IN-SITU STUDIES ON HEART-RATE AND LOCOMOTOR-ACTIVITY IN THE FRESH-WATER 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 int ensity. 2. All sixteen crayfish investigated were found to express cir cadian 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 rh ythmicity in locomotor activity with a 24 +/- 2h periodicity and, for 67% of the animals, this rhythmicity was significant (P < 0.05). In ge neral, the expression of circadian rhythms was associated with nocturn al behaviour, heart rates and locomotor activity levels being higher a t night than by day. 3. A positive correlation was observed between he art rate and locomotor activity. Temperature and light intensity exert ed positive and negative influences on heart rate in A. astacus, respe ctively. 4. In the first experiment, 84% of the variation in heart rat e 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.