ABSTRACT
Numerous water-breathers exhibit a gas-exchange regulation strategy that maintains O2 partial pressure, 
, in the arterial blood within the range 1–3 kPa at rest during the daytime. In a night-active crustacean, we examined whether this could limit the rate of O2 consumption 
of locomotor muscles and/or the whole body as part of a coordinated response to energy conservation. In the crayfish Astacus leptodactylus, we compared the in vitro relationship between the 
of locomotor muscles as a function of the extracellular 
and 
and in vivo circadian changes in blood gas tensions at various values of water 
. In vitro, the 
of locomotor muscle, either at rest or when stimulated with CCCP, was O2-dependent up to an extracellular 
of 8–10 kPa. In vivo, the existence of a night-time increase in arterial 
of up to 4 kPa at water 
values of 20 and 40 kPa was demonstrated, but an experimental increase in arterial 
during the day did not lead to any rise in whole-body 
. This suggested that the low blood 
in normoxia has no global limiting effect on daytime whole-body 
. The participation of blood O2 status in shaping the circadian behaviour of crayfish is discussed.
