Energetics of Hermit Crabs during Locomotion: the Cost of Carrying a Shell

Oxygen consumption (VOZ) was measured as hermit crabs (Coenobita compressus) walked at controlled velocities on a motor-driven treadmill inside a small respirometer. The crabs displayed an aerobic response to exercise with a rapid increase in VQ2 reaching a steady state in about 5-6 min followed by a rapid recovery. The highest VQ2 was four times the resting rate. V^ was directly dependent on the velocity of travel (V): VQ2 = 0*29+1*98V. Metabolic rate was increased significantly in crabs with bilateral leg ablation. The cost of shell carrying was evaluated by comparing Voj of crabs with and without their protective snail shells at different velocities; the absolute difference was constant (0-17mlO2g h"), suggesting that the cost of shell support was constant per unit of time regardless of speed. The cost of transport dropped dramatically with speed for crabs both with and without snail shells. Crabs carrying shells used twice as much O2 per gram per kilometre as did 'nude' crabs walking slowly at 0-02kmh~\ but the difference decreased to 1-3 times when velocity was increased 10-fold. Hermit crabs did not increase their VQ2 proportionately with load: the VQ2 loaded/unloaded ratio was consistently less than the mass loaded/unloaded ratio. This apparent increase in efficiency may be due to the fact that crabs carrying heavy shells alter their leg position and tend to drag their shell. Crabs with and without shells have the same minimum cost of transport CM, so travel at the highest velocity is theoretically the most economical way to cover a given distance. Appropriately, crabs on the beach average a fast 0-23 km h" which produces a low cost of transport only 1-3 times higher than CM. The CM of six-legged hermit crabs is comparable to that of mammals, birds, crabs and insects of similar size and indicates that leg number does not affect fuel economy.