Effects of rapid cooling on heart rate of the Japanese lobster in vivo

Seasonal changes of the effects of rapid cooling on the in vivo heart rate of the lobster, Panulirus japonicus, were studied. Cardiac activity was monitored with chronically implanted electrodes. The mean heart rate was 45.5 plus or minus 6.0 bts/min during winter and 99.5 plus or minus 7.5 bts/min during the other seasons at the same acclimation temperature (20 plus or minus 1 degree C). When the ambient temperature was lowered (0.1-1.0 degree C/min, dT = 5-6 degree C), the heart rate decreased along a linear line in spring, summer and fall. The correlation ratio of the heart rate and temperature was 0.98 with the minimum rate of 67.0 plus or minus 12.0 bts/min. The Q sub(10) value was 2.2. In contrast, the heart rate in winter decreased only for the initial few minutes of cooling and little for the later phase with the minimum rate of 28.8 plus or minus 9.5 bts/min. Q sub(10) was 2.6. The correlation plots for the heart rate and temperature appeared to regress on two linear lines. The ratio was 0.88 in the range of 19-21 degree C and 0.41 in the range of 15-19 degree C. These data may suggest that the lobsters in winter have some compensatory mechanisms for a heart rate drop by cooling as well as for the mean heart rate, which are different from those in the other seasons.