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

Abstract

Coenobita compressus (H. Milne Edwards) walk forward on six legs using analternating tripod gait similar to that of insects. The first walking leg provides thedriving force for locomotion aided secondarily by the second walking leg, while thechelipeds act largely as supports. The left appendages are longer and heavier than theright, 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 anabdomen 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 gaitwas adopted. Walking was poor in crabs without chelae (L1 and R1) because of problems of balance. Crabs without their first walking legs (L2 and R2) were themost accomplished amputee walkers. Crabs lacking legs L3 and R3 showed the mostgait diversity. Velocity of travel is a function of crab size and the substrate walked upon. Largecrabs 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 genusNerita 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.