Respiratory and acid–base responses during migration and to exercise by the terrestrial crabDiscoplax (Cardisoma) hirtipes, with regard to season, humidity and behaviour

Abstract

SUMMARYThe terrestrial crab Discoplax hirtipes (formerly Cardisoma hirtipes) exhibits a seasonally dichotomous activity pattern governed by the seasonal rainfall on Christmas Island, with a breeding migration in the wet season. Greater activity in the wet season reflects a release of constraints on ion and water balance independent of changes in exercise physiology. The respiratory responses to walking exercise by D. hirtipes were assessed with regard to humidity and season. In the laboratory, crabs walked 38% faster when humidity was high, while the O2 uptake across the lung became diffusion limited despite a Ca2+-induced increase in O2 affinity of the haemocyanin. Crabs walking in 90% relative humidity exhibited a larger metabolic acidosis while re-oxidation of l-lactate was only 0.49 mmol l–1 h–1. The wet-season crabs were more active and exhibited a respiratory acidosis compared with the quiescent dry-season crabs. The migration was close to the limit of the aerobic scope of the crabs, and the pulmonary oxygen partial pressure(PO2) was depressed, the O2 uptake diffusion limited and the arterial–venous PO2 difference diminished. Additional, enforced exercise induced a metabolic acidosis. The optimum strategy for migration would be walking continuously but at a speed within the aerobic scope. D. hirtipes is influenced by seasonal rain and responds to lower ambient humidity by limiting exercise. The behavioural response is paramount since the changes in respiratory status were determined primarily by the differences in commitment to, and investment in, walking and not by direct effects of humidity on respiratory physiology.