In situ studies on heart rate and locomotor activity in the freshwater crayfish, Astacus astacus (L.) in relation to natural fluctuations in temperature and light intensity

1. By the aid of a computer technique, the in situ variations in heart rate and locomotor activity were recorded continuously for 7 days in sixteen individuals of the crayfish, Astacus astacus (Linnaeus 1758), in association with natural changes in water temperature and light intensity. 2. All sixteen crayfish investigated were found to express circadian rhythmicity in heart rate with a periodicity of approximate to 24 h. For 88% of the animals investigated, rhythmicity was significant (P < 0.05). Ninety-three per cent of the crayfish showed circadian rhythmicity in locomotor activity with a 24 +/- 2h periodicity and, for 67% of the animals, this rhythmicity was significant (P < 0.05). In general, the expression of circadian rhythms was associated with nocturnal behaviour, heart rates and locomotor activity levels being higher at night than by day. 3. A positive correlation was observed between heart rate and locomotor activity. Temperature and light intensity exerted positive and negative influences on heart rate in A. astacus, respectively. 4. In the first experiment, 84% of the variation in heart rate could be explained by the changes in locomotor activity, temperature and light intensity. In the second experiment 35% of the variation in the heart rate was explained using these parameters.