Strategies for migration in the terrestrial Christmas Island red crab Gecarcoidea natalis: Intermittent versus continuous locomotion

The terrestrial red crab Gecarcoidea natalis undertakes an annual breeding migration and must sustain locomotion for prolonged periods. The migrating crabs must travel a specific distance in a fixed time and can either walk a t a constant speed or walk faster for short periods and then pause to feed or rest, To simulate the potential differences between continuous and inter mittent locomotion during the migration, red crabs were sampled after walki ng at a voluntary speed for 5 or 20 min without pausing or after 20 min of enforced walking intermittently at approximately twice that speed. The resp iratory and metabolic status of the crabs was investigated during the diffe rent exercise regimes to assess which strategy might be more advantageous d uring the migration. The gills and lungs appeared to function similarly in gas exchange, and the P-O2 in the haemolymph was 8.2 kPa which fully saturated the haemocyanin w ith Oz. The uptake of O-2 by red crabs was diffusion-limited and the diffus ion coefficient (L-diff) varied from 0.53 in resting crabs to 0.8 post-exer cise, Post-exercise, red crabs experienced a mixed respiratory/metabolic ac idosis which was greatest (0.2 pH units) in crabs walking intermittently, i .e. at a higher speed. Haemolymph L-lactate concentrations peaked at 5 mmol l(-1) immediately post-exercise in the intermittent exercise group, wherea s after 20 min of continuous exercise haemolymph L-lactate continued to inc rease, reaching a maximum of 2.5 mmol l(-1) at Ih post-exercise. L-Lactate recovered slowly to basal levels within 5 h, The maximum rate of L-lactate clearance from the haemolymph was only 1.75 mmol l(-1) h(-1), and short pau ses in exercise were insufficient for substantial L-lactate reoxidation, Ex ercise regimes in the laboratory were within the locomotor speeds determine d for migrating red crabs, which overall have a mean walking speed close to their aerobic limit but periodically pause and also exceed this limit by t hree- to fourfold.