The Christmas Island red crab Gecarcoidea natalis undergoes extreme changes in metabolic status, ranging from inactivity during
the dry season, to a spectacular annual breeding migration at the start of the wet season. The dramatic change in metabolic
physiology that this polarisation entails should be reflected in changes in endocrine physiology, particularly that of the
crustacean hyperglycaemic hormone (CHH), of which we know relatively little. CHH levels were measured using a novel
ultrasensitive time-resolved fluoroimmunoassay (TR-FIA), together with metabolites (glucose, lactate), in the field at several
scales of temporal resolution, during migratory activities (wet season) and during the inactive fossorial phase (dry season).
Release patterns of CHH were measured during extreme (forced) exercise, showing for the first time an unexpectedly rapid
pulsatile release of this hormone. A seasonally dependent glucose-sensitive negative-feedback loop was identified that might be
important in energy mobilisation during migration. Haemolymph lactate levels were strongly correlated with CHH levels in both
field and experimental animals. During migration, CHH levels were lower than during the dry season and, during migration,
daytime CHH levels (when most locomotor activity occurred) increased. However, the intense dawn activity in both dry and wet
seasons was not always associated with repeatable hyperglycaemia or CHH release. The results obtained are discussed in
relation to the life history and behaviour of G. natalis.