Exercise in the Terrestrial Christmas Island Red Crab Gecarcoidea Natalis - Energetics of Locomotion

The respiratory and circulatory physiology of exercising Christmas Island red crabs Gecarcoidea natalis were investigated with respect to their annual breeding migration. Red crabs were allowed to walk for up to 45 min. During this exercise period, the functioning of the circulatory system in gas transport and the energy status of the red crabs were quantified. Energy production during exercise required both aerobic and anaerobic contributions. The aerobic scope of G. natalis was low, with only a doubling of the resting rate of oxygen consumption (resting M(dot)O2=95±15 µmol kg-1 min-1). Maximal O2 consumption was attained within the first 5 min of exercise and the level remained stable thereafter. The anaerobic contribution to energy production was directly related to the speed of locomotion. l-lactate levels in blood and leg muscle were similar throughout the exercise period; blood lactate concentration was 33.39±2.29 mmol l-1 after 45 min of exercise. Heart rate in resting animals was 56±7 beats min-1. At the onset of exercise, heart rate also doubled, but without a significant increase in cardiac output. Increased O2 delivery was facilitated by increased extraction from the blood. During the 45 min of exercise, glucose levels increased rapidly in the muscle tissue (from 2.30±0.54 to 8.78±1.20 mmol l-1) and subsequently in the blood (from 1.22±0.26 to 2.12±0.17 mmol l-1), fuelling increased glycolysis during locomotion. The energy production from stored glucose/glycogen was sufficient to support the energetic needs of locomotion, since the energy charge remained stable at 0.82. Haemolymph l-lactate levels in crabs sampled in the field after migration were high compared with levels in many crustacean species but equivalent to l-lactate levels in laboratory animals exercised for less than 10 min. During their migration, therefore, the red crabs avoid exceptional l-lactate build-up in the blood by either walking very slowly or intermittently. However, G. natalis are exceptionally well adapted to cope with exhaustive locomotion and the resultant severe metabolic acidosis.