride length.