BLANK, J.M.; MORRISSETTE, J.M.; LANDEIRA-FERNANDEZ, A.M.; BLOCK, B.A.; Hopkins Marine Station, Stanford University; Hopkins Marine Station, Stanford University; Hopkins Marine Station, Stanford University; Hopkins Marine Station, Stanford University: Cardiac Performance of Pacific Bluefin Tuna in Response to Temperature

Tunas are endothermic teleosts with high metabolic rates and exceptional cardiac performance. Previous physiological studies of tuna cardiovascular function have been conducted on juvenile tropical tunas, but previous work on bluefin tunas has been restricted to biochemical experiments and field observations. We report on the cardiac physiology of the Pacific bluefin tuna (Thunnus orientalis) as measured with an in situ heart preparation at temperatures of 30°C to 2°C. Heart rates ranged from 156 bpm at 30°C to 13 bpm at 2°C. Maximal stroke volumes were 1.1 ml/kg at 25°C and 1.3 ml/kg at 2°C. Maximal cardiac outputs were 106 ml/kg/min at 25�C and 18.1 ml/kg/min at 2°C. These data indicate that cardiovascular function in the Pacific bluefin tuna exhibits a strong temperature dependence, but cardiac function is retained at temperatures colder than those tolerated by tropical tunas (7-11°C). Tunas rely on an elaborate system of countercurrent heat exchangers, called retia mirabilia, to maintain endothermy of the slow-twitch muscles and viscera, however the aerobic myocardium must operate at ambient temperature. Comparisons of temperature sensitivity of yellowfin and bluefin tunas suggest that the bluefin tuna�s cardiac performance in the cold may be a key adaptation supporting the broad thermal niche of bluefin tuna in the wild. Cellular and biochemical studies suggest that cold-tolerance of bluefin tuna hearts depends on enhanced SR Ca²⁺ cycling in cardiac myocytes. Funded by NSF, AHA, Pew Foundation, and Monterey Bay Aquarium.