Dissociation of heart rate and locomotor activity during the expression of rhythmic behaviour in the shore crab Carcinus maenas

Expression of endogenous rhythms in heart rate and locomotor activity were examined in male Carcinus maenas (Brachyura, Crustacea). Heart rate was monitored in immobilised and free moving crabs using a non-invasive, infra-red transducer system. Temporal relations between changes in heart rate and alterations in locomotor activity were investigated using a computer-aided video system to track movements of crabs whilst simultaneously monitoring cardiac activity. Mean heart rates of immobilised and free moving crabs were 63.9 +/- 23.7 beats per minute and 44.3 +/- 13.4 beats per minutes respectively. Endogenous circatidal rhythms in heart rate were detected in both immobilised and free moving individuals. Highest heart rates occurred at expected high tide times. A circadian heart rate rhythm was also evident with highest rates occurring during the hours of darkness. Some crabs, however, failed to express endogenous rhythms. When crabs became active, heart rate usually increased during the following 60 seconds. In some cases, however, heart rate increased 1-3 minutes prior to the initiation of locomotor activity. The findings are discussed with regard to the behaviour of C. maenas in the field.