Heart rate responses to temperature in free-swimming Pacific bluefin tuna (Thunnus orientalis)

The bluefin tuna heart remains at ambient water temperature (T-a) but must supply blood to warm regions of the body served by countercurrent vascular heat exchangers. Despite this unusual physiology, inherent difficulties have precluded an understanding of the cardiovascular responses to T-a in free-swimming bluefin tunas. We measured the heart rate (fH) responses of two captive Pacific bluefin tunas (Thunnus orientalis; 9.7 and 13.3. kg) over a cumulative period of 40. days. Routine fH during fasting in the holding tank at a T-a of 20 degrees C was 45.1 +/- 8.0 and 40.7 +/- 6.5. beats. min(-1) for Tuna 1 and Tuna 2, respectively. f(H) decreased in each fish with a Q(10) temperature coefficient of 2.6 (Tuna 1) and 3.1 (Tuna 2) as Ta in the tank was slowly decreased to 15 degrees C (similar to 0.4 degrees Ch(-1)), despite a gradual increase in swimming speed. The same thermal challenge during digestion revealed similar thermal dependence of fH and indicated that the rate of visceral cooling is not buffered by the heat increment of feeding. Acutely decreasing Ta from 20 to 10 degrees C while Tuna 1 swam in a tunnel respirometer caused a progressive increase in tail-beat frequency and oxygen consumption rate ((M) over dot(O2)). fH of this fish decreased with a Q(10) of 2.7 as T-a decreased between 20 and 15 degrees C, while further cooling to 10 degrees C saw a general plateau in f(H) around 35. beats. min(-1) with a Q(10) of 1.3. A discussion of the relationships between f(H), (M) over dot(O2) and haemoglobin-oxygen binding sheds further light on how bluefin cardiorespiratory systems function in a changing thermal environment.