| Tunas are noted for traits that enhance physiological performance, including the ability to maintain elevated temperatures in swimming muscle, viscera, brain and eye tissues, however the heart operates at ambient temperature. Among tunas, bluefin maintain the highest internal temperatures and have the broadest thermal niches, making their physiological performance and its response to temperature particularly interesting. This study presents measurements of heart function and oxygen consumption of Pacific bluefin and yellowfin tunas at temperatures encompassing those encountered in the wild.; Cooling reduces heart rates and cardiac outputs of bluefin and yellowfin hearts in situ, suggesting a physiological limit in cold waters. However, bluefin hearts are less temperature sensitive than those of yellowfin, matching the wider thermal niche of bluefin. Bluefin hearts beat rhythmically at 2°C, corresponding to the lowest ambient temperatures observed in tracks of wild bluefin tuna. The cold tolerance of bluefin hearts is associated with increased cardiac sarcoplasmic reticulum Ca2+ cycling.; Minimum metabolic rates of bluefin and yellowfin tunas swimming in a swim-tunnel at 20°C after a two day fast and overnight acclimation period were 222 +/- 24 mg O2 kg-1 hr-1 and 162 +/- 19 mg O2 kg-1 hr-1, respectively. The higher VO2 of bluefin accords with biochemical and tissue-level physiological characteristics. VO2 results lower than those in the literature for yellowfin suggest that previous measurements may not represent a basal physiological state.; A remarkable U-shaped relationship of VO2 to temperature was observed in bluefin swimming at a fixed speed of 1.0 length s-1. A minimum VO2, or potential thermoneutral zone, was evident at 15--20°C, with increases in VO2 at 8--10°C as well as 25°C. At higher speeds, VO2 was similar among different temperatures, suggesting that bluefin increase muscular heat production to thermoregulate at low ambient temperatures. Muscle temperature measurements will be required to test this hypothesis.; The finding that bluefin have high metabolic rates and enhanced heart function relative to tropical tunas supports an evolutionary linkage of endothermy, increased metabolic rate, high heart rates, and intracellular Ca2+ cycling in heart cells. |
