Respiratory gas transport, metabolic status, and locomotor capacity of theChristmas Island red crab Gecarcoidea natalis assessed in the field with respect to dichotomous seasonal activity levels

Red crabs, Gecarcoidea natalis, exhibit seasonal activity patterns: low act ivity during the dry season when they shelter in burrows to avoid dehydrati on, and high activity during the wet season. Red crabs were examined in sit u in the rainforest of Christmas Island to determine if there were underlyi ng seasonal differences in the capacity for exercise and associated metabol ism. During both seasons, free-ranging (FR) crabs engaged in their normal a ctivities and, together with crabs induced to exercise for 5 min, were samp led for haemolymph and muscle tissue. Respiratory gases in the haemolymph a nd key metabolites were measured to assess differences in metabolic status of FR and exercised crabs. Actively foraging FR crabs during the wet season exhibited a relative haemolymph hypoxia (2.9 kPa) and accumulated an extra 3 mmol.litre(-1) of CO2 compared to the relatively inactive FR crabs durin g the dry season. Wet-season crabs appeared to be in a state of relative re spiratory acidosis compared to dry-season animals. This hypercapnia may ari se as a consequence of a relative hypoventilation in animals with a relativ ely higher metabolic rate during the wet season. Oxygenation of pulmonary a nd arterial haemolymph was similar and remained high after 5 min of exercis e, indicating that the gills and lungs functioned similarly in gas exchange in both FR and exercised crabs. During exercise, venous O-2 reserves decre ased and red crabs experienced a mixed respiratory/metabolic acidosis. Simi lar changes, after 5 min of enforced exercise, in metabolite concentrations , pH and respiratory gas status in the haemolymph during both sampling seas ons suggest that the crabs maintain similar capacity to increase exercise d uring the wet and the dry seasons, despite the differences in underlying ph ysiological status. This is important since after prolonged inactivity duri ng the dry season, with the arrival of monosoonal rains, red crabs must eng age in their annual breeding migration. J. Exp. Zool. 286:552-562, 2000. (C ) 2000 Wiley-Liss, Inc.