Figure 1. Hypothermic anaesthesia leads to cessation of opercular movements of medaka but maintains heartbeats.

(a) A medaka chamber used in the hypothermic anaesthesia experiment. The water temperature was measured using a thermocouple near the transparent medaka (STIII). Heartbeats and opercular movements were videotaped from the side of the chamber. (b) The heart rate and breathing rate are correlated with water temperature. At approximately 5 °C, the breathing rate dropped to zero, but the heart rate did not. With or without water in a tube, heartbeats existed and opercular movements ceased. Sudden temperature increases at 5,500 and 7,484 seconds corresponded to the draining of water and returning the transparent medaka to a tank, respectively. (c) During the 60 minutes in which the opercular movements were lost, the heart rate decreased slightly with a temperature decrease. (d) Although the temperature increased during the 60 minutes, the heart rate stayed constant. (e) In the case of constant temperature during the 60 minutes, the heart rate also stayed constant. A sudden change in the temperature at the beginning of the 60 minutes decreased the constant heart rate. All transparent medaka in (b–e) recovered uneventfully after the experiments.