Abstract

By the use of computer techniques, the circadian rhythmicities in heart rate and locomotor activity were investigated in noble crayfish, Astacus astacus, exposed to different regimes of hypoxia. In one experiment, noble crayfish were exposed to normoxia for 5 days, then exposed to hypoxia during night time for 6 days (2-3 mg O₂ l^-1), followed by a 5 day period with continuous hypoxia. In another experiment, crayfish were also exposed to 5 days of normoxia, then to 6 days of hypoxia (1-4 mg O₂ l^-1), and then again 3 days of normoxia. Crayfish were found to express a circadian rhythmicity in both heart rate and locomotor activity during normoxia with heart rate and locomotor activity levels being high during night time and low during day time. When the oxygen tension was kept low during night time, time series for heart rate and locomotor activity became less rhythmic, and the rhythm disappeared completely when crayfish were exposed to long-term hypoxia. When crayfish were exposed to long-term hypoxia following normoxia, heart rate and locomotor activity levels decreased, and the circadian rhythmicity in heart rate and locomotor activity were less pronounced but could still be observed. When crayfish were returned to normoxic conditions, their heart rate and locomotor activity levels increased again and a circadian rhythmicity with a periodicity of approximately 24 hours could be observed. A tight coupling was found between heart rate and locomotor activity. This coupling however, was weaker during hypoxia since the crayfish were less active during hypoxia. The experiments show, that hypoxia with oxygen tensions of 2-3 mg O₂ l^-1 can be tolerated for several days, yet the nocturnal behaviour of the crayfish is affected.

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